Astrophysics in the Next Decade - The James Webb Space Telescope

Astrophysics in the Next Decade

Astrophysics and Space Science Proceedings Advisory Editors: W.B. Burton; National Radio Astronomy Observatory, Charlottesville, Virginia USA and University of Leiden, The Netherlands Lars Lindberg Christensen; ESA/Hubble, Garching bei M¨unchen, Germany

For other titles published in this series, go to www.springer.com/series/7395

Astrophysics in the Next Decade The James Webb Space Telescope and Concurrent Facilities

Edited by

H.A. Thronson NASA Goddard Space Flight Center, Greenbelt, MD, U.S.A.

M. Stiavelli Space Telescope Science Institute, Baltimore, MD, U.S.A. and

A.G.G.M. Tielens NASA Ames Research Center, Moffett Field, CA, U.S.A.

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Editors Harley A. Thronson NASA Goddard Space Flight Center (GSFC) Greenbelt MD 20771 USA [email protected]

A.G.G.M. Tielens NASA Ames Research Center Mail Stop 239-4 Moffett Field CA 94035-1000 USA [email protected]

Massimo Stiavelli Space Telescope Science Institute (STScI) 3700 San Martin Dr. Baltimore MD 21218 USA [email protected]

ISBN: 978-1-4020-9456-9

e-ISBN: 978-1-4020-9457-6

DOI 10.1007/978-1-4020-9457-6 Library of Congress Control Number: 2008939015 c Springer Science+Business Media B.V. 2009  No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Printed on acid-free paper 9 8 7 6 5 4 3 2 1 springer.com

Preface

The conference that eventually became Astrophysics in the Next Decade: JWST and Concurrent Facilities was originally proposed by Matthew Greenhouse in 2006. He argued that the astronomical community would be well served by a meeting (and proceedings) that compiled in one place the breadth of scientific and technical information available for the James Webb Space Telescope. As planning for the meeting evolved, several individuals urged that the conference be broadened to include other major astronomical missions of the coming decade. The conference was held September 24–27, 2007 at the Marriott Starr Pass in Tucson, Arizona, and was attended by over 200 astronomers from around the world. The facilities were outstanding and the Sonoran Desert was stunning in early autumn. Under the general guidance of the local organizers, the Science Organizing Committee selected topics and arranged for speakers to address the key goals of the astronomical community in the next decade. While the conference emphasized astronomical observations from the visible to the sub-millimeter, several sessions dealt with key science goals enabled by dramatic new capabilities in other bands, particularly when these goals were complementary to those of Webb observers. The breadth of the final program and these proceedings underscore the importance of the Webb mission to almost all areas of modern astrophysics research. Harley A. Thronson Massimo Stiavelli Alexander G.G.M. Tielens

Greenbelt, Maryland Baltimore, Maryland Moffett Field, California

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Acknowledgements

The conference could not have been successful without the participation from a number of individuals and institutions. In addition to the committee membership, Marcia Rieke assisted with logistics in Tucson and Eric Smith provided essential support from the NASA Science Mission Directorate. Mario Livio and other scientists from the Space Telescope Science Institute, the JWST Project, Johns Hopkins University, and NASA Goddard Space Flight Center Flight Programs Directorate contributed institutional and financial support. Of course, the meeting and these proceedings would not be possible without the commitment of the speakers, moderators, and authors, to whom we are indebted. Science Organizing Committee Chairperson: Alan Dressler LOC Liaison: Peter Stockman Other Members: Crystal Brogan, Dale Cruikshank, Ewine van Dishoeck, Richard Ellis, Rob Kennicutt, Rolf Kudritzki, Avi Loeb, John Mather, Yvonne Pendleton, Massimo Stiavelli, Leonardo Testi, Xander Tielens, Meg Urry, Jeff Valenti. Local Organizing Committee Chairperson: Peter Stockman Other Members: Jonathan Gardner, Matt Greenhouse, Heidi Hammel, John Mather, Neill Reid, Massimo Stiavelli, Harley Thronson. Photography Harley Thronson and Dale Cruikshank Administrative Support Personnel Karyn Keidel, Tina Schappell, Darlene Spencer, Lee Tinnin Kirsten Theunissen Springer Science + Business Media B.V. Deivanai Loganathan and Team, Integra Software Services Pvt. Ltd, India. vii

Contents

1 The James Webb Space Telescope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jonathan P. Gardner and the JWST Science Working Group

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2 Beyond the Hubble Space Telescope: Early Development of the Next Generation Space Telescope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Robert W. Smith and W. Patrick McCray 3 The Kuiper Belt and Other Debris Disks . . . . . . . . . . . . . . . . . . . . . . . . . . 53 David Jewitt, Amaya Moro-Mart´ın and Pedro Lacerda 4 The Future of Ultracool Dwarf Science with JWST . . . . . . . . . . . . . . . . 101 Mark S. Marley and S.K. Leggett 5 Transiting Exoplanets with JWST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 S. Seager, D. Deming and J.A. Valenti 6 The Unsolved Problem of Star Formation: Dusty Dense Cores and the Origin of Stellar Masses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Charles J. Lada 7 Accretion Disks Before (?) the Main Planet Formation Phase . . . . . . . . 167 C. Dominik 8 Astrochemistry of Dense Protostellar and Protoplanetary Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Ewine F. van Dishoeck 9 Extreme Star Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 Jean L. Turner ix

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10 Prospects for Studies of Stellar Evolution and Stellar Death in the JWST Era . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 Michael J. Barlow 11 Origin and Evolution of the Interstellar Medium . . . . . . . . . . . . . . . . . . 271 A.G.G.M. Tielens 12 Astrophysics in the Next Decade: The Evolution of Galaxies . . . . . . . . . 309 Alice Shapley 13 The Co-Evolution of Galaxies and Black Holes: Current Status and Future Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 Timothy M. Heckman 14 The Intergalactic Medium at High Redshifts . . . . . . . . . . . . . . . . . . . . . . 357 Steven R. Furlanetto 15 Observing the First Stars and Black Holes . . . . . . . . . . . . . . . . . . . . . . . . 385 Zolt´an Haiman 16 Baryons: What, When and Where? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 Jason X. Prochaska and Jason Tumlinson 17 Observational Constraints of Reionization History in the JWST Era . 457 Xiaohui Fan 18 The Frontier of Reionization: Theory and Forthcoming Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481 Abraham Loeb Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511

Contributors

Michael J. Barlow Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK, [email protected] D. Deming NASA Goddard Space Flight Center, Planetary Systems Branch, Code 693 Greenbelt, MD 20771, USA, [email protected] C. Dominik Sterrenkundig Instituut “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, NL-1098 SJ, Amsterdam; and Afdeling Sterrenkunde, Radboud Universiteit Nijmegen, Faculteit NWI, Postbus, 9010, NL-6500 GL Nijmegen, [email protected] Xiaohui Fan Steward Observatory, University of Arizona, Arizona, USA, [email protected] Steven R. Furlanetto University of California, Los Angeles, CA, USA, [email protected] Jonathan P. Gardner Observational Cosmology Laboratory, Code 665, NASA Goddard Space Flight Center, Greenbelt MD 20771, USA, [email protected] Zolt´an Haiman Department of Astronomy, Columbia University, New York, NY 10027, USA, [email protected] Timothy M. Heckman Center for Astrophysical Sciences, Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA, [email protected] David Jewitt Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, USA, [email protected] Pedro Lacerda Institute for Astronomy, University of Hawaii, USA, [email protected] Charles J. Lada Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA, [email protected]

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S.K. Leggett Gemini Observatory, 670 North A’ohoku Place Hilo, HI 96720, USA, [email protected] Abraham Loeb Harvard University, CfA, MS 51, 60 Garden Street, Cambridge MA 02138, USA, [email protected] Mark S. Marley NASA Ames Research Center, Mail Stop 245-3, Moffett Field CA 94035, USA, [email protected] W. Patrick McCray Department of History, University of California, Santa Barbara CA, USA, [email protected] Amaya Moro-Mart´ın Department of Astronomy, Princeton University, Princeton, NJ, USA, [email protected] Jason X. Prochaska University of California Observatories - Lick Observatory, University of California, Santa Cruz, CA 95064, USA, [email protected] S. Seager Department of Earth, Atmospheric, and Planetary Sciences, Dept. of Physics, Massachusetts Institute of Technology, 77 Massachusetts Ave, 54-1626, Cambridge, MA, 02139, USA, [email protected] Alice Shapley Princeton University, Peyton Hall, Ivy Lane, Princeton, NJ 08544, USA, [email protected] Robert W. Smith Department of History and Classics, University of Alberta, Edmonton, Alberta, Canada, [email protected] A.G.G.M. Tielens MS 245-3, Space Sciences Division, NASA Ames Research Center, Moffett Field, CA 94035, USA, [email protected] Jason Tumlinson Yale Center for Astronomy and Astrophysics, Yale University, New Haven, CT, 06520, USA, [email protected] Jean L. Turner Department of Physics and Astronomy, UCLA, Los Angeles CA 90095-1547 USA, [email protected] J.A. Valenti Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA, [email protected] Ewine F. van Dishoeck Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands and Max-Planck Institute f¨ur Extraterrestrische Physik, Garching, Germany, [email protected]

Chapter 1

The James Webb Space Telescope Jonathan P. Gardner and the JWST Science Working Group1

Abstract The James Webb Space Telescope (JWST) is a large (6.6 m), cold (<50 K), infrared (IR)-optimized space observatory that will be launched early in the next decade into orbit around the second Earth–Sun Lagrange point. The observatory will have four instruments: a near-IR camera, a near-IR multi-object spectrograph, and a tunable filter imager that will cover the wavelength range, 0.6 < ␭ < 5.0 ␮m, while the mid-IR instrument will do both imaging and spectroscopy from 5.0 < ␭ < 29 ␮m. The JWST science goals are divided into four themes. The End of the Dark Ages: First Light and Reionization theme seeks to identify the first luminous sources to form and to determine the ionization history of the early universe. The Assembly of Galaxies theme seeks to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, and active nuclei within them evolved from the epoch of reionization to the present day. The Birth of Stars and Protoplanetary Systems theme seeks to unravel the birth and early evolution of stars, from infall onto dust-enshrouded protostars to the genesis of planetary systems. The Planetary Systems and the Origins of Life theme seeks to determine the physical and chemical properties of planetary systems including our own, and investigate the potential for the origins of life in those systems. To enable these science goals, JWST consists of a telescope, an instrument package, a spacecraft, and a sunshield. The telescope primary mirror is made of 18 beryllium segments, some of which are deployed. The segments will be brought into optical alignment on-orbit through a process of periodic wavefront sensing and control. The instrument package contains the four science instruments and a fine guidance sensor. The spacecraft provides pointing, orbit maintenance, and communications. The sunshield provides passive thermal control. The JWST operations plan is based on that used for previous space J.P. Gardner (B) Observational Cosmology Laboratory, Code 665, NASA Goddard Space Flight Center, Greenbelt MD 20771, USA e-mail: [email protected] 1 John C. Mather, Mark Clampin, Rene Doyon, Kathryn A. Flanagan, Marijn Franx, Matthew A. Greenhouse, Heidi B. Hammel, John B. Hutchings, Peter Jakobsen, Simon J. Lilly, Jonathan I. Lunine, Mark J. McCaughrean, Matt Mountain, George H. Rieke, Marcia J. Rieke, George Sonneborn, Massimo Stiavelli, Rogier Windhorst and Gillian S. Wright.

H.A. Thronson et al. (eds.), Astrophysics in the Next Decade, Astrophysics and Space Science Proceedings, DOI 10.1007/978-1-4020-9457-6 1,  C Springer Science+Business Media B.V. 2009

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observatories, and the majority of JWST observing time will be allocated to the international astronomical community through annual peer-reviewed proposal opportunities.

1.1 Science 1.1.1 The End of the Dark Ages: First Light and Reionization The James Webb Space Telescope (JWST; Fig. 1.1; Gardner et al. 2006) seeks to identify the first luminous sources to form and to determine the ionization history of the early universe. The emergence of the first sources of light in the universe marks the end of the “Dark Ages” in cosmic history, a period characterized by the absence of discrete sources of light (Rees 1997). After the appearance of the first sources of light, hydrogen in the intergalactic medium was reionized. Results from the Wilkinson Microwave Anisotropy Probe (Kogut et al. 2003) combined with data on quasars at z ∼ 6 from the Sloan Digital Sky Survey (Fan et al. 2002) show that this reionization had a complex history (e.g. Gnedin 2004). Although there are indications that galaxies produced the majority of the ultraviolet reionizing radiation, the contribution of quasars could be significant. What Are the First Galaxies? To identify a sample of high-redshift galaxies, JWST will make an ultra-deep imaging survey using several broad-band filters. The Lyman break technique will identify objects at increasing redshifts up to z = 30 or higher. For dwarf galaxies with 106 M of zero-metallicity massive stars at

Optical Telescope Element (OTE)

Integrated Science Instrument Module (ISIM )

Secondary Mirror Sunshield Primary Mirror

Spacecraft

Fig. 1.1 The JWST Observatory. The optical telescope element (OTE) contains the primary and secondary mirrors, the integrated science instrument module (ISIM) element contains the instrumentation and the spacecraft element consists of the spacecraft and the sunshield

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15 < z < 30, the expected AB magnitude at emitted wavelengths just longward of Lyman ␣ is ∼31 mag. This ultra-deep survey will allow the derivation of galaxy counts as a function of redshift, seeing a drop at redshifts beyond the formation of the first sources. The intensity of the non-ionizing continuum yields the star formation rate as a function of redshift. The deep broad-band imaging in the near infrared will enable follow-up low-resolution spectroscopy and mid-infrared imaging of promising sources. The imaging could be timed to search for supernovae. When and How Did Reionization Occur? JWST will obtain high signal-tonoise, R = 1000, near-infrared spectra of high-redshift QSOs, galaxies or GRBs in order to determine the presence of a Gunn-Peterson trough or of a Lyman α damping wing (Fig. 1.2; X. Fan, private communication). The targets will be the brightest known high-redshift objects, perhaps found in a wide-area Spitzer warm mission survey (Gardner et al. 2007). A damping wing should be present for a few million years, before the ionizing radiation is sufficient to create a large Str¨omgren sphere around each ionizing source. In order to measure the ionizing continuum of a class of sources, we will measure their hydrogen and helium Balmer lines. Comparison between these lines gives an estimate for the steepness, or hardness of the ionizing continuum, providing a measurement of the rate of production of ionizing photons for any given class of sources under the assumption that the escape fraction is known. Identification of the nature of ionizing sources requires a combination of diagnostics: line shapes, line widths, line ratios, and the shape of the continuum. We expect the intrinsic line shapes and widths of AGN-powered sources to be broader than those of sources ionized by stellar radiation.

Fig. 1.2 A simulated spectrum of a z = 9.4 quasar observed with JWST’s NIRSpec for 300 ksec. In the top figure, a faint quasar is observed with spectral resolution R = 1000, while in the bottom figure a brighter quasar is observed at higher resolution. The simulation includes an early reionization, followed by a recombination and a second dark age. (X. Fan, private communication.)

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1.1.2 The Assembly of Galaxies JWST seeks to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, and active nuclei within them evolved from the epoch of reionization to the present day. Theory and observation show that galaxies are assembled through a process of the hierarchical merging of dark matter concentrations, accompanied by chemical evolution of the gas and dust, and resulting in the Hubble Sequence of galaxies (e.g. White & Frenk 1991, Barkana & Loeb 2001). However, we do not really know how galaxies are formed, what controls their shapes, and what makes them form stars. We do not know how the chemical elements are generated and redistributed through the galaxies, and whether the central black holes exert great influence over the galaxies. We do not know the global effects of violent events as small and large parts join together in collisions. How did the Hubble Sequence and Heavy Elements Form? To trace the formation of the Hubble Sequence of galaxies, JWST will determine the morphologies, stellar populations, and star-formation rates in a very large sample of galaxies observed in deep-wide imaging and spectroscopic surveys, as a function of their mass, environment, and cosmic epoch. JWST will also determine when the long-lived stars in a typical galaxy were formed, whether in situ or in smaller galaxies that subsequently merged together to form a large galaxy. Direct characterization of the merging rate of galaxies will provide another angle on this question. What Physical Processes Determine Galaxy Properties? Despite the variety of galaxy properties observed today, galaxies obey a number of remarkably tight scaling relations between basic properties of luminosity, size, kinematics and metal enrichment. These include the Tully-Fisher relation for disk galaxies and the “fundamental plane”, and projections thereof, for spheroids (Tully & Fisher 1977, Bender et al. 1992). More recently, a surprising relationship between the mass of the central black hole and the properties of the surrounding spheroid (e.g., the velocity dispersion) has been established (Magorrian et al. 1998, Gebhardt et al. 2000). JWST will investigate these processes through deep imaging for structural parameters plus high-resolution spectroscopy for kinematical data. The dark matter will be revealed through gravitational weak-lensing surveys reaching z ∼ 2.5. JWST will also use mid-infrared spectroscopy to diagnose the energy sources in the interior of bright systems, as starbursts have strong PAH features, while AGN have strong [NeVI] 7.66 ␮m lines (Armus et al. 2004, Soifer et al. 2004).

1.1.3 The Birth of Stars and Protoplanetary Systems JWST seeks to unravel the birth and early evolution of stars, from infall onto dustenshrouded protostars, to the genesis of planetary systems. The formation of stars and planets is a complex process, even in the well-developed paradigm for a single, isolated low-mass star (Shu et al. 1987). Things are even more complicated,

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as stars very rarely form in isolation. The current picture of star formation starts on large scales, as molecular cloud cores cool and fragment to form highly dynamic clusters of protostars, spanning the mass spectrum from O stars to planetary-mass brown dwarfs. Within those clusters, individual young sources are often encircled by disks of warm gas and dust, where material aggregates to form proto-planetary systems. These disks are the source of highly-collimated jets and outflows, which transfer energy and angular momentum from the infalling material into the surrounding medium, and clear away the remainder of the birth core. On larger scales, the intense ultraviolet flux and strong winds of the most massive stars can disperse an entire molecular cloud, while simultaneously ionizing and evaporating the circumstellar disks of the surrounding lower-mass stars. How do Clouds Collapse and form Protoplanetary Systems? Stars form in small (∼0.1 pc) regions undergoing gravitational collapse within larger molecular clouds. Standard theory predicts that these cores collapse from the inside out (Shu 1977, Terebey et al. 1984), but there are alternative theories which predict different density distributions in the cores. By measuring the extinction to background stars, JWST will map those density distributions in order to understand the relative roles that magnetic fields, turbulence, and rotation play while the clouds collapse to form stars. Once self-gravitating molecular cloud cores have formed, they collapse to form protostellar seeds, which gain material via continuing accretion. These protostars will begin radiating in the 10 ␮m to 20 ␮m region, since radiation from the warm central source is scattered off dust grains in the inner envelope into the line-of-sight. Direct imaging of disks around low-mass stars reveal their internal density and temperature structure, and show how they are affected by their environment. Since circumstellar disks are both a product and a mediator of the star-formation process, as well as the progenitors of planetary systems, it is clear that a fuller understanding of the evolution of circumstellar disks will play a key role in our understanding of these central topics. Spectra of circumstellar disks will trace the formation history of planetesimals and other solid bodies, their composition, their processing, and their possible future evolution (Fig. 1.3). How do Very High and Very Low-Mass Stars Form? The formation of massive stars produces intense winds and ionizing radiation which impacts the surrounding molecular cloud material and the nascent circumstellar disks of adjacent low-mass stars. The mechanism by which massive stars form is not yet known. The standard disk-accretion scenario appropriate for low-mass stars cannot be simply scaled up, since predictions are that the radiation pressure from the growing central source would build up so quickly that no more material could accrete, limiting the mass of the source (Yorke & Sonnhalter 2002). Likewise, the low-mass opacitylimited cutoff for star formation is predicted to be 3 to 10 MJUP , although a more detailed consideration of the role of magnetic fields may bring this down to 1 MJUP (Boss 2001). JWST will conduct imaging and spectroscopic surveys of young starforming regions to establish the both the high and low-mass limits of the initial mass function, and to determine the processes that operate at the extremes of star formation.

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Fig. 1.3 A series of ISO SWS mid-IR spectra of young stars and circumstellar disks at different stages in their evolution (Malfait et al. 1998, Gibb et al., 2000, van den Ancker et al., 2000). From top to bottom, in a rough evolutionary sequence, the spectra change from being dominated by solid-state absorption features and shocked gas emission lines, to PAH features and photodissociation region lines, to amorphous and crystalline silicates with HI recombination lines. An ISO SWS spectrum of Comet Hale-Bopp is shown for comparison (Crovisier et al. 1997)

1.1.4 Planetary Systems and the Origins of Life JWST seeks to determine the physical and chemical properties of planetary systems including our own, and to investigate the potential for the origins of life in those systems. How do Planets Form? The formation of multiple objects is a common outcome of star formation, including binary or higher-order star systems, a central star orbited by brown dwarfs and/or planets, or a star with a remnant disk of particulates. Brown dwarfs and giant planets might arise from two different formation mechanisms, or planets could be the low-mass tail of a single star-formation process (Chabrier et al. 2000, Lunine et al. 2004). The formation of giant planets is a signpost, detectable with JWST, of a process that may also generate terrestrial planets. In contrast, direct collapse formation of brown dwarfs may signal systems in which terrestrial planet formation is rare or impossible, because of the required disk mass, angular momentum and subsequent disk evolution. JWST will conduct coronagraphic surveys of nearby stars to find mature Jovian companions, and of more distant systems to find young planets (Fig. 1.4). Spectroscopy of isolated or widely separated planets or sub-brown dwarfs will reveal basic physical parameters such as gravity, composition, temperature and the effect of clouds. The remnant of the circumstellar disk that formed our solar system is observable today as the smaller planets, moons, asteroids and comets, along with the zodiacal light and interplanetary gas and dust. JWST spectroscopy of comets will reveal how circumstellar disks are like our own solar system.

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Fig. 1.4 Dusty debris disks. We show the Fomalhaut debris disk as it appears to Spitzer at 24 ␮m (right; Staplefeldt et al. 2004), and in a simulated JWST image. Structure within the disk is resolvable by JWST

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Fomalhaut at 24 microns

Simulated JWST image

Spitzer image

How are Habitable Zones Established? Some geochemical evidence suggests that Earth’s water did not come from locally formed planetesimals (Morbidelli et al. 2000). The source of water is uncertain; it could have been delivered through impacts of asteroids and comets, although isotopic ratios call the latter source into question. By measuring isotopic ratios in comets and larger Kuiper Belt bodies, JWST can solve this part of the puzzle, removing a major uncertainty in the source of water for our own planet. JWST will also monitor icy and organic-rich bodies in the outer solar system, such as Titan, in order to investigate ongoing organic evolution as well as constrain chemical processes in the early solar system.

1.2 Implementation To make these scientific measurements, JWST will be a large cold telescope, with a wide field of view (FOV), exceptional angular resolution and sensitivity, and wide wavelength coverage in both imaging and spectroscopy. It will be launched early in the next decade to an orbit around the Earth-Sun second Lagrange point (L2). The JWST Project is organized into three segments: observatory, ground and launch. The observatory is composed of an optical telescope element, an integrated science instrument module (ISIM) containing the scientific instruments, a spacecraft, and a sunshield (Fig. 1.1). The telescope (Feinberg 2004, Atkinson et al. 2006) is a deployable optical system that provides diffraction-limited performance at 2 ␮m using active wavefront sensing and control (WFS&C). The ISIM (Greenhouse et al. 2006) contains four science instruments (SIs): a near-infrared camera (NIRCam; Horner & Rieke 2004), a near-infrared multi-object spectrograph (NIRSpec; Zamkotsian & Dohlen 2004), a mid-infrared camera (MIRI; Wright et al. 2004), and a near-infrared tunable filter imager (TFI; Rowlands et al. 2004b). The ISIM also contains a fine guidance sensor (FGS; Rowlands et al. 2004a) to provide active control of pointing. The spacecraft provides the pointing platform and housekeeping

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functions for the observatory. The wavelength range of JWST and the SIs spans 0.6 to 29 microns, limited at the short end by the gold coatings on the primary mirror and at the long end by the detector technology. The sunshield shades the telescope and ISIM from solar illumination to allow zodiacal-light-limited performance at ␭ < 10 ␮m and high sensitivity out to 29 ␮m, and provides a stable thermal environment for the telescope and ISIM.

1.2.1 Observatory The primary mirror uses semi-rigid primary mirror segments mounted on a stable and rigid backplane composite structure. The architecture is termed “semi-rigid”, because it has a modest amount of flexibility that allows for on-orbit compensation of segment-to-segment radius of curvature variations. Semi-rigid segments have a high degree of inherent optical quality and stability, allowing verification of optical performance by 1 g end-to-end ground testing before launch. The telescope optics are made of beryllium. Spitzer and IRAS had beryllium mirrors and its material properties are known at temperatures as low as 10 K. Beryllium has an extremely small variation in its coefficient of thermal expansion over temperatures of 30 to 80 K, making the telescope optics intrinsically stable to small temperature variations. Beryllium fabrication and figuring procedures were designed using the results from the Advanced Mirror System Demonstrator (AMSD) program (Feinberg 2004, Stahl et al. 2004). An engineering demonstration unit will be used as a pathfinder for the flight mirror processing. The sunshield enables passive cooling and provides a stable cryogenic environment by minimizing the amount of solar energy incident onto the telescope and ISIM. The observatory will not use active wavefront control during observations.

1.2.2 Observatory Performance (Table 1.1) Image Quality: The imaging performance of the telescope will be diffraction limited at 2 ␮m, defined as having a Strehl ratio >0.80 (e.g., B´ely 2003). JWST will achieve this image quality using image-based wavefront sensing and control (WFS&C) of the primary mirror. There will also be a fine guidance sensor in the focal plane and a fine steering mirror to maintain pointing during observations. The allocated top-level wavefront error (WFE) is 150 nm root-mean-squared (rms) through to the NIRCam focal plane, and includes both the effect of 7.0 milliarcsec image motion, most of which is line-of-sight jitter, and 51 nm of drift instability. Sky Coverage and Continuous Visibility: Field of regard (FOR) refers to the fraction of the celestial sphere that the telescope may point towards at any given time. JWST’s FOR is limited by the size of the sunshield. A continuous viewing zone within 5◦ of both the north and south ecliptic poles is available throughout the

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Table 1.1 The predicted performance for the JWST Observatory Parameter

Capability

Wavelength Image quality Telescope FOV Orbit Celestial sphere coverage

0.6 to 28.5 ␮m Strehl ratio of 0.8 at 2 ␮m Instruments share ∼166 arcmin2 FOV Lissajous orbit about Sun-Earth L2 100% annually 39.7% at any given time 100% of sphere has >51 contiguous days visibility Continuous viewing zone <5◦ from ecliptic pole Observatory ∼80%. Overall efficiency >70% Commissioning at launch + 6 months 5 yr minimum lifetime after commissioning 10 yr fuel carried for station keeping

Observing efficiency Mission life

year. Thirty percent of the sky can be viewed continuously for at least 197 continuous days. All regions of the sky have at least 51 days of continuous visibility per year. The architecture provides an instantaneous FOR at any epoch of approximately 40% of the sky. This FOR extends 5◦ past the ecliptic pole, and provides 100% accessibility of the sky during a one-year period. In addition, a nominal 5◦ angular safety margin will be maintained when determining the allowable observatory pointing relative to the Sun. In order to take full advantage of the FOR, JWST will be able to observe any point within it at any allowable roll angle, with a probability of acquiring a guide star of at least 95% under nominal conditions. This will ensure that most of the required targets will be observable without special scheduling or other procedures. Sensitivity and Stray Light Limitations: Many JWST observations will be background limited. The background is a combination of in-field zodiacal light, scattered thermal emission from the sunshield and telescope, scattered starlight, and scattered zodiacal light. Over most of the sky, the zodiacal light dominates at wavelengths ␭ < 10 ␮m, so broad-band imaging will be zodiacal-light limited at these wavelengths. The flux sensitivities of the JWST instruments are given in Section 1.2.4. Observational Efficiency: The observational efficiency is the fraction of the total mission time spent actually “counting photons,” rather than occupied by overhead activities such as slewing, stabilizing, calibrating, and adjusting parts of the observatory. The overall efficiency is expected to be 70%, although this depends on the specific observing programs that are implemented. In comparison to missions in low-Earth orbit, there are significant efficiency advantages of the L2 orbit. JWST will use parallel instrument calibration (i.e., darks) to achieve this overall efficiency. The JWST architecture is optimized in the sense that the overheads associated with pointing to targets are the dominant contributors to inefficiency. The spacecraft will carry enough propellant to maintain science operations for a 10-year lifetime.

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1.2.3 Observatory Design Description 1.2.3.1 Optical Telescope Element (OTE) The telescope has 132 degrees of freedom of adjustment mechanisms and a composite structure so that passive figure control and passive disturbance attenuation are possible. Figure 1.5 shows an isometric view of the telescope’s primary and secondary mirrors. The optical telescope has a 25 m2 collecting area, and is a three-mirror anastigmat, made up of 18 hexagonal segments. The telescope has an effective f/number of 20, and an effective focal length of 131.4 m. Each ISIM instrument reimages the telescope focal plane onto its detectors, allowing for independent selection of detector plate scale for sampling of the optical PSF. A fine steering mirror (FSM) is used for image stabilization. Figure 1.6 shows the placement of the individual ISIM instrument detectors on the telescope’s field of view. The NIRCam detectors are placed in a spatial region with the lowest residual WFE to take full advantage of imaging performance. The telescope mirrors are gold coated, providing a broad spectral bandpass, from 0.6 to 29 ␮m. Secondary Mirror support & baffle

Glare Stop

Primary Mirror Segment (1 of 18)

Fig. 1.5 Isometric view of the JWST telescope. The telescope is a three-mirror anastigmat with an f/20 design, made up of 18 hexagonal Beryllium segments totaling 25 m2 collecting area

Aft Optical Subsystem (Tertiary mirror & fine steering mirror)

Primary Mirror Backplane

Folding secondary mirror support strut

Deployment Motor

1.2.3.2 Mirror Segments The primary mirror is composed of 18 individual beryllium mirror segments. When properly phased relative to each other, these segments act as a single mirror. Primary

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Vertical Field Position (arcmin in +V3 Axis)

– 5.7

TFI

1.15

FGS

Region in which RMS WFE <131nm

NIRCam NIRSpec MIRI

+3.4 –9.1

0

9.1

Horizontal Field Position (arcmin in +V2 Axis)

Fig. 1.6 Placement of the science instruments in the telescope field of view

mirror phasing is achieved via six degree-of-freedom rigid body motion of the individual segments, and an additional control for the mirror segment radius of curvature. The six degrees of freedom are decenter (x, y), tip, tilt, piston (z), and clocking. Each mirror segment is hexagonal with a 1.32 m flat-to-flat dimension. There are three separate segment types with slightly different aspheric prescriptions depending on placement. All segments within a single type are completely interchangeable. Figure 1.7 shows the rear portion of the mirror segments and the seven actuators. The secondary mirror has 6 actuators which provide 6 degree-offreedom rigid body control, although only 5 degree-of-freedom control is actually needed.

Fig. 1.7 A mirror segment with the hexapod support structure and actuators is prepared for acoustic and vibration testing

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1.2.3.3 Integrated Science Instrument Module The ISIM contains the science instruments for the observatory, and the support electronics for the science instruments: NIRCam, NIRSpec, TFI, and MIRI (described in Section 1.2.4). A cryo-cooler will be used for cooling MIRI and its Si:As detectors. The near-infrared detector arrays in the other instruments are passively cooled HgCdTe. In addition to the science instruments, the ISIM contains the fine guidance sensor (FGS) and the computer that directs the daily science observations based on plans received from the ground. The science instruments and FGS have non-overlapping FOVs as shown in Fig. 1.6. Simultaneous operation of all science instruments is possible; this capability will be used for parallel calibration, including darks and possibly sky flats. FGS is used for guide star acquisition and fine pointing. Its FOV and sensitivity are sufficient to provide a greater than 95% probability of acquiring a guide star for any valid pointing direction and roll angle. 1.2.3.4 Wavefront Sensing and Control Subsystem (WFS&C) The ability to perform on-orbit alignment of the telescope’s 18 segments is one of the enabling technologies that allow JWST to be built. The WFS&C subsystem aligns these segments so that their wavefronts match properly, creating a diffractionlimited 6.6 m telescope, rather than overlapping images from 18 individual 1.3 m telescopes. WFS&C uses a set of deterministic algorithms that control the fundamental and critical initial telescope alignment operation (Acton et al. 2004). Determination of the wavefront error and the necessary telescope mirror commands is done on the ground using the downlinked image data. The algorithm uses least-squares fits to images taken in and out of focus, at different wavelengths, and at multiple field points. The focus is adjusted using weak lenses in the NIRCam filter wheel. This algorithm was used to diagnose and measure the spherical aberration of the HST primary mirror. Mirror adjustment commands are uplinked to the observatory. Figure 1.8 summarizes the WFS&C process. The initial telescope commissioning process occurs in four phases: (1) image capture and identification, (2) coarse alignment, (3) coarse phasing, and (4) fine phasing. To accomplish commissioning, the telescope is pointed at a celestial region with specific characteristics at the 2 ␮m NIRCam operation wavelength. A bright (magnitude 10 or brighter) isolated commissioning star is located on the NIRCam detector. A second star (guide star), with magnitude and position restrictions relative to the commissioning star, is used with the fine guiding sensor. 1.2.3.5 Sunshield The sunshield (Fig. 1.9) enables the passively-cooled thermal design. It is a 5-layer “V” groove radiator design, which is sized (about 19.5 m × 11.4 m) and shaped to limit solar radiation pressure induced momentum buildup. It reduces the ∼200 kW incident solar radiation that impinges on the sunshield to milliwatts incident on the

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After Step 1

Initial Capture 18 individual 1.6-m diameter aberrated sub-telescope images PM segments: < 1 mm, < 2 arcmin tilt SM: < 3 mm, < 5 arcmin tilt

1. Segment Image Capture 2. Coarse Alignment Secondary mirror aligned Primary RoC adjusted

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After Step 2

Final Condition PM segments: < 100 µm, < 2 arcsec tilt SM: < 3 mm, < 5 arcmin tilt

Primary Mirror segments: < 1 mm, < 10 arcsec tilt Secondary Mirror : < 3 mm, < 5 arcmin tilt

WFE < 200 µm (rms)

After Step 3 3. Coarse Phasing - Fine Guiding (PMSA piston)

WFE: < 250 µm rms

WFE < 1 µm (rms)

WFE: < 5 µm (rms)

WFE < 110 nm (rms)

WFE: < 150 nm (rms)

WFE < 110 nm (rms)

After Step 4 4. Fine Phasing

5.

Image-Based Wavefront Monitoring

After Step 5

Fig. 1.8 A summary of the WFS/C commissioning and maintenance phases

Fig. 1.9 The JWST sunshield is the size of a tennis court

telescope and ISIM. The orientation and angular separation of the individual layers can be seen in the artist’s rendition of the JWST observatory in Fig. 1.1. 1.2.3.6 Spacecraft Bus The spacecraft provides the housekeeping function of the observatory, including data handling, orbit maintenance, solar array power, coarse star tracking, and reaction wheels for fine star tracking. It has a 471 Gbit solid state recorder to hold the

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science and engineering data collected between and during the daily contacts with the ground station. The downlink operates at Ka band and has a selectable rate of 7, 14 or 28 Mbps. A pair of omni-directional antennas (at S band) provides nearly complete spherical coverage for emergency communications.

1.2.3.7 Integration and Test All of the JWST flight elements will be qualified to operational environments during their integration and test. Performance verification starts at the sub-system level and continues as the observatory is integrated. Integration and test culminates in an endto-end cryogenic optical test of the OTE and ISIM, where the telescope’s cryogenic performance is verified. The telescope and ISIM combination will be cryogenically tested at Chamber A at NASA’s Johnson Space Center (Fig. 1.10) and verified prior to integration to the spacecraft and sunshield. This telescope/ISIM cryogenic test will be the end-to-end optical performance verification for the observatory and will include tests to verify the alignment of the OTE to ISIM, an alignment check of the individual telescope elements (primary, secondary, aft optics system to each other), a test of the full 18-segment phased primary mirror, and model validation of the thermal and structural stability. Crosschecks will be also be performed on the aft-optics system and science instrument wavefront error, and end-to-end wavefront error over portions of the telescope pupil, pupil alignment, and stray light levels. The test chamber is 16.8 m diameter and 35.7 m high, giving adequate room to configure cost-effective assembly, test, and verification equipment. The telescope will be tested with the optical axis vertical to minimize gravity effects. The sunshield deployment will be tested at room temperature, and a scale version of the sunshield will be tested in the deployed configuration with a helium-cooled shroud to validate thermal models.

Fig. 1.10 The JWST observatory pictured in the thermal vacuum test chamber at NASA’s Johnson Space Center. The test chamber is 16.8 m in diameter and 35.7 m high. The end-to-end optical performance verification for the observatory will test alignment, the full 18 segment phased primary mirror, and validate the thermal and structural stability models

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1.2.4 Instrumentation Table 1.2 lists the main parameters of the science instruments. 11.5 Table 1.3 lists the instrument sensitivities. Sensitivity is defined to be the brightness of a point source detected at 10σ in 10,000 s. Longer or shorter exposures are expected to scale as the square root of the exposure time. Targets at the North Ecliptic Pole are assumed. The sensitivities in this table represent the best estimate at the time of writing and are subject to change. Although stated as the sensitivity achievable for 10,000 s exposures, it is expected that cosmic ray hits will limit the maximum exposure time for an individual integration to about 1000 s, and that longer total exposure times will be achieved through

Table 1.2 Science instrument characteristics Instrument λ (␮m) Detector mas/pixel Field of view NIRCam Short 0.6 to 2.3 Eight 2048 × 2048 32 2.2 × 4.4 arcmin Longa 2.4 to 5.0 Two 2048 × 2048 65 2.2 × 4.4 arcmin NIRSpec MSAb 0.6 to 5.0 Two 2048 × 2048 100 3.4 × 3.1 arcmin Slitsc ∼0.2 × 4 arcsec IFU 3.0 × 3.0 arcsec MIRI Imaging 5.0 to 27.0 1024 × 1024 110 1.4 × 1.9 arcmin Slitd 5.0 to 10.0 0.2 × 5 arcsec IFU 5.0 to 28.5 Two 1024 × 1024 200 to 470 3.6 × 3.6 to 7.5 × 7.5 arcsec TFI 1.6 to 4.9e 2048 × 2048 65 2.2 × 4.4 arcmin a Use of a dichroic renders the NIRCam long-wavelength FOV cospatial with the short-wavelength channel; the two channels acquire data simultaneously. b NIRSpec includes a microshutter assembly (MSA) with four 365 × 171 microshutter arrays. The individual shutters are each 203 (spectral) × 463 (spatial) milliarcsec clear aperture on a 267 × 528 milliarcsec pitch. c NIRSpec also includes several fixed slits and an integral field unit (IFU)’ which provide redundancy and high contrast spectroscopy on individual targets. d MIRI includes a fixed slit for low-resolution (R ∼ 100) spectroscopy over the 5 to 10 ␮m range, and an IFU for R ∼ 3000 spectroscopy over the full 5 to 28.5 ␮m range. The long wavelength cutoff for MIRI spectroscopy is set by the detector performance, which drops longward of 28.0 ␮m. e The wavelength range for the TFI is 1.6 to 2.6 ␮m and 3.1 to 4.9 ␮m. There is no sensitivity between 2.6 and 3.1 ␮m. Table 1.3 Instrument sensitivities Instrument/mode NIRCam TFI NIRSpec/Low Res. NIRSpec/Med. Res. MIRI/Broadband MIRI/Broadband MIRI/Spect. MIRI/Spect.

λ (␮m)

Bandwidth

Sensitivity

2.0 3.5 3.0 2.0 10.0 21.0 9.2 22.5

R=4 R = 100 R = 100 R = 1000 R=5 R = 4.2 R = 2400 R = 1200

11.4 nJy, AB = 28.8 126 nJy, AB = 26.1 132 nJy, AB = 26.1 1.64 × 10−18 erg s−1 cm−2 700 nJy, AB = 24.3 8.7 ␮Jy, AB = 21.6 1.0 × 10−17 erg s−1 cm−2 5.6 × 10−17 erg s−1 cm−2

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co-adding. Based on experience with Hubble data, for example in the Hubble Deep Field, we expect the errors to scale as the square root of the exposure time in co-adds as long as 105 or even 106 s. The absolute photometric accuracy is expected to be 5% for imaging and 10 to 15% for coronagraphy and spectroscopy, based on calibration observations of standard stars. 1.2.4.1 Near-Infrared Camera NIRCam provides filter imaging in the 0.6 to 5.0 ␮m range with wavelength multiplexing. It includes the ability to sense the wavefront errors of the observatory. NIRCam consists of an imaging assembly within an enclosure that is mounted in the ISIM. The imaging assembly consists of two fully redundant, identical optical trains mounted on two beryllium benches, one of which is shown in Fig. 1.11. The incoming light initially reflects off the pick-off mirror. Subsequently it passes through the collimator and the dichroic, which is used to split the light into the short (0.6 to 2.3 ␮m) and long (2.4 to 5.0 ␮m) wavelength light paths. Each of these two beams then passes through a pupil wheel and filter wheel combination, each beam having its own pair of pupil and filter wheel. After this, the light passes through the camera corrector optics and is imaged (after reflecting off a fold flat in the short wavelength beam) onto the detectors. The instrument contains a total of ten 2048 × 2048 detector chips, including those in the identical redundant optical trains. The short wavelength arm in each optical train contains a 2 × 2 mosaic of these detectors, optimized for the 0.6–2.3 ␮m wavelength range, with a small gap (∼ 3 mm =∼ 5 arcsec) between adjacent detectors. The long wavelength arm in each optical train contains a single detector

Fig. 1.11 The NIRCam engineering test unit optical bench

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Fig. 1.12 The NIRSpec optical bench

covering 2.4–5.0 ␮m. The detectors arrays are HgCdTe of HAWAII II heritage built by Rockwell Science Center. The detectors will all have thinned substrates to avoid cosmic ray scintillation issues, as well as to extend their sensitivity below 0.85 ␮m. Each optical train contains a dual filter and pupil wheel, containing a range of wide-, medium- and narrow-band filters and the WFS&C optics. Coronagraphy: To enable the coronagraphic imaging, each of the two identical optical trains in the instrument also contains a traditional focal plane coronagraphic mask plate held at a fixed distance from the detectors, so that the coronagraph spots are always in focus at the detector plane. Each coronagraphic plate is transmissive, and contains a series of spots of different sizes, including linear and radial-sinc occulters, to block the light from a bright object. Each coronagraphic plate also includes a neutral density spot to enable centroiding on bright stars, as well as calibration sources at each end that can send light through the optical train of the imager to enable internal alignment checks. Normally these coronagraphic plates are not in the optical path for the instrument, but they are selected by rotating into the beam a mild optical wedge that is mounted in the pupil wheel, which translates the image plane so that the coronagraphic masks are shifted onto the active detector area. Diffraction can also be reduced by apodization at the pupil mask, thus the pupil wheels will be equipped with both a classical and an apodized pupil with integral wedges in each case. Current models predict a contrast of ∼104 at 0.5 arcsec, at a wavelength of 4.6 ␮m.

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1.2.4.2 Near-Infrared Spectrograph NIRSpec (Fig. 1.12) is a near-infrared multi-object dispersive spectrograph capable of simultaneously observing more than 100 sources over a field-of-view (FOV) larger than 3 × 3 arcmin. In addition to the multi-object capability, it includes fixed slits and an integral field unit for imaging spectroscopy. Six gratings will yield resolving powers of R ∼ 1000 and ∼2700 in three spectral bands, spanning the range 1.0 to 5.0 ␮m. A single prism will yield R ∼ 100 over 0.6 to 5.0 ␮m. Figure 1.13 shows a layout of the instrument. The region of sky to be observed is transferred from the JWST telescope to the spectrograph aperture focal plane by a pick-off mirror and a system of fore-optics that includes a filter wheel for selecting bandpasses and introducing internal calibration sources. Targets in the FOV are normally selected by opening groups of shutters in a micro-shutter assembly (MSA) to form multiple apertures. The micro-shutter assembly itself consists of a mosaic of 4 subunits producing a final array of 730 (spectral) by 342 (spatial) individually addressable shutters with 203 × 463 milliarcsec openings and 267 × 528 milliarcsec pitch. Sweeping a magnet across the surface of the micro shutters opens all of the shutters. Individual shutters may then be addressed and released electronically, and the return path of the magnet closes the released shutters. The minimum aperture size is 1 shutter (spectral) by 1 shutter (spatial) at all wavelengths. Multiple pointings may be required to avoid placing targets near the edge of a shutter and to observe targets with spectra that would overlap if observed simultaneously at the requested roll angle. The nominal slit length is 3 shutters in all wavebands. In the open configuration, a shutter passes light from the fore-optics to the collimator. A slitless mode can be configured by opening all of the micro shutters. As the shutters are individually addressable, long slits, diagonal

Active MSA Area (4 x 171 x 365 Shutters)

Mounting Frame

Fixed Slits and IFU Aperture 3.4’

Fig. 1.13 Schematic layout of the NIRSpec microshutter array mask overlaid on the detector array and projected to the same angular scale

Detector Array (2 x 2048 x 2048 pixels)

3.6’

Direction of Dispersion

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slits, Hadamard-transform masks (Riesenberg & Dillner 1999), and other patterns can also be configured with them. In addition to the slits defined by the micro-shutter assembly, NIRSpec also includes five fixed slits that can be used for high-contrast spectroscopy. They are placed in a central strip of the aperture focal plane between sub-units of the microshutter assembly and also provide redundancy in case the micro-shutters fail. Three fixed slits are 3.5 arcsec long and 200 milliarcsec wide. One fixed slit is 4 arcsec long and 400 milliarcsec wide for increased throughput at the expense of spectral resolution. One fixed slit is 2 arcsec long and 100 milliarcsec wide for brighter targets. The strip between micro-shutter sub-units also contains the 3 by 3 arcsec entrance aperture for an integral field unit (IFU). The IFU has 30 slices, each 100 milliarcsec wide. The IFU relay optics introduce a 2:1 anamorphic magnification of each slicer such that the matching projected virtual slits are properly sampled on the detector by two 50 milliarcsec pixels in the dispersion direction and at the (nominal) 100 milliarcsec per pixel in the spatial direction. The aperture focal plane is re-imaged onto a mosaic of two NIR detectors by a collimator, a dispersing element (gratings or a double-pass prism) or an imaging mirror, and a camera. Three gratings are used for first-order coverage of the three NIRSpec wavebands at R ∼ 1000 (1.0 to 1.8 ␮m; 1.7 to 3.0 ␮m; 2.9 to 5.0 ␮m). The same three wavebands are also covered by first-order R ∼ 2700 gratings for objects in a fixed slit or in the IFU. The prism gives R ∼ 100 resolution over the entire NIRSpec bandpass (0.6 to 5 ␮m), but can optionally be blocked below 1 ␮m with one of the filters. Any of the aperture selection devices (micro-shutter assembly, fixed slits or IFU) can be used at any spectral resolution. The focal plane array is a mosaic of two detectors (see Fig. 1.13), each 2k × 2k, forming an array of 2k × 4k 100 milliarcsec pixels. The detectors will be thinned HgCdTe arrays built by Rockwell Science Center. NIRSpec contains a calibration unit with a number of continuum and line sources. 1.2.4.3 Mid-Infrared Instrument The Mid-Infrared Instrument (MIRI) on JWST provides imaging and spectroscopic measurements over the wavelength range 5 to 29 ␮m. MIRI consists of an optical bench assembly (Fig. 1.14) with associated instrument control electronics, actively cooled detector modules with associated focal plane electronics, and a cryo-cooler with associated control electronics. The cryo-cooler electronics interface with the spacecraft command and telemetry processor, while the instrument control electronics interface with the ISIM command and data handling. The optical bench assembly contains two actively cooled subcomponents, an imager and an Integral Field Unit (IFU) spectrograph, plus an on-board calibration unit. Imaging: The imager module provides broad-band imaging, coronagraphy, and low-resolution (R ∼ 100, 5 to 10 ␮m) slit spectroscopy using a single 1024 × 1024 pixel Si:As detector with 25 ␮m pixels. Three quarters of the detector is available for imaging, while the remaining quarter is devoted to the coronagraphic masks

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Fig. 1.14 MIRI verification model going into thermal vacuum testing

and the slit for the low-resolution spectrometer. The coronagraphic masks include three phase masks for a quadrant-phase coronagraph and one opaque spot for a Lyot coronagraph. The coronagraphic masks each have a square field of view of 26 × 26 arcsec and are optimized for particular wavelengths. The imager’s only moving part is an 18-position filter wheel. Filter positions are allocated as follows: 12 filters for imaging, 4 filter and diaphragm combinations for coronagraphy, 1 ZnS-Ge double prism for the low-resolution spectroscopic mode and 1 dark position. The imager

Fig. 1.15 Test data from the MIRI verification model, showing a spectrum (left) and point spread function of the imager (right)

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will have a pixel scale of 0.11 arcsec/pixel and a total field of view of 113 × 113 arcsec; however, the field of view of its clear aperture is 84 × 113 arcsec because the coronagraph masks and the low-resolution spectrograph are fixed on one side of the focal plane. Integral-Field Spectroscopy: The integral-field spectrograph obtains simultaneous spectral and spatial data on a small region of sky. The spectrograph field of view is next to that of the imager so that accurate positioning of targets will be possible by locating the image with the imager channel and off-setting to the spectrograph (Fig. 1.15). The light is divided into four spectral ranges by dichroics, and two of these ranges are imaged onto each of two detector arrays. A full spectrum is obtained by taking exposures at each of three settings of the grating wheel. The spectrograph uses four image slicers to produce dispersed images of the sky on two 1024 × 1024 detectors, providing R ∼ 3000 integral-field spectroscopy over the full 5 to 29 ␮m wavelength range, although the sensitivity of the detectors drops longward of 28 ␮m. The IFUs provide four simultaneous and concentric fields of view. The slice widths set the angular resolution perpendicular to the slice. The pixel sizes set the angular resolution along the slice. The spectral window of each IFU channel is covered using three separate gratings (i.e., 12 gratings to cover the four channels). Each grating is fixed in orientation and can be rotated into the optical path using a wheel mechanism (there are two wheel mechanisms which each hold 3 pairs of gratings). The optics system for the four IFUs is split into two identical sections (in terms of optical layout). One section is dedicated to the two short wavelength IFUs and the other handles the two longer wavelength IFUs, with one detector for each section. The two sections share the wheel mechanisms (each mechanism incorporates three gratings for one of the channels in the short wavelength section and three gratings for one of the channels in the long wavelength section). The image slicers in the MIRI IFU dissect the input image plane. The dispersed spectra from two IFU inputs are placed on one detector side-by-side. The spatial information from the IFU is spread out into two adjacent rows with the information from each slice separated by a small gap. Coronagraphy: MIRI features a coronagraph designed for high contrast imaging in selected mid-infrared bandpasses. Three quadrant phase masks (Boccaletti et al. 2004, Gratadour et al. 2005) provide high contrast imaging to an inner working angle of ␭/D, with bandpass of ␭/20, centered at 10.65 ␮m, 11.4 ␮m, and 15.5 ␮m respectively. A fourth, traditional Lyot mask of radius 0.9 arcsec, will provide R ∼ 5 imaging at a central wavelength of 23 ␮m. Simulations predict that the quadrant phases masks will achieve a contrast of ∼104 at 3 ␭/D. The Lyot stop is predicted to deliver a contrast of 2 × 103 at 3 ␭/D.

1.2.4.4 Tunable Filter Imager The tunable filter imager (TFI; Fig. 1.16) provides narrow-band near-infrared imaging over a field of view of 2.2 × 2.2 arcmin2 with a spectral resolution R ∼ 100. The etalon design allows observations at wavelengths of 1.6 ␮m to 2.6 ␮m and 3.1 ␮m

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Fig. 1.16 The FGS/TFI optical bench

to 4.9 ␮m. The gap in wavelength coverage allows one channel to reach more than one octave in wavelength. The TFI uses dielectric-coated Fabry-Perot etalon plates with a small air (vacuum) gap. The finesse is about 30 and the filters are used in third order. The finesse was chosen to be a compromise between the surface figure requirements and the need to minimize the number of blocking filters, while providing a contrast ratio of about 100. The filters are scanned using piezo-electric actuators, consisting of lead-zirconia-titanite ceramic transducers. The air (vacuum) gap ranges from 2.0 to 8.0 ␮m plate spacings. The Fabry-Perot operates at a nominal temperature of ∼35 K. To demonstrate the feasibility of a cryogenic Fabry-Perot etalon that meets the requirements, Rowlands et al. (2004b) fabricated a prototype. The etalon surface figure is the most critical requirement, and is influenced by the coatings, as well as structural and cryogenic issues. The TFI incorporates four coronagraphic occulting spots permanently to one side of the field of view, and occupying a region 20 by 80 arcsec. A set of selectable apodization masks is located at the internal pupil images of each channel by the filter wheels. The coronagraph will deliver a contrast ratio of ∼104 (10␴) at 1 arcsec separation. The sensitivity is limited by speckle noise. Contrast ratios of 105 may be achievable at sub-arcsec scales using roll or spectral deconvolution techniques (Doyon et al. 2004, Sparks & Ford 2002).

1.2.4.5 Fine Guidance Sensor The fine guidance sensor (FGS) instrument uses two fields of view in the JWST focal plane to provide fine guidance for the telescope. The field of view locations are chosen to provide optimum lever arms to all instruments for roll about a single guide star. Roll is sensed by star trackers on the spacecraft bus. The FGS consists of an optical assembly and a set of focal plane and instrument control electronics. The optical assembly of the FGS instrument consists of two channels, imaging separate regions of the sky onto independent 2k × 2k detectors.

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The detectors will be HgCdTe, similar to those in NIRCam. The plate scale is 67 milliarcsec/pixel and the field of view is 2.3 × 2.3 arcmin2 . The FGS will provide continuous pointing information to the observatory that is used to stabilize the line of sight, allowing JWST to obtain the required image quality. Each of the independent FGS channels provides > 90% probability of obtaining a useable guide star for any observatory pointing and roll angle. With both channels, the probability is >95%. The wavelength region and pixel size have been optimized so that in fine-guidance mode the FGS will provide pointing information to a precision of <5 milliarcsec updated at 16 Hz. The guide stars will be chosen from existing catalogs with AB < 19.5 mag in the J band. Fainter stars may be used for location identification. In the event that a suitable guide star is not available for a particular desired pointing and roll angle combination, alternate choices of roll angles can be considered during scheduling. JWST will be capable of relative pointing offsets with an accuracy of 5 milliarcsec rms, which will enable sub-pixel dithering and coronagraphic acquisition. Absolute astrometric accuracy will be limited to 1 arcsec rms by the accuracy of the guide star catalog. JWST will be capable of moving object tracking, at linear rates up to 30 milliarcsec s−1 relative to the fixed guide stars, although some degradation of the image quality may occur. This will enable observations of Kuiper Belt objects, the moons of the outer planets and comets that come as close to the Sun as Mars. The planets themselves would saturate many of the observing modes. The FGS is designed to be completely redundant in terms of the guiding function. The loss of any single component would at most result in the loss of one FGS channel. This would reduce the probability of guide star acquisition to ∼90% if using current catalogs.

1.2.5 Launch, Orbit and Commissioning Launch and Orbit: JWST will be launched on an Arianespace Ariane 5 Enhanced Capability-A rocket into orbit about the second Sun-Earth Lagrange (L2) point in the Earth-Sun system, approximately 1.5 × 106 km from the Earth. The orbit (shown in Fig. 1.17) lies in a plane out of, but inclined slightly with respect to, the ecliptic plane. This orbit avoids Earth and Moon eclipses of the Sun, ensuring continuous electrical power. JWST will have a 6-month orbit period about the L2 point in the rotating coordinate system moving with the Earth around the Sun. Station-keeping maneuvers are performed after the end of each orbit determination period of 22 days. The total observatory mass is 6500 kg, including station-keeping propellant sized for a 10-year lifetime. Deployment: The observatory has the following five deployments: (1) deploy spacecraft bus appendages, including solar arrays and the high-gain antenna, (2) deploy sunshield, (3) extend telescope tower, (4) deploy secondary mirror support structure, and (5) deploy primary mirror wings. The deployment mechanism design

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J.P. Gardner and JWST Science Working Group 1 week interval

(L + 6 months) Commissioning Complete (L + 113 da ys ) Initiate Observatory Commissioning

(L+ 41 min) S olar Array De ployme nt

(L + 4 days ) OTE De ployme nt

(L + 28 da ys) Initial WFS&C Commissioning

(T0) Launch (L + 14 hr) High Gain Antenna De ployme nt

(L + 2 days ) Suns hield De ployme nt

(L + 63 da ys ) Cool down ne ar ste ady state; Repeat WFS&C Commissioning

Stowed Observatory in 5meter shroud

Fig. 1.17 The launch and orbit of JWST

includes heaters and other protections that eliminate the need for time-critical events and allow for unlatching and re-latching to relieve any residual long-term stress in the structure. The secondary-mirror deployment and primary-mirror deployment sequence are shown in Fig. 1.18. Commissioning: There will be pre-commissioning activities during the transfer to L2. About 28 days after launch (Fig. 1.17), the observatory will cool down to a temperature that permits pre-commissioning activities to begin. Almost continuous ISIM availability for preliminary science observations is provided from this time until 106 days after launch, when a final trajectory burn is required to achieve orbit about L2. The intrinsic passive-thermal stability of the semi-rigid mirror segment and the two-chord-fold primary allows early operation of science instruments, by providing a stable optical image to the ISIM. These pre-commissioning activities will develop an operational experience database that allows formal commissioning and science operations to be conducted efficiently. A final checkout of all systems is initiated after the L2 orbit is achieved. Commissioning (complete 6 months after launch) includes ISIM, telescope, sunshield, and spacecraft operations that were not feasible during the transfer to L2. It also repeats

OTEinfolded configuration

Rotateandlatchprimary mirrorchords Latchsecondarymirror supportstructure

Deploysecondarymirror

Fig. 1.18 The secondary and primary mirror deployment sequence of JWST

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selected operations performed during the transfer, to ensure adequate knowledge of system performance in the final orbital-thermal conditions and allow comparisons with previous measurements. Examples of repeated operations include final optical distortion mappings for the observatory and the characterization of the wavefront sensing and control actuators’ transfer function. Allotting 76 days for ISIM precommissioning activities allows for operations rehearsal and increases familiarity with observatory operations without reducing operational availability. Mission Lifetime: JWST will operate with all science instruments for at least five years after completion of commissioning. In order to exploit the full scientific potential of the mission, a lifetime of ten years or longer is desired. Although we will not require mission assurance to guarantee a lifetime greater than five years, JWST will maintain the possibility of a longer mission lifetime, and will carry propellant sized for at least 10 years of operation after launch. There are no other consumables which would limit lifetime.

1.2.6 Operations JWST will be operated from a Science and Operations Center (S&OC) located at the Space Telescope Science Institute (STScI), the organization that operates HST for NASA. Although the capabilities of JWST are being developed to address science themes discussed in the previous sections, almost all of the observing time on JWST will be competitively selected. Approximately 10% of the observing time for the first 5 years of the mission has already been awarded to the science instrument teams and to other members of the science working group. An additional 5% will be director’s discretionary time allocated by the director of the S&OC. The remaining ∼85% of the observing time will be awarded through a series of proposal solicitations, which will be open to any astronomer in the world. The proposal solicitations will begin a year or two before the anticipated launch of JWST. The scope of JWST’s competitively-selected investigations will range from large legacy-style projects, which last months and address a range of science goals simultaneously, to small programs that target important but very specific science objectives.

1.2.6.1 Operations Concept JWST will be located at Sun-Earth L2 in order to allow effective cooling of the telescope and ISIM. The distance to the observatory is approximately the same throughout the mission, so the data rate to and from the observatory can be maintained with the same large ground antennae and 1 m class downlink antenna. Operations will be simpler than for HST, as HST’s low-Earth orbit means that target visibilities are interrupted every 95 min by Earth occultation. Astronomical targets from an L2 orbit are visible for long periods of time once or twice a year. L2 is a saddle point in the gravitational potential of the Earth-Sun system, and so propellant is required to maintain JWST at this position, and to dump angular momentum that builds up

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due to solar pressure on the sunshield. Since propellant is limited, its usage will be carefully monitored to meet the design goal of a 10-year operational lifetime. From an operations perspective, the instruments on JWST are comparable in complexity to those on HST, but more complex than those on Spitzer. A relatively small number of instrument configurations will be used by observers, with a limited selection of readout and dither patterns. Limiting the number of observing configurations also allows better calibrations of the instruments and assures consistency in the archived data. In order to achieve 70% observational efficiency, most calibration exposures will be done in parallel; that is, one instrument will be taking darks while another instrument is doing science observations. 1.2.6.2 Event-Driven Architecture Commanding HST and most other low-Earth-orbiting satellites is based upon absolute time. In contrast, JWST’s observing program will have few absolute time requirements. The commanding concept for JWST is like a command queue; commands in the queue are executed sequentially and the next command starts when an indication is received that the previous command is completed (successfully or unsuccessfully). This approach sacrifices certainty, but simplifies the software system that is needed for operations planning and lowers operations cost, in both the mission’s development and operational phases, and improves observational efficiency. The event-driven architecture for JWST will be implemented through a construct of visits and an observation plan. A visit is a logically grouped series of activities and conditions, including a slew to a new target, the steps required to finely point at the target, the setup of the instrument for the observation itself, and the acquisition of all of the science data at that pointing position. The observation plan constitutes the single queue that orders the visits. If a visit ends early because of a failure to identify the appropriate guide stars, for example, the on-board software starts the next visit in the sequence. The observation plan is uploaded to the observatory weekly, and fits within a long-range plan. Orbit maintenance is the only routine activity which will require real-time commanding, because managing its risk requires real-time monitoring. The S&OC will solicit proposals for JWST observations annually, on behalf of NASA, ESA and CSA. Those proposing observations will use an integrated planning tool, consisting of a graphical user interface, exposure-time calculators and tools for importing sky maps and accurately positioning the JWST apertures on targets. When writing a proposal, the astronomer will complete those portions required for scientific and technical assessment, including any special requirements for timing and/or fixed orientation of the observatory (Phase I). As with Spitzer, time will effectively be allocated in “wall-clock time”, including estimates for slew and setup times. Once the proposal is selected, the scientist will fill in the remaining details that would be required to execute the approved observations on JWST (Phase II). Target of opportunity observations, which interrupt the JWST observing plan for time-critical events such as supernovae or gamma-ray burst sources, will be enabled within 2 days of the decision to make these observations.

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The ground operations software will be constructed around the largely commercialoff-the-shelf command and telemetry system that will have been used in JWST’s integration and testing. The calibration pipeline will include cosmic ray removal, the factor which limits the exposure times for the detectors, and processing to remove the dark current and flat field and any other image artifacts. The S&OC will also manage the wavefront sensing and control observations, which will be implemented using the same system as the science observations. The WFS/C observations will be analyzed and corrections to the mirror positions will be implemented at the next observatory contact. An archive of all of the data obtained from the observatory will be maintained in the S&OC, using the capabilities of the Multi-Mission Archive at Space Telescope. Processing of the raw data will be done “on-the-fly” as calibrated data are requested over the Internet.

References Acton, D. S., Atcheson, P. D., Cermak, M., Kingsbury, L. K., Shi, F., & Redding, D. C. 2004, in J. C. Mather, ed., Optical, Infrared, & Millimeter Space Telescopes, Proceedings of SPIE, 5487, (SPIE: Bellingham WA), 887 Armus, L. et al. 2004, ApJS, 154, 178 Atkinson, C., Texter, S., Hellekson, R., Patton, K., Keski-Kua, R., & Feinberg, L. 2006, in J. C. Mather, H. A. MacEwen, & M. W. M. de Graauw, eds., Space Telescopes and Instrumentation I: Optical, Infrared and Millimeter, Proceedings of SPIE, 6265, (SPIE: Bellingham WA), 62650T Barkana, R., & Loeb, A. 2001, Physics Reports, 349, 125 B´ely, P. Y. 2003, The Design and Construction of Large Optical Telescopes, (Springer: New York) Bender, R., Burstein, D., & Faber, S. M. 1992, ApJ, 399, 462 Boccaletti, A., et al. 2004, PASP, 116, 1061 Boss, A. P. 2001, ApJ, 551, L167 Chabrier, G., Baraffe, I., Allard, F., & Hauschildt, P. 2000, ApJ, 542, L119 Crovisier, J., et al. 1997, Science, 275, 1904 Doyon, R., et al. 2004, in J. C. Mather, ed., Optical, Infrared, and Millimeter Space Telescopes, Proceedings of SPIE, 5487, (SPIE: Bellingham WA), 746 Fan, X., et al. 2002, AJ, 123, 1247 Feinberg, L. 2004, in J. C. Mather, ed., Optical, Infrared, and Millimeter Space Telescopes, Proceedings of SPIE, 5487, (SPIE: Bellingham WA), 814 Gardner, J. P., et al. 2006, Space Sci. Rev., 123, 485 Gardner, J. P., Fan, X., Wilson, G. & Stiavelli, M. 2007, in L. J. Storrie-Lombardi, & N. A. Silberman, eds., The Science Opportunities of the Warm Spitzer Mission Workshop, AIP Conference Proceedings, (Springer-Verlag: New York) Gebhardt, K., et al. 2000, ApJ, 539, L13 Gibb, E. L., et al. 2000, ApJ, 536, 347 Gnedin, N. Y. 2004, ApJ, 610, 9 Gratadour, D., et al. 2005, A&A, 429, 433 Greenhouse, M. A., et al. 2006, in J. C. Mather, H. A. MacEwen, & M. W. M. de Graauw, eds., Space Telescopes and Instrumentation I: Optical, Infrared and Millimeter, Proceedings of SPIE, 6265, (SPIE: Bellingham WA), 626513 Horner, S. D., & Rieke, M. J., 2004, in J. C. Mather, ed., Optical, Infrared, and Millimeter Space Telescopes, Proceedings of SPIE, 5487, (SPIE: Bellingham WA) 628 Kogut, A., et al. 2003, ApJS, 148, 161

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Lunine, J. I., Coradini, A., Gautier, D., Owen, T.C., & Wuchterl, G. 2004, in F. Bagenal, T. Dowling, & W. McKinnon, eds., Jupiter, (Cambridge University Press: Cambridge) 19 Magorrian J., et al. 1998, AJ, 115, 2285 Malfait, K., Waelkens, C., Waters, L. B. F. M., Vandenbussche, B., Huygen, E., & de Graauw, M. S. 1998, A&A, 332, L25 Morbidelli, A., Chambers, J., Lunine, J. I., Petit, J. M., Robert, F., Valsecchi, G. B., Cyr, K. E. 2000, Meteoritics Pl. Sci. 35, 1309 Rees, M. J. 1997, in N. R. Tanvir, A. Aragon-Salamanca, & J. V. Wall eds., The Hubble Space Telescope and the High Redshift Universe, (World Scientific: Singapore), 115 Riesenberg, R., & Dillner, U. 1999, in S. S. Shen ed., Imaging Spectrometry V, Proceedings of SPIE, 3753, (SPIE: Bellingham WA), 203 Rowlands, N., et al. 2004a, in J. C. Mather, ed., Optical, Infrared, and Millimeter Space Telescopes, Proceedings of SPIE, 5487, (SPIE: Bellingham WA), 664 Rowlands, N., et al. 2004b, in J. C. Mather, ed., Optical, Infrared, and Millimeter Space Telescopes, Proceedings of SPIE, 5487, (SPIE: Bellingham WA), 676 Shu, F. H. 1977, ApJ, 214, 488 Shu, F. H., Adams, F. C., & Lizano, S. 1987, ARA&A, 25, 23 Soifer, B. T., et al. 2004, ApJS, 154, 151 Sparks, W. B., & Ford, H. C. 2002, ApJ, 578, 543 Stahl, H. P., Feinberg, L., & Texter, S. 2004, in J. C. Mather, ed., Optical, Infrared, and Millimeter Space Telescopes, Proceedings of SPIE, 5487, (SPIE: Bellingham WA), 818 Terebey, S., Shu, F. H., & Cassen, P. 1984, ApJ, 286, 529 Tully, R. B., & Fisher, J. R., 1977, A&A, 54, 661 van den Ancker, M. E., Tielens, A. G. G. M., & Wesselius, P. R. 2000, A&A, 358, 1035 White, S. D. M., & Frenk, C. S. 1991, ApJ, 379, 52 Wright, G. S., et al. 2004, in J. C. Mather, ed., Optical, Infrared, and Millimeter Space Telescopes, Proceedings of SPIE, 5487, (SPIE: Bellingham WA) 653 Yorke, H. W., & Sonnhalter, C. 2002, ApJ, 569, 846 Zamkotsian, F., & Dohlen, K. 2004, in J. C. Mather, ed., Optical, Infrared, and Millimeter Space Telescopes, Proceedings of SPIE, 5487, (SPIE: Bellingham WA), 635

Xander Tielens, Massimo Stiavelli, and Massimo Robberto pose for the camera

Chapter 2

Beyond the Hubble Space Telescope: Early Development of the Next Generation Space Telescope Robert W. Smith and W. Patrick McCray

Abstract In this paper we investigate the early history of what was at first called the Next Generation Space Telescope, later to be renamed the James Webb Space Telescope. We argue that the initial ideas for such a Next Generation Space Telescope were developed in the context of the planning for a successor to the Hubble Space Telescope. Much the most important group of astronomers and engineers examining such a successor was based at the Space Telescope Science Institute in Baltimore. By the late 1980s, they had fashioned concepts for a successor that would work in optical, ultraviolet and infrared wavelengths, concepts that would later be regarded as politically unrealistic given the costs associated with them. We also explore how the fortunes of the planned Next Generation Space Telescope were intimately linked to that of its “parent,” the Hubble Space Telescope.

2.1 Introduction Very large-scale machine-centered projects have been a central element in the physical sciences since World War II, especially in North America, Europe, and Japan. Built with the support of national governments, often working together in international partnerships, these endeavors cost hundreds of millions or even billions of dollars and engage the efforts of armies of scientists and engineers. The biggest of these projects have typically taken decades to bring to fruition. For scientists, their construction has been in large part an act of faith that new and powerful new scientific instruments will surely lead to novel and exciting scientific results.1 The journey from conception to completion for such endeavors has usually been fraught with assorted challenges and difficulties, and in some cases these have led to a project’s demise years after detailed work has begun. A striking example of this is the 1993 cancellation of the Superconducting Super Collider, a high-energy physics accelerator, after the Department of Energy spent over $4 billion on its design and construction.2 In general, however, scientists and scientific communities have R.W. Smith (B) Department of History and Classics, University of Alberta, Edmonton, Alberta, Canada e-mail: [email protected] H.A. Thronson et al. (eds.), Astrophysics in the Next Decade, Astrophysics and Space Science Proceedings, DOI 10.1007/978-1-4020-9457-6 2,  C Springer Science+Business Media B.V. 2009

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become increasingly adept at enlisting broad involvement in proposed programs, thereby building stronger bases of support that enable advocates to better resist threats of cancellation. Such very large-scale efforts have also resulted in unique and extremely powerful tools that have greatly expanded as well as helped to intellectually reconstitute scientific disciplines. A leading example of this phenomenon is the Hubble Space Telescope. The space and scientific agencies, as well as the scientific and engineering communities, engaged in such enterprises have often faced a number of critical and sensitive issues. One is when to initiate serious design work on new machines that will replace those in operation, being built, or being planned. Another is the question of when to decommission instruments already doing productive research. Given the very long lead times from conception to operation, engineers and scientists have often wanted (or been forced) to begin planning the next big machine years or decades before securing any scientific results from the one under construction, results that of course might well have the potential to shape or revise design decisions. In this paper we will examine how a scientific community and its constituent sub-communities took the first steps towards the construction of what was initially called the Next Generation Space Telescope (renamed by NASA in 2002 as the James Webb Space Telescope in honor of NASA’s administrator between 1961 and 1968). Scientists and engineers initially conceived the NGST in the mid-1980s as a successor to the Hubble Space Telescope (HST), some years before this observatory began scientific operations in orbit around the Earth in 1990. There were initially two parallel tracks to the NGST’s early history. In one there were a range of developments in infrared astronomy that would prove later to be crucial for NGST planning. In the second track, advocates explored a successor to the Hubble Space Telescope that would operate in ultraviolet, optical, and infrared wavelengths, just as the Hubble was supposed to do. These two tracks would ultimately come together in the mid-1990s and prompt NASA to issue a study contract in October 1995 for feasibility studies for a Next Generation Space Telescope. In this paper, our focus is on the second of these two tracks, and our main narrative thread is provided by the way in which scientists and engineers examined a wide range of design options for the NGST early in its life. The largest of large-scale scientific tools require not just the enthusiastic endorsement of small groups of scientists and engineers, but the whole-hearted support of entire scientific communities, generally in more than one country because projects of the largest scale typically involve international collaborations. We therefore ask how advocates worked to form a consensus around some basics of the design of the NGST. This effort helped to create a favorable climate of opinion, a key step towards winning broad approval for an NGST by persuading colleagues it might be not just technically feasible but also perhaps politically feasible. As we will see, space astronomy has been in one respect a remarkable adventure of the human spirit, but in the U.S. it has also been pursued in a highly competitive environment with often intense debates and conflicts over resources and priorities. That is, the resultant mission is rarely a consensus design, but rather the “winner” of a contentious process.

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2.2 A Successor to the Hubble Space Telescope In its early years, NASA supported space astronomy in various wavelength regions, although the agency gave the edge to UV and optical astronomy from the start. Studies in the different wavelength regions nevertheless ran a similar course in that research generally started with survey missions, leading in time to very versatile but complex and costly spacecraft. The pace at which a particular wavelength region reached the stage of what would later be called “Great Observatories” or “Flagship” missions differed. As UV and optical investigations had gained an early lead, the large-scale observatory in this region came first in the shape of the Hubble Space Telescope (HST). From the early 1970s on, the HST was the key space telescope in NASA’s planning.3 By 1974, this was a joint effort of NASA and the European Space Agency, although NASA was the dominant partner. NASA, of course, has been primarily a technical management agency. When it comes to pursuing astronomy, NASA has mostly provided money, facilities and management expertise. The design and construction of hardware and software themselves has come very largely from industrial contractors that NASA oversees and coordinates. NASA, of course, has its own institutional interests, of which astronomers outside the agency must be mindful to get built the tools they desire. NASA, however, is not a monolithic agency. Different groups within the space agency often have somewhat different or even contradictory interests. NASA’s history is replete with many examples of the tensions, for example, between its different several field centers as well as between the field centers and NASA Headquarters in Washington D.C.4 The idea of building a successor to HST also seemed obvious to some, but by no means all, astronomers almost from the start of serious planning for Hubble. Detailed design and construction of HST began in 1978 following White House and Congressional approval of the project, by which time astronomers had generally accepted the view that “the whole history of science, and particularly of Astronomy in recent years, tells us that progress depends on the development of new instruments which give us new ways of looking at the world.”5 The main advocates in the 1980s of a successor to the Hubble Space Telescope, as we shall see, were based at the AURA-managed Space Telescope Science Institute (STScI) in Baltimore. There was therefore an aspect of “institutional maintenance” too to this effort. Once the HST’s mission was over, so was the Institute’s, unless new business, perhaps in the form of a successor to the HST, was in the offing. Thus, long range scientific planning meshed nicely with institutional maintenance. Even before the Hubble Space Telescope was launched, the central question for many astronomers was not “Should a successor to the Space Telescope be built?”, but rather how much the successor to the Hubble Space Telescope should differ from the HST itself. Of special importance was whether they should simply scale up Hubble to what seemed likely to be a “do-able” size, given whatever technology would be available at that future time. By the mid-1980s, Hubble’s mirror, 2.4 m in diameter, was quite small by the standards of state-of-the-art ground-based telescopes, either under construction or in planning. Scaling up to a bigger mirror size

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and fashioning a generally larger version of Hubble was therefore an attractive option to at least explore. The limit to how big a mirror they might reasonably argue for would likely be set by the perceptions of what cost its patrons in the White House and Congress would support. On the other hand, instead of a simple scaling-up, supporters of the new space telescope could be even bolder and press for radical, and therefore quite probably more risky, technological choices. This could also include a major mission that would operate at different wavelengths than Hubble, thus opening a new “window” in exploring the Universe. Addressing the question of how far to push the technological boundaries as well as simply determining what they were and what the “political system” would support demanded in part that astronomers and engineers integrate various elements from existing state-of-the-art telescope designs into their planning. It also required that eventually they take the expertise of industrial contractors and the military into account as they had very extensive experience in both planning and building complex satellites for a variety of national security purposes.6 The various contractors who might be involved in planning for, managing or building such a machine also had self-interest in proposing some sort of successor to the Hubble Space Telescope. For example, in 1980, NASA’s Marshall Space Flight Center considered a plan to launch an 8 m space telescope into low-Earth orbit in the external tank of the space shuttle. In the following year, the company responsible for the optical elements of the HST, Perkin-Elmer, published an article on “Space Astronomy.” Included here was a concept for an optical-ultraviolet telescope in space with a mirror 8 m in diameter (a very big step up from the 2.4 m diameter mirror for the HST). Malcolm Longair, one of the leading European astronomers working closely on the HST and at the time Astronomer Royal for Scotland, carried this idea further when in 1983 he argued that many “exciting projects are being converted from gleams in the eye of the astronomer into feasible astronomical projects, the only limits being those of the imagination of the astronomer and the more important limits of funding and manpower. Examples of these types of project include a very large space telescope for optical and ultraviolet observations. An aperture of up to 10 m . . . would represent a huge increase in scientific capability over even the Space Telescope.”7 During the 1980s, the idea of such a successor to the HST was pressed most enthusiastically by a group of staff members at the Space Telescope Science Institute. Key in this respect was the Institute’s director, Riccardo Giacconi. Giacconi, arguably the leading figure in the establishment of x-ray astronomy, shared the Nobel Prize for Physics in 2002 for this work. From hard experience of flying and planning x-ray satellites, Giacconi knew that the lead times for large-scale space observatories could be counted in decades. If there was not to be big gap between the end of the life of the Hubble Space Telescope and its successor, it was essential to get planning underway well before HST was launched. By 1986, a small group at the Institute was thinking hard about such a successor. The group included astronomers Garth Illingworth and Peter Stockman, as well as Pierre Bely, an engineer with experience of large ground-based telescopes as well as space astronomy. In that year, Bely wrote a paper that laid out some of the

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details for a 10 m optical telescope in space. He considered, among other things, the size, cost, and location of such a telescope. In line with Giacconi’s own thinking, Bely contended that although the Hubble Space Telescope was not yet launched, its limited operational life of 15 years and experience that it takes from ten to fifteen years to complete a large astronomical telescope, meant “it soon will be time to start making serious plans for its successor.”8 Bely reported that several designs had already been advanced. He promoted a general purpose observatory in space that would be as “unspecialized”9 as HST. By “unspecialized,” Bely meant that the new observatory, like HST, should have a wide, rather than a narrow, range of capabilities – a general-purpose observatory, which would also have political appeal to a broad range of astronomical communities. The new telescope, however, should be a much more powerful scientific tool than the HST: among other things, it should have a bigger mirror and greater resolving power, and cover the wavelength range from the ultraviolet through the optical to the infrared. Bely considered costs, where such a telescope should operate from, whether or not it should be an international venture, the options for the size and type of main mirror, and a number of other issues. In the end, he advocated further studies of a telescope with a primary 10 m in diameter placed into geosynchronous orbit; that is, an orbit some 22,000 miles or so above the Earth, so that it would always be above one part of the planet. This location was sufficiently far from the Earth that the Earth would not block out large sections of the sky, a serious handicap for the Hubble Space Telescope designed to orbit only a few hundred miles above the Earth, which was at the time the limit to the altitude to which a larger optical system could be launched by the system – the Space Shuttle – that was available to the astronomers. Bely’s initial design criteria proved to have significant longevity as the NGST began to take shape.

2.3 The Space Studies Board Since the American government established NASA in 1958, the agency has maintained important links with the National Academy of Sciences. Charged, among other things, with providing advice to the government, the National Academy of Sciences had formed the Space Sciences Board (later renamed as the Space Studies Board [SSB]) at the dawn of the Space Age with the specific intent of using the Board to provide advice to NASA. Although the relationship between the SSB and NASA has sometimes been fraught, in the opinion of some critics, the National Academy quickly became a form of “shadow government” whose backing was often reckoned to be critical if a new project was to proceed. The Space Studies Board’s recommendations therefore generally carried clout, and its reports can become crucial political resources. If a successor to the HST was to come into being, it would surely need the strong backing of the National Academy. In 1988, the Space Studies Board released a report that detailed key scientific issues they anticipated that space scientists would tackle from roughly 1995 to 2015.

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Space Science in the Twenty-First Century: Imperatives for the Decades 1995–2015, outlined three main areas for astronomy and astrophysics, one being “large area and high-throughput telescopes.”10 The SSB also recommended NASA consider an optical telescope with a mirror 8–16 m in diameter: “A large aperture space telescope for the ultraviolet, optical, and infrared regions has immense scientific potential. The need for such a telescope will be very high after 10 to 20 years use of HST and ground-based 8-to 10 m-class telescopes,” the report contended. “Even now we see that some of the most fundamental of all astronomical questions will require the power of a filled-aperture telescope of 8- to 16-m diameter designed to cover a ˚ to 30 ␮m [that is, from the ultraviolet to the mid-infrared] wavelength range of 912 A with ambient cooling to 100 K to maximize the infrared performance.” The report also noted that both the Hubble Space Telescope and SIRTF (the Space Infrared Telescope Facility, a medium-aperture infrared telescope at this time slated to be carried into space intermittently by the Space Shuttle), which had yet to be launched, with the expected wealth of date yet to be analyzed so “it is difficult but not premature to formulate a detailed concept of such a large-scale telescope for the ultraviolet, optical and infrared regions.” The report’s authors nevertheless extolled the increased performance of such an instrument compared to the Hubble Space Telescope. In their opinion, there was a wide range of scientific problems that could be tackled only by a telescope of this type. The extra light-gathering power and resolution, combined with advanced instruments and detectors, “would lead to a quantum leap in our understanding of some of the most fundamental questions in astronomy.”11 But they stressed it would “not simply be a scaled-up HST.” Unlike the HST, the new telescope’s optics could be cooled to “at least the lower limit of passive radiation methods, about 100 K12 ”, and so its infrared performance optimized. [The great challenge with infrared observations is to ensure that the thermal emission from the telescope itself does not swamp the radiation the telescope is detecting from astronomical objects.] For an 8 m class telescope, a large launch vehicle could carry it to orbit, while a 16 m telescope would require the segments of its mirror to be lofted into space and then assembled onto a structure. Also, the group reckoned that an 8–16 m telescope was “within closer reach than a simple extrapolation form HST would suggest.”13

2.4 Beyond the Hubble Space Telescope In 1988 STScI scientist Garth Illingworth gave a presentation on “The Next Generation: An 8–16 m Space Telescope” as part of the International Astronomical Union’s General Assembly in Baltimore in 1988. He posed the basic question “What is the UV-Visible-Optical Observatory that will follow HST?” First, he made the case for a successor, what he referred to as “Son of HST” or maybe “Daughter of HST.” He presented two arguments in making his case: continuity and discoveries. HST offered broad capabilities and the scientific case for such an observatory would be as true in the future as it had been for HST back in the 1970s, both to carry out

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major observing programs and to support other missions. New facilities, such as a next generation telescope, would “open up new ‘discovery space’ by a significant amount.” About the same time, a popular book and accompanying articles on Cosmic Discovery by Cornell astronomer Martin Harwit was widely quoted among astronomers. According to Harwit, an increase in instrument sensitivity of 2–3 orders of magnitude were typically required to achieve major “discoveries;” that is, an increase in telescope diameter of 3–5 along with new generations of instruments. In line with the 1988 Space Studies Board report, Illingworth described the observatory he would prefer to study, one with a 16 m primary (perhaps with a segmented design made up of four 8 m parts) that would be passively cooled to around 100 K, be in geosynchronous orbit, and operate in the wavelength range from 0.1 ␮m to longer than 10 ␮m, thereby providing an extremely powerful telescope that could operate from the ultraviolet well into the infrared. As to timing, he noted that in 1962, the National Academy of Sciences had held a workshop that produced “A Review of Space Science,” and recommended a large diffraction-limited space telescope, in effect a recommendation for what became HST. This meant there had been more than 25 years from a major recommendation to launching HST (he was speaking about a year before its launch). Hence, to ensure a successor to the HST in 15–20 years – that is, around 2005–2010 – “now is clearly the time to move.”14 Even more sobering, what would become the Space Infrared Telescope Facility (SIRTF) and today the Spitzer infrared “Great Observatory” was first proposed to NASA in the summer of 1971 and launched slightly more than 30 years later. A year after Illingworth gave his presentation at the International Astronomical Union, over 130 astronomers, engineers, and science managers met in Baltimore at the Space Telescope Science Institute to plan what was now called “The Next Generation Space Telescope.” This gathering proved a key step in moving beyond the Hubble Space Telescope to the ‘Next Big Machine’ for space astronomy. The Baltimore meeting was of a kind that astronomers frequently held. Such meetings offer a convenient forum for members of the science and engineering communities to acquaint themselves with what their colleagues are doing, exchange ideas, advocate particular choices, impress potential patrons and possible partners, build a base of support among scientists, and generally move a project forward. As one participant explained, “No one goes away from these meetings and says ‘I’ve seen the Holy Grail and so-and-so has it.’ They are to inform people and get everyone up to the same level before they can move on to the next level.”15 Attendees at the 1989 workshop learned of several diverse designs already vying to become the Next Generation Space Telescope. Some were relatively new while versions of others had circulated throughout the astronomy community for years, again a common practice in astronomy by this date. Garth Illingworth chaired the meeting’s science committee. He described his colleagues’ goals as “very ambitious and challenging, but realistic extrapolations of current technology.”16 The organizers of the Baltimore meeting presented their colleagues with two “straw man” designs to focus their discussion – one was a space telescope with a 10 m mirror orbiting the Earth. The second was even more exotic: a 16 m telescope on the lunar surface, its proposed location reflecting NASA’s very

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short-lived commitment to a new policy announced just weeks earlier by President George H.W. Bush on 20 July 1989, the twentieth anniversary of astronauts landing on the Moon. This proposed program, known as the Space Exploration Initiative (SEI), envisaged a relatively swift return by the United States to the surface of the Moon and the start of planning for a human expedition to Mars.17 NASA and the astronomy community responded to Bush’s proposal (and the hope of massive funding that might become available) by rapidly developing optimistic plans for Moon-based astronomy facilities. One idea suggested by NASA’s Marshall Space Flight Center (at the time the lead NASA center for the development of the Hubble Space Telescope) was a Large Lunar Telescope. These schemes pictured a 16 m telescope (with a segmented mirror) that would be assembled robotically or perhaps by teams of astronauts. In the planning for the Hubble Space Telescope some two decades earlier, the option of a large Moon-based telescope had also been raised. For a time the name of the proposed telescope was the Large Orbital Telescope, but this was changed to the more neutral Large Space Telescope so as to leave open the possibility of basing it on the Moon. Bush’s ambitious, but politically unrealistic, plan faded swiftly away and carried off with it the idea of a lunar telescope. Marshall’s planning initiatives nevertheless underlined the space center’s interest in some sort of role in the NGST project. Astronomers considered other locations for the proposed Next Generation Space Telescope. One option that engineers advanced was a low-Earth orbit achieved via the Space Shuttle, which is where the HST would be located. Another involved flying the telescope to one of the Lagrangian points. Located about a million miles from Earth, this piece of cosmic real estate was reckoned by many advocates of NGST as having excellent qualities for a space observatory: it is very cold and dark and a long way from the Earth. But placing a telescope at a Lagrangian point would also eliminate the option of having visiting astronauts service and upgrade it, at least so long as the Shuttle was the only means to flying U.S. astronauts into space. The proposed location for an NGST was, as we shall see, a critical element in the efforts to develop a telescope that would be far less expensive than a scaled-up version of the Hubble Space Telescope. The debate and negotiations that ensued on the possible location of an NGST as well as other issues revealed several critical problems that astronomers and engineers reckoned they needed to resolve if they were to make serious progress towards a Next Generation Space Telescope. One pressing issue was the design and size of the telescope’s primary mirror. Astronomers generally regard the primary mirror as the single most important component of any telescope. Its size and quality determine how much light the telescope can collect and the worth of the data it produces. At the Baltimore meeting engineers and industrial contractors touted the optical quality of Hubble’s 2.4 m mirror, which at the time was generally reckoned to be superb (which of course was not in fact the case). A 10 m mirror would collect twenty times as much light as HST and, other things being equal, yield much superior images and even more exciting data. Even a 16 m mirror was discussed. Not widely appreciated at the time was a prescient design by a Swales engineer, Philip J. Tulkoff, that proposed a 10 m space

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Fig. 2.1 Artist’s concept of a 10 m next generation space telescope as envisaged in 1989

aperture telescope that could be passively cooled to between 70 and 100 K, permitting very sensitive observations well into long infrared wavelengths and a modest step in breaking the engineering paradigm of the time that infrared space telescopes required complex and heavy liquid cryogen systems. As already noted, by 1989 the size of Hubble’s mirror was decidedly modest by comparison with ground-based telescope. By the 1980s, the standard size of telescope mirrors for ground based instruments was in the 4 m range while astronomers and engineers were already well-along in building telescopes with mirrors that sported mirrors as large as 10 m (Fig. 2.1). Astronomers reckoned, however, that HST’s location far above the turbulent atmosphere would allow it to collect images of exquisite detail while working 24 hours a day and these factors would more than offset the fact that in terms of size its mirror was far from the state-of-the-art in 1989. HST’s mirror was also relatively heavy in comparison to the new lightweight technologies NASA and the military desired. Putting objects into orbit in the era of the Space Shuttle was – and remains – extremely expensive, and designers of scientific spacecraft in the 1980s and 1990s, therefore, saw reducing weight wherever possible as a crucial problem. Designers also knew that the heavier a scientific satellite generally the more it would cost. Developing the capability to make very big lightweight mirrors, sometimes called “gossamer optics,”18 was clearly generally reckoned by the participants at the Baltimore meeting to be the most significant technical obstacle to building the NGST. The design of NGST’s primary mirror certainly became one of the project’s so-called tall poles,19 just as it had been for the Hubble Space Telescope. That is to say, designing and fashioning the primary mirror to the required specifications might entail such difficulties that it would hold up the project (aka, the “tent”) and other problems would tend to get lost beneath the canvas – problems that would be present, certainly, but not so pressing or so visible. If astronomers were going to launch any Next Generation Space Telescope, it was already clear to its advocates in 1989 that they needed to reduce radically the weight of the mirror from what

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they could expect if they simply scaled-up the HST mirror design. Such a procedure would lead to a hugely expensive telescope, and a key concern at the meeting was the overall cost of an NGST. Hanging over the proceedings was the need to “break” the cost curve of the HST as an extrapolation of this curve would mean, unless the political context changed radically, an impossibly expensive telescope that would never be built. George Field, a prominent astronomer who had been a champion of the Hubble Space Telescope in the 1970s, suggested at the Baltimore meeting that the price of a telescope in low Earth orbit with a mirror of diameter D (meters) would be about $3.8(D/10)1.7 billion in 1986 dollars.20 If a 16 m telescope were placed on the Moon, the new estimate for the telescope alone would be $8.4 billion in 1986 dollars. As Garth Illingworth noted in his presentation, one scaling law used for the cost of telescopes reckoned that the cost rose as the 2.7th power of the diameter. HST’s cost for design and development was around $2 billion, so applying “such a factor for a 10–16 m class telescope based on HST’s cost leaves one gasping.” But Illingworth contended that recent large ground-based telescopes had “broken” this “cost curve,” which had been established by telescopes built in the 1950s to the 1970s, by a factor of 4, with more gains in the “pipeline.” Critical was savings in weight, which translated into cost savings. Illingworth judged that as “we can see from the discussion at this meeting, this is an area where major improvements in fabrication and polishing techniques are occurring. The combination of improved performance and lower weight for the optical segments will directly and dramatically affect the final cost of the NGST.”21 In the published report of the Workshop, five statements and recommendations were presented as representing the spirit of the collective opinion of the participants and are worth quoting here at length. There were: 1. Scientific Objectives: There will be a definitive need to continue and extend the observational capability offered by HST beyond its predicted lifetime. A gap of more than 5 years would be a blow to the vitality of forefront astronomical research. The scientific potential of an HST follow-up mission with enhanced flux collecting power and spatial resolution, and with spectral coverage extended through the near-infrared is enormous. . .. An observatory providing high sensitivity and high-throughput spectroscopic capability at diffraction-limited spatial resolution from the UV to beyond 10 microns is vital for the study of the most fundamental questions of astrophysics. These include the formation and evolution of galaxies, stars and planets, and the nature of the young universe. 2. Technological Readiness: A telescope in the 10–16 m class is not an unrealistically large step beyond the current state of technological development. While development and demonstration programs are clearly needed, many of the core technologies are maturing to the point where the required goals appear to be within a very reasonable extrapolation of the current stateof-the-art. In particular, advances in the fabrication of lightweight optics and new techniques for polishing have the potential for very substantial weight savings and hence cost savings, while offering optical performance beyond what was possible in the past.

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3. Siting: Both the Moon and high Earth orbits are suitable sites for [the] next generation space telescope. Low Earth orbits are undesirable because of high disturbance levels, insufficient passive cooling and low observing efficiency. . .. Space-based and lunar-based designs should be pursued in parallel for the next few years to clarify the observational, technical, space logistical and cost tradeoffs. 4. Programmatic approach: A 10–16 m (space-based) to 16 m (lunar-based) aperture is considered a realistic goal. Future workshops should concentrate on further definition of the scientific objectives, review of preliminary studies and the identification of critical technologies. Strawman designs should be prepared to refine the various concepts and ideas and to focus discussion. . .. In projects of this complexity, efficient design is the result of many compromises that can only be developed by successive iterations and by system-level analyses. The importance of this iterative process involving astronomers, physicists and engineers in the science-engineering tradeoffs and in defining the requirements was emphasized by many participants. The involvement of these different groups needs to occur during all phases of the project, from concept to development, through technology development and fabrication, and finally during system-level testing. Once clearly identified by the preliminary design process, the development of the key enabling technologies should be integrated with the appropriate long-term program of the national and international Space Agencies. . .. 5. International cooperation: Like HST, the next generation space telescope project should be carried out cooperatively as an international program. Cost sharing renders such major missions more affordable for each participating country, and international collaboration often enhances quality and performance. Complex and pioneering space missions also benefit from the exchange of ideas and variety of approaches afforded by multicultural associations.22

Co-operation between NASA and the European Space Agency was very much “in the air” at the Workshop. Duccio Machetto, an ESA astronomer based at the Space Telescope Science Institute, spoke on “ESA[’s]” Long Term Plans and Status. He noted that “There is a large interest in the astronomical community in Europe for HST and also in a future HST. It is therefore important to include the European astronomical scientific community in a possible joint venture in a 10–16 m next generation space telescope.”23 Further, one page of the published report of the Workshop also carried a section headed “Sage Advice.” Here John Bahcall, a leading and very influential astronomer who had played a pivotal role in winning Congressional approval for the Hubble Space Telescope in the 1970s and so making the HST politically feasible, was quoted as arguing that “International cooperation may be critical for such a major project.”24 Ultimately, the James Webb Space Telescope would be a cooperative venture of NASA, the European Space Agency, and the Canadian Space Agency. At this stage, however, none of the astronomers and engineers who attended the workshop was based at Canadian institutions, although there was already strong European interest. Indeed, as we shall see in subsequent work, the European space agencies were in many cases offering the astronomy communities more frequent opportunities at this time to fund advanced post-HST concepts than NASA.

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2.5 The Decade of the Infrared As astronomers and engineers met in Baltimore to discuss an NGST, work was already proceeding on what became known as the Bahcall Committee report, as this very large scale effort at planning was chaired by John Bahcall. We have already discussed the importance of advice from the National Academy of Sciences in shaping NASA’s priorities. Hence a significant hurdle came for the NGST when in early 1989 the National Research Council commissioned the Astronomy and Astrophysics Survey Committee to review the field and produce a series of recommendations on new ground-based and space-based programs and observatories. The Bahcall Committee’s recommendations were based on studies by fifteen advisory panels that represented various wavelength regions and particular areas of astrophysics. In all, advice came from over 300 astronomers who served on these panels, and another 600 or so wrote letters, essays, or delivered oral presentations at various open meetings. In line with the usual thinking of astronomers by this point, the Committee argued in its final report that “Progress in astronomy often comes from technological advances that open new windows on the universe or make possible large increases in sensitivity or resolution. During the 1990s, arrays of infrared detectors, the ability to build large optical telescopes, improved angular resolution at a variety of wavelengths, new electronic detectors, and the ability of computers to process large amounts of date will make possible an improved view of the universe.”25 The Committee went on to package their conclusions by proclaiming the 1990s as “The Decade of the Infrared” and expected that the “technological revolution in detectors at infrared wavelengths will increase the power of telescopes by factors of thousands.”26 The panel on ultraviolet and optical astronomy, which was chaired by Garth Illingworth, who we have seen was an active champion of a successor to the HST, strongly recommended building what the panel termed the “Large Space Telescope.” This would be a 6 m telescope operating in the ultraviolet, the infrared, and the optical, and the panel urged that it be flown within a few years of the end of the expected 15 year life of the HST (then assumed to be 2005). The panel therefore proposed starting the Large Space Telescope in 1998 so that it could be completed by 2009. The Large Space Telescope would later be followed by “a telescope of astonishing power,” the 16 m Next Generation Space Telescope, which the panel judged should naturally be located at a lunar outpost,27 again assuming a major NASA program capable of placing and operating complex facilities on the lunar surface at a cost and timeframe acceptable to the astronomy community. With the Bahcall Committee charged to recommend the most important new initiatives for the decade 1990 to 2000, it in fact advocated neither the Large Space Telescope nor the Next Generation Space Telescope. Even the Large Space Telescope was costed at $2 billion and in 1991 very large scale astronomy projects were standing in the shadow of the Hubble Space Telescope, which had been launched in 1990 but, as we shall see later, its early performance had failed badly to live up to its advance billing. While the Bahcall Committee did not give explicit reasons why it

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chose not to back the Large Space Telescope, they certainly judged it premature to give it a top priority. In particular, the Bahcall Committee assessed what it reckoned to be the critical technological initiatives for the 1990s so as to “form the basis for frontier science in the decade 2000–2010.” $50 million, the Committee argued, should be spent developing technologies for large space telescopes.28 Contending that “we must begin now the conceptual planning and technological development for the next generation of astronomy missions to follow the Great Observatories,” they cited as one example the 6 m Large Space Telescope operating in the ultraviolet to the infrared. They discussed other possible missions too, including a very large x-ray telescope and a submillimeter observatory consisting of a deployable 10 m telescope. It did not, however, make a choice between the different options, judging that the “scientific imperatives and the infrastructure available at the time of selection will influence which missions are chosen.” Among the technical issues, they reckoned, would be the construction and control of lightweight systems, the capability of launch vehicles, advances in robotic constructions techniques, as well as the possible availability of facilities on the Moon. “The technology development programs listed [in this report],” the Bahcall Committee claimed, “will provide part of the factual basis required for decisions about future astronomical missions.”29

2.6 A Road Not Traveled By 1991, as Bahcall’s committee deliberated its recommendations for astronomy’s next decade, the advocates for some sort of Next Generation Space Telescope (what had also been called the Large Space Telescope in the Bahcall Committee deliberations) had made considerable progress in nursing their project along. But they were still a very long way from a go-ahead to start detailed feasibility studies, let alone serious design work or actual construction. The kind of issues that could derail plans for a big space telescope are provided by the story of another of the large space telescopes touched on by the Bahcall Committee, the Large Deployable Reflector (LDR).30 Far from being a side story, the LDR offers an example of NASA sets about developing new missions and its fate was a warning at the time to NGST advocates. In the late 1970s, engineers and scientists from two NASA field centers, the Jet Propulsion Laboratory and Ames Research Center, both in California, had proposed the LDR. This mission sprang from JPL’s studies that began in 1976 of a space observatory to make observations in the submillimeter wavelength range. These studies led to a series of workshops in 1977 involving American and European astronomers. They recommended pursuing a 10 m space telescope to operate in the infrared and sub-millimeter regions of the spectrum. As the possible designs matured, they suggested one path to bigger yet lighter mirrors. Rather than using a single, massive piece of material for the primary mirror, engineers by the early 1980s proposed a Large Deployable Reflector with a 20 m mirror (Fig. 2.2).

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Fig. 2.2 A concept that never was. The planned large deployable reflector with a 20 m primary

Assembled from small lightweight hexagonal segments, this would fit together to make one giant light collector. The mirror could be collapsed to a smaller size to fit inside a rocket or into a spacecraft that would fit into the Space Shuttle’s cargo bay (the first flight of which was in 1981), and then, once in space, open like the petals of a flower. In later studies engineers examined assembling the LDR in space using the space station, NASA’s flagship program, which had been approved for construction by President Reagan in 1983, illustrating the often ad hoc nature of plans for new missions as advocates borrow from existing projects or other planned missions and work within the overall context of NASA’s broader institutional goals. The idea of a segmented primary mirror was attractive to many managers, engineers and scientists, one major reason being the advances astronomers were making in designing ground-based telescopes that exploited segmented mirrors.31

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Designs for segmented primary mirrors for space projects were based, in part, on new schemes astronomers were proposing for ground-based telescopes. During the 1980s, for example, engineers and astronomers at Caltech and University of California developed segmented mirror technologies for the 10 m Keck Telescope project in Hawaii. The first Keck telescope went into operation in 1991 and a second one soon followed. Other telescope projects were also demonstrating how computers linked with mechanical systems could accurately control lightweight mirrors. This knowledge helped boost the confidence of NASA staffers that segmented mirror technologies could be developed for space telescopes.32 The Jet Propulsion Laboratory and Ames sustained the Large Deployable Reflector program with modest funding throughout the 1980s. The two NASA centers held conferences on it every two years which drew dozens of participants. In 1982, for example, a week-long workshop in California, similar in many respects to the Baltimore NGST workshop that was to be held in 1989, attracted around one hundred scientists and engineers to develop the science rationale for the LDR and to formulate what its observational capabilities should be.33 The National Academy of Sciences twice recommended the Large Deployable Reflector as a high priority.34 While it existed only on paper, the Large Deployable Reflector was a serious project whose advocates initially had high hopes of advancing it through the NASA bureaucracy and the White House and Congress to the stage where it would indeed be built. Reflecting their seriousness, NASA supported Lockheed Martin and Kodak to perform studies of the Large Deployable Reflector concept. Although the Bahcall Committee had declared the 1990s as the decade of the infrared, in the end it did not support the LDR. Instead the Committee gave its strong backing to three other infrared projects instead,35 although none were missions considered capable enough to be considered the successor to Hubble. Hence after several years of funding, NASA’s upper management decided not to pursue the Large Deployable Reflector past the initial design stage. Advances in ground-based telescopes, its anticipated very high cost, plus the fact that somewhat similar missions were being pursued by the European Space Agency, kept it limited to the drawing board. One common pattern in NASA’s strategy in the 1980s and 1990s for developing new missions is clear here. The agency sponsored studies of various depths for many possible projects. Relatively few survived to be built, but elements of them lived on in various ways in other programs. In the case of the Large Deployable Reflector, the deployable primary mirror concept became a central feature in NASA’s later plans for Next Generation Space Telescope,36 although the direct influence on the designs of the NGST was probably negligible.

2.7 Spherical Aberration In April 1990, before the results of the deliberations of the Bahcall Committee were complete, the Hubble Space Telescope was launched amid an enormous blaze of publicity. Expectations of the quality of the images it would return were extremely

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high, among astronomers and the general public. When NASA released the first images to the public, however, its spokesmen had the grim job of announcing that the telescope suffered from an optical defect known as spherical aberration. This meant that its images were not nearly as good as expected and that HST’s scientific performance would be crippled, at least until some kind of repair or technological fix could be put into place. Derided by late night comedians and in editorial cartoons, proclaimed to be a “technoturkey” by one U.S. Senator, Hubble swiftly became a national symbol of technological failure.37 The space agency put into place a team to locate and review what had gone wrong. It soon concluded that the primary mirror was the culprit. Due to a mistake that had not been caught in assembling a test device, the primary mirror had been polished too flat at its edges.38 Cast in the now dubious role of successor to the flawed and for a time publicly ridiculed Hubble Space Telescope, the consequences for the Next Generation Space Telescope were severe. Progress on its planning slowed to a crawl. It was not, however, brought to a halt. In early 1991, there was a two-day workshop with 79 participants at the Jet Propulsion Laboratory on “Technologies for Large Filled-Aperture Telescopes in Space,” one result of funding by NASA Headquarters to advance the technologies engineers and scientists reckoned to be needed for new astrophysics missions between 1995 and 2015. A number of the presentations centered on possible successors to the Hubble Space Telescope but, to judge from the conference proceedings, the likely cost of such a Next Big Machine was even more of a concern for the participants than they had they had been two years earlier in Baltimore. In the executive summary of the two-day workshop, Garth Illingworth described the main conclusions drawn by the participants. As he put it, an 8 m class telescope “in high Earth orbit would have unprecedented power for problems as diverse as planet searches around nearby stars to the way in which galaxies formed in the young universe. It will build upon the discoveries and astronomical understanding of many decades of research with astronomical observatories, and is the natural successor to the Hubble Space Telescope (HST) and the new generation of large 10 m class ground-based telescopes.”39 He reckoned that while the gains with the HST are “impressive,” those with the NGST would be “truly astonishing.”40 Illingworth also claimed that operating an NGST in high Earth orbit would lead to savings in weight, size, power, and the complexity of its operations. These savings would in turn mean an 8 m NGST would be comparable in weight to the HST (12 tons), thereby “breaking away” from the HST cost curve; that is, securing a telescope considerably less expensive than might be anticipated just from the HST experience. But there was also now a key development. In the discussion of one of the papers, Rodger Thompson of the University of Arizona and a Principal Investigator for one of the instruments slated to fly aboard the HST later in the 1990s, challenged the assumptions underlying the planning directed towards a single, multipurpose and multiwavelength telescope that would cost a great deal of money, but not be serviced on orbit. He asked if “maybe a series of identical, let’s say 6 m telescopes with individual instrumentation, all sort of lined up to go for specific purposes, might

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be better. One might be cryogenic, one might be just for spectroscopy or something else. This might be a better way to save money. These are production line types of telescopes.”41 Illingworth protested that while there was “a significant level of rationality in that argument,” politically it would be one that “you’d never be able to sell. Having one of something that was closely similar to the rest of them would essentially kill off the rest, given the cost. In the minds of the folks that are funding these things – and I think Congress in a sense – they’re looking at this and saying: Here’s astronomers out there wanting the world. We’ll give them one and that’s it.”42 Thompson replied that HST’s fate had changed attitudes. Other participants also spoke in favor of Thompson’s idea. Although various concerns were expressed, in the panel discussion of the chairs of the various working groups, the debate had now shifted the consensus to planning for two telescopes. But the discussants also accepted that no studies had been done on this concept. Basic information was lacking so serious studies were needed. As Illingworth put it, “If it turns out it’s do-able, the cost to do two of them, then you know you’re not in a position of really selling it. If there are some cost savings to be made, we may be in a much more advantageous position.” One of the possible telescopes now being mentioned was what one speaker referred to as a “super Edison;” that is, a larger and more powerful version of a concept for an ambitious passively cooled “next generation” infrared telescope developed by a team led by University of Wyoming astronomer Harley Thronson in collaboration with a group at the Royal Observatory, Edinburgh.43 Thronson was in attendance at the workshop. But only later, as we shall discuss elsewhere, would schemes for infrared telescopes mesh with the planning for the NGST. Hence in the executive summary of the workshop, the conclusion now was that “While we have discussed NGST as being a single all-purpose UV to mid-IR telescope, it has been suggested that it may well be cheaper to design and configure two spacecraft, one for UV-Visible and the other for the Visible-IR. This is not obviously the case. Technical feasibility studies need to be combined with cost trade-off analyses to establish the most cost-effective and timely route to fruition of the program. The current baseline is to consider NGST as a single [high Earth orbit] telescope”44 operating in the uv-visible and infrared.

2.8 Conclusions Years before the Hubble Space Telescope was launched in 1990 a number of astronomers and engineers in the US and Europe were thinking hard about a possible successor to the HST as well as working to engage a broad community of researchers in the design of such a new observatory. That the launch of any such successor was likely to be many years away was also widely accepted. However, the fiasco of Hubble’s spherical aberration had a serious effect on the pace at which plans were advancing for the Next Generation Space Telescope. Thus crucially for

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the dynamics of building the “Next Big Machine,” the fate of the offspring was intimately tied to that of the parent. In fact, as we will describe in later papers, it was only when in the mid-1990s that the NGST planning was remade by the incorporation of a series of technology developments in infrared astronomy that NASA threw its institutional weight and money behind the development of a Next Generation Space Telescope: until that time, the American space agency had been generally standing on the sidelines as the major astronomical space telescope of the early 21st Century was being debated. The efforts between the mid 1980s and the early 1990s were nevertheless critical to the establishment and success of the later endeavor. Without a set of committed advocates in these years willing to work away often as individuals or small groups to raise the consciousness of their colleagues about the possibilities of a successor to the HST even before its launch and eventual success, those later efforts would surely have been postponed, if pursued at all. Acknowledgments We are grateful to Harley Thronson for comments on an earlier draft of this paper. The research for this paper has in part been made possible by the Canadian Space Agency (9F007-060458/001/ST) and NASA (NAG5-13604) and we are most grateful for this support. Robert W. Smith also completed part of his research during his tenure as the Lindbergh Professor of Aerospace History at the Smithsonian Institution in 2007 and he greatly appreciates the Institution’s support and the collegial atmosphere provided by the Division of Space History at the National Air and Space Museum.

Notes 1. Smith, R.: Engines of Discovery: Scientific Instruments and the History of Astronomy and Planetary Science in the United States in the Twentieth Century. Journal for the History of Astronomy, 28, 49–77 (1997). 2. See, for example, Riordan, M.: A Tale of Two Cultures: Building the Superconducting Super Collider, 1988–1993. Historical Studies in the Physical and Biological Sciences, 32, 125–144 (2001). 3. What eventually became the Hubble Space Telescope in 1983, was also known for lengthy periods as the Large Space Telescope and the Space Telescope. 4. This is an implicit theme, for instance, in Dunar, A.J., and Waring, S.J., Power to Explore. A History of Marshall Space Flight Center 1960–1990. NASA, Washington DC (1999). 5. Brown, R.J.: The Size, Shape and Temperature of Stars. In West, R.M. (ed.) Understanding the Universe. The Impact of Space Astronomy, pp. 73–92. D. Reidel, Dordrecht (1983). On this point, see also Smith, R.W.: Engines of Discovery: Scientific Instruments and the History of Astronomy and Planetary Science in the United States in the Twentieth Century. Journal for the History of Astronomy, 28, 49–77 (1997). 6. On the development of satellites for national security purposes, see, among others, Day, D.A., Logsdon, J.M., Latell, B. (eds.): Eye in the Sky. The Story of the Corona Spy Satellites. Smithsonian Institution Press, Washington DC and London (1998). Burrows, W.E.: Deep Black. Space Espionage and National Security. Random House, New York (1986). 7. Longair, M.: Space Science and Cosmology. In: West. R.M. (ed.): .The Impact of Space Astronomy, pp. 129–226. D. Reidel, Dordrecht (1983). 8. Bely, P.Y.: A Ten-Meter Optical Telescope in Space. Advanced Technology Optical Telescopes III, in SPIE, 628, 188–195, (1986). 9. Ibid.

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10. Space Science in the Twenty-First Century: Imperatives for the Decade 1995–2015. Washington DC, National Academy Press (1988), in seven volumes, volume 1, Overview, 46. Quotations are from Field, G: Summary of Space Science Board 1995-2015 Study, pp. 11–15. In: Bely, P.Y., Burrows, C.J., Illingworth, G.J. (eds.). The Next Generation Space Telescope. Proceedings of a Workshop Jointly Sponsored by the National Aeronautics and Space Administration and the Space Telescope Science Institute and held at the Space Telescope Science Institute, Baltimore, Maryland, 13–15 September 1989. Space Telescope Science Institute, Baltimore (1990). 11. Ibid. 12. This widely quoted apparent limit to a low temperature achievable via passive (aka, radiative) cooling alone became for some years a major hindrance to even more aggressive use of the natural cold of space in achieving sensitive observations at long infrared wavelengths. 13. Space Science in the Twenty-First Century: Imperatives for the Decade 1995–2015. Washington DC, National Academy Press (1988), in seven volumes, volume 1, Overview, 46. Quotations are from Field, G: Summary of Space Science Board 1995–2015 Study, pp. 11–15. In: Bely, P.Y., Burrows, C.J., Illingworth, G.J. (eds.). The Next Generation Space Telescope. Proceedings of a Workshop Jointly Sponsored by the National Aeronautics and Space Administration and the Space Telescope Science Institute and held at the Space Telescope Science Institute, Baltimore, Maryland, 13–15 September 1989. Space Telescope Science Institute, Baltimore (1990). 14. Illingworth, G.J.: The Next Generation: An 8–16 m Space Telescope. Highlights of Astronomy, 8, 449–453 (1989). 15. James Breckinridge; July 17, 2001 interview. 16. After HST, What Next?: Sky & Telescope. February 1990, p. 129. 17. On the fate of the SEI, see Hogan, T.: Mars Wars. The Rise and Fall of the Space Exploration Initiative. NASA, Washington DC (2007). For a discussion of some of the policy issues involved, see also Roy, S.A.: The Origin of the Smaller, Faster, Cheaper Approach in NASA’s Solar System Exploration Program. Space Policy, 14, 153–171 (1998). 18. Angel, R.: Future Optical and Infrared Telescopes. Nature, 409, 427–431 (2001). 19. This is discussed in Smith, R.W. (with contributions by Hanle, P., Kargon, R., Tatarewicz, J.): The Space Telescope: A Study of NASA, Science, Technology and Politics. Expanded paperback edition, Cambridge, University Press, New York (1993), p. 69. 20. Field, G.: Summary of Space Science Board 1995–2015 Study, pp. 11–15. In: Bely, P.Y., Burrows, C.J., Illingworth, G.J. (eds.). The Next Generation Space Telescope. Proceedings of a Workshop Jointly Sponsored by the National Aeronautics and Space Administration and the Space Telescope Science Institute and held at the Space Telescope Science Institute, Baltimore, Maryland, 13–15 September 1989. Space Telescope Science Institute, Baltimore (1990). 21. Illingworth, G.J.: The Next Generation UV-Visible-IR Space Telescope. In: Bely, P.Y., Burrows, C.J., Illingworth, G.J. (eds.). The Next Generation Space Telescope. Proceedings of a Workshop Jointly Sponsored by the National Aeronautics and Space Administration and the Space Telescope Science Institute and held at the Space Telescope Science Institute, Baltimore, Maryland, 13–15 September 1989, pp. 31–36. Space Telescope Science Institute, Baltimore (1990). 22. Conclusions of the Workshop: In: Bely, P.Y., Burrows, C.J., Illingworth, G.J. (eds.). The Next Generation Space Telescope. Proceedings of a Workshop Jointly Sponsored by the National Aeronautics and Space Administration and the Space Telescope Science Institute and held at the Space Telescope Science Institute, Baltimore, Maryland, 13–15 September 1989, pp. 5–6. Space Telescope Science Institute, Baltimore (1990). 23. Macchetto, D.: ESA Long Term Plans and Status. In: Bely, P.Y., Burrows, C.J., Illingworth, G.J. (eds.). The Next Generation Space Telescope. Proceedings of a Workshop Jointly Sponsored by the National Aeronautics and Space Administration and the Space Telescope Science Institute and held at the Space Telescope Science Institute, Baltimore, Maryland, 13–15 September 1989, pp. 27–30. Space Telescope Science Institute, Baltimore. 24. Bahcall, J.: In: Bely, P.Y., Burrows, C.J., Illingworth, G.J. (eds.). The Next Generation Space Telescope. Proceedings of a Workshop Jointly Sponsored by the National Aeronautics and Space Administration and the Space Telescope Science Institute and held at the Space Telescope Science

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25. 26. 27.

28. 29. 30.

31. 32. 33.

34. 35. 36. 37.

38. 39. 40. 41. 42. 43.

44.

R.W. Smith and W.P. McCray Institute, Baltimore, Maryland, 13–15 September 1989, p. 7. Space Telescope Science Institute, Baltimore (1990). National Research Council Astronomy and Astrophysics Survey Committee: The Decade of Discovery in Astronomy and Astrophysics. National Academy Press, Washington DC (1991), p. 3. Ibid., p. 10. National Research Council Astronomy and Astrophysics Survey Committee: The Decade of Discovery in Astronomy and Astrophysics. Working Papers. . .: National Academy Press, Washington DC (1991), pp. IV–1. National Research Council Astronomy and Astrophysics Survey Committee: The Decade of Discovery in Astronomy and Astrophysics. National Academy Press, Washington DC (1991), p. 20. Ibid., p. 27. See, for example, Swanson, P.N., Gulkis, S., Kuiper, T.B.H., Kiya, M.: Large Deployable Reflector (LDR): A Concept for an Orbiting Submillimeter-Infrared Telescope for the 1990s. Optical Engineering, 22, 725–731 (1983). Pierre Y. Bely; July 18, 2001 interview. Pierre Y. Bely; July 18, 2001 interview. According to a number of sources, in an ironic Spoonerism, long-remembered by scientists who originally had high hopes for LDR, at a presentation on the mission at a workshop at the CalTech at this time, one speaker inadvertently referred to the mission as “the Large Deplorable Reflector.” The audience winced. Astronomy Survey Committee: Astronomy and Astrophysics for the 1980’s. Report of the Astronomy Survey Committee. National Research Council, Washington DC (1982). National Research Council Astronomy and Astrophysics Survey Committee: The Decade of Discovery in Astronomy and Astrophysics. National Academy Press, Washington DC (1991), 75–80. Pierre Y. Bely; July 18, 2001, interview. The finding of spherical aberration and its aftermath is discussed in Smith, R.W. (with contributions by Hanle, P., Kargon, R., Tatarewicz, J.N.): Cambridge University Press, New York (1993), expanded paperback edition, pp. 399–425. The Hubble Space Telescope Optical Systems Failure Report. NASA, Washington DC (1990). Illingworth, G.J.: Executive Summary, in Workshop Proceedings: Technologies for Large FilledAperture Telescopes in Space (JPL D-8541, vol. 4, 1991), pp. 3–16. Ibid., p. 5. Ibid., comments in question session, p. 161. Ibid., p. 161. For a brief account of Edison, see Davies, J.K.: It will never work! An idea that changed infrared astronomy from space. The Space Review, 21 August 2006. www.thespacereview.com/article/688/1. Accessed 21 March 2008. Ibid., p. 12.

Michael Shull, Matthew Greenhouse, and Chick Woodward embarrass Harley Thronson, who obviously dressed inappropriately for a meeting in Arizona

Chapter 3

The Kuiper Belt and Other Debris Disks David Jewitt, Amaya Moro-Mart´ın and Pedro Lacerda

Abstract We discuss the current knowledge of the solar system, focusing on bodies in the outer regions, on the information they provide concerning solar system formation, and on the possible relationships that may exist between our system and the debris disks of other stars. Beyond the domains of the terrestrial and giant planets, the comets in the Kuiper belt and the Oort cloud preserve some of our most pristine materials. The Kuiper belt, in particular, is a collisional dust source and a scientific bridge to the dusty “debris disks” observed around many nearby main-sequence stars. Study of the solar system provides a level of detail that we cannot discern in the distant disks while observations of the disks may help to set the solar system in proper context.

3.1 Introduction Planetary astronomy is unusual among the astronomical sciences in that the objects of its attention are inexorably transformed by intensive study into the targets of other sciences. For example, the Moon and Mars were studied telescopically by astronomers for nearly four centuries but, in the last few decades, these worlds have been transformed into the playgrounds of geologists, geophysicists, meteorologists, biologists and others. Telescopic studies continue to be of value, but we now learn most about the Moon and Mars from in-situ investigations. This transformation from science at-a-distance to science up-close is a forward step and a tremendous luxury not afforded to the rest of astronomy. Those who study other stars or the galaxies beyond our own will always be forced by distance to do so telescopically. However, the impression that the solar system is now only geology or meteorology or some other science beyond the realm of astronomy is completely incorrect when applied to the outer regions. The outer solar system (OSS) remains firmly entrenched within the domain of astronomy, its contents accessible only to telescopes. Indeed, major components of the OSS, notably the Kuiper belt, were discovered D. Jewitt (B) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, USA e-mail: [email protected] H.A. Thronson et al. (eds.), Astrophysics in the Next Decade, Astrophysics and Space Science Proceedings, DOI 10.1007/978-1-4020-9457-6 3,  C Springer Science+Business Media B.V. 2009

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(telescopically) less than two decades ago and will continue to be best studied via astronomical techniques for the foreseeable future. It is reasonable to expect that the next generation of telescopes in space and on the ground will play a major role in improving our understanding of the solar system, its origin and its similarity to related systems around other stars. In this chapter, we present an up-to-date overview of the layout of the solar system and direct attention to the outer regions where our understanding is the least secure but the potential for scientific advance is the greatest. The architecture of the Kuiper belt is discussed as an example of a source-body system that probably lies behind many of the debris disks of other stars. Next, the debris disks are discussed based on the latest observations from the ground and from Spitzer, and on new models of dust transport. Throughout, we use text within grey boxes to highlight areas where the next generation telescopes are expected to have major impact.

3.2 The Architecture of the Solar System It is useful to divide the solar system into three distinct domains, those of the terrestrial planets, the giant planets, and the comets. Objects within these domains are distinguished by their compositions, by their modes of formation and by the depth and quality of knowledge we possess on each.

3.2.1 Terrestrial Planets The terrestrial planets (Mercury, Venus, Earth and Moon, Mars and most main-belt asteroids) are found inside 3 AU. They have refractory compositions dominated by iron (∼35% by mass), oxygen (∼30%), silicon and magnesium (∼15% each) and were formed by binary accretion in the protoplanetary disk. About 95% of the terrestrial planet mass is contained within Venus and Earth (∼1 M⊕ = 6 × 1024 kg, each). The rest is found in the small planets Mercury and Mars (0.1 M⊕ combined) with trace amounts (∼3 × 10−4 M⊕ ) in the main-belt asteroids located between Mars and Jupiter. The largest asteroid is Ceres (∼900 km diameter). The terrestrial planets grew by binary accretion between solid bodies in the protoplanetary disk of the Sun. While not all details of this process are understood, it is clear that sticking and coagulation of dust grains, perhaps aided at first by hydrodynamic forces exerted from the gaseous component of the disk, produced larger and larger bodies up to the ones we see now in the solar system. The gaseous component, judged mainly by observations of other stars, dissipated on timescales from a few to ∼10 Myr. Measurements of inclusions within primitive meteorites show that macroscopic bodies existed within a few Myr of the origin. The overall timescale for growth was determined, ultimately, by the sweeping up of residual mass from the disk, a process thought to have taken perhaps 40 Myr in the case of the Earth. No substantial body is found in the asteroid belt although it is likely that sufficient mass existed there

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to form an object of planetary class. This is thought to be because growth in this region was interrupted by strong perturbations, caused by the emergence of nearby, massive Jupiter (at ∼5 AU).

3.2.2 Giant Planets Orbits of the giant planets (Jupiter, Saturn, Uranus and Neptune) span the range 5–30 AU. The giants are in fact of two compositionally distinct kinds.

3.2.2.1 Gas Giants Jupiter (310 M⊕ ) and Saturn (95 M⊕ ) are so-called gas giants because, mass-wise, they are dominated by hydrogen and helium. Throughout the bulk of each planet these gases are compressed, however, into a degenerate (metallic) liquid that supports convection and sustains a magnetic field through dynamo action. The compositional similarity to the Sun suggests to some investigators that the gas giants might form by simple hydrodynamic collapse of the protoplanetary gas nebula (Boss 2001). In hydrodynamic collapse the essential timescale is given by the freefall time, and this could be astonishingy short (e.g. 1000 yrs). Details of this instability, especially related to the necessarily rapid cooling of the collapsing planet, remain under discussion (Boley et al. 2007, Boss 2007). In fact, measurements of the moment of inertia, coupled with determinations of the equation of state of hydrogen-helium mixtures at relevant pressures and temperatures, show that Saturn (certainly) and Jupiter (probably) have distinct cores containing 5 M⊕ to 15 M⊕ of heavy elements (the case of Jupiter is less compelling than Saturn because of its greater mass and central hydrostatic pressure, leading to larger uncertainties concerning the self-compressibility of the gas). The presence of a dense core is the basis for the model of formation through “ nucleated instability”, in which the core grows by binary accretion in the manner of the terrestrial planets, until the gravitational escape speed from the core becomes comparable to the thermal speed of molecules in the gas nebula. Then, the core traps gas directly from the nebula, leading to the large masses and gas-rich compositions observed in Jupiter and Saturn. Historically, the main sticking point for nucleated instability models has been that the cores must grow to critical size before the surrounding gas nebula dissipates (i.e. 5 M⊕ to 15 M⊕ cores must grow in much less than 10 Myr). The increase in the disk surface density due to the freezing of water as ice outside the snowline is one factor helping to decrease core growth times. Another may be the radial jumping motion of the growing cores, driven by angular momentum and energy exchange with planetesimals (Rice and Armitage 2003). In recent times, a consensus appears to have emerged that Jupiter’s core, at least, could have grown by binary accretion from a disk with Σ ∼ 50 to 100 kg m−2 on timescales ∼1 Myr (Rice and Armitage 2003). The collapse of nebular gas onto the core after this would have been nearly instantaneous.

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Recent data show that the heavier elements (at least in Jupiter, the better studied of the gas giants) are enriched relative to hydrogen in the Sun by factors of ∼2 to 4 (Owen et al. 1999), so that wholesale nebular collapse cannot be the whole story (and may not even be part of it). The enrichment applies not only to species that are condensible at the ∼100 K temperatures appropriate to Jupiter’s orbit, but to the noble gases Ar, Kr and Xe, which can only be trapped in ice at much lower temperatures, <30 K (e.g. Bar-Nun et al. 1988). Therefore, Owen et al. suggest that the gas giants incorporate substantial mass from a hitherto unsuspected population of ultracold bodies, presumably originating in the outer solar system. In a variant of this model, cold grains from the outer solar system trap volatile gases but drift inwards under the action of gas drag, eventually reaching the inner nebula when they evaporate in the heat of the Sun and enrich the gas (Guillot and Hueso 2006). 3.2.2.2 Ice Giants Uranus (15 M⊕ ) and Neptune (17 M⊕ ), in addition to being an order of magnitude less massive than the gas giants are compositionally distinct. These planets contain a few M⊕ of H and He, and a much larger fraction of the “ices” H2 O, CH4 and NH3 . For this reason they are known as “ice giants”, but the name is misleading because they are certainly not solid bodies but are merely composed of molecules which, if they were much colder, would be simple ices. In terms of their mode of formation, the difference between the ice giants and the gas giants may be largely one of timescale. It is widely thought that the ice giants correspond to the heavy cores of Jupiter and Saturn, but with only vestigial hydrogen/helium envelopes accreted from the rapidly dissipating gaseous component of the protoplanetary disk. While qualitatively appealing, forming Uranus and (especially) Neptune on the 10 Myr timescale associated with the loss of the gas disk has been a major challenge to those who model planetary growth. The problem is evident from a simple consideration of the collision rate between particles in a disk or surface density Σ(R) kg m−2 , where R is the heliocentric distance. The probability of a collision in each orbit varies in proportion to Σ(R), while the orbital period varies as R 3/2 . Together, this gives a collision timescale varying as R 3/2 /Σ(R) which, with Σ(R) ∝ R −3/2 gives tc ∝ R 3 . A giant planet core that takes 1 Myr to form at 5 AU would take 63 ∼ 200 Myr to form at 30 AU, and this considerably exceeds the gas disk lifetime. Suggested solutions to the long growth times for the outer planets include augmentation of the disk density, Σ(R), perhaps through the action of aerodynamic drag, and formation of the ice giants at smaller distances (and therefore higher Σ) than those at which they reside. The latter possibility ties into the general notion that the orbits of the outermost three planets have expanded in response to the action or torques between the planets and the disk. Still another idea is that Uranus and Neptune are gas giants whose hydrogen and helium envelopes were ablated by ionizing radiation from the Sun or a nearby, hot star (Boss et al. 2002).

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3.2.3 Comets Comets are icy bodies which sublimate in the heat of the Sun, producing observationally diagnostic unbound atmospheres or “comae”. For most known comets, the sublimation is sufficiently strong that mass loss cannot be sustained for much longer than ∼104 yr, a tiny fraction of the age of the solar system. For this reason, the comets must be continually resupplied to the planetary region from one or more low temperature reservoirs, if their numbers are to remain in steady state. In the last half century, at least three distinct source regions have been identified.

3.2.3.1 Oort Cloud Comets The orbits of long period comets are highly elliptical, isotropically distributed and typically large (Fig. 3.1), suggesting a gravitationally bound, spheroidal source region of order 100,000 AU in extent (Oort 1950). Comets in the cloud are scattered randomly into the planetary region by the action of passing stars and also perturbed by the asymmetric gravitational potential of the galactic disk (e.g. Higuchi et al. 2007). Unfortunately, the cloud is so large that its residents cannot be directly counted. The population must instead be inferred from the rate of arrival of comets from the cloud and estimates of the external torques. A recent work gives the number larger than ∼1 km in radius as 5 × 1011 , with a combined mass in the range 2M⊕ –40M⊕ (Francis 2005). Oort cloud comets are thought to have originated in the Sun’s protoplanetary disk in the vicinity of the giant planets and were scattered out by interactions with the growing, migrating planetary embryos. Although most were lost, a fraction of the ejected comets, perhaps from 1 to 10%, was subsequently deflected by external perturbations that lifted the perihelia out of the planetary region, effectively decoupling these comets from the rest of the system (Hahn and Malhotra 1999). Over ∼1 Gyr, the orbits of trapped comets were randomized, converting their distribution from a flattened one reflecting their disk source to a spherical one, compatible with the random directions of arrival of long period comets. In this model, which is qualitatively unchanged from that proposed by Oort (1950), the scale and population of the Oort cloud are set by the external perturbations from nearby stars and the galactic disk. If the Sun formed in a cluster, then the average perturbations from cluster members would have been bigger than now and a substantial population of more tightly bound, so-called “inner Oort cloud” comets could have been trapped. It is possible that the Halley family comets, whose orbits are predominantly but not exclusively prograde (Fig. 3.2) are delivered from the inner Oort cloud (Levison et al. 2001). Some 90 to 99% of the comets formed in our system eluded capture and now roam the interstellar medium. If this fraction applies to all stars, then ∼1023 to ∼1024 “interstellar comets” exist in our galaxy, containing several ×105 M of metals. Interstellar comets ejected from other stars might be detected with all-sky surveys in the Pan STARRS or Large Synoptic Survey Telescope (LSST) class (Jewitt 2003).

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Fig. 3.1 Orbits of the nearly 200 long period comets (orbital period, P > 200 yrs) listed in the JPL small-body database. The highly eccentric orbits of many LPCs appear as nearly radial lines on the top (wide angle) view, showing a cube 20,000 AU on a side. The bottom panel shows a narrow angle view of a 100 AU cube. Prograde orbits are shown in cyan while retrograde orbits are in magenta. Numbers along the axes are distances in AU from the Sun at (0,0,0). The orbits of the giant planets are shown in white

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Fig. 3.2 Halley family comets. Their non-isotropic inclination distribution, with more prograde (cyan) orbits than retrograde (magenta), points to an origin in the inner Oort cloud, where stellar and galactic perturbations have been too small to randomize the orbits. Following the classical definition, we plot the 44 comets in the JPL small-body database having Tisserand parameters with respect to Jupiter ≤2 and orbital periods 20 < P(yrs) < 200. The orbits of the giant planets are shown in white

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0 50 20 0 20 40 50 0 50

Such objects would be recognized by their strongly hyperbolic orbits relative to the Sun, quite different from any object yet observed. 3.2.3.2 Kuiper Belt Comets The orbits of the Jupiter family comets (JFCs) have modest inclinations, with no retrograde examples, and most have eccentricities much less than unity (Fig. 3.3). They are dynamically distinct from the long period Oort comets, and they interact strongly with Jupiter. For many years it was thought that the JFCs were captured from the long period population by Jupiter but increasingly numerical detailed work in the 1980’s showed that this was not possible (Fern´andez 1980, Duncan et al. 1988). The source appears to be the Kuiper belt, although this is not an ironclad conclusion and the particular region or regions in the Kuiper belt from which the JFCs originate has yet to be identified. About 200 JFCs are numbered (meaning that their orbits are very well determined) and a further 200 are known. Their survival is limited by a combination of volatile depletion, ejection from the solar system, or impact into a planet or the Sun. Dynamical interactions alone give a median lifetime near 0.5 Myr (Levison and Duncan 1994), which matches the volatile depletion lifetime for bodies smaller than r ∼ 40 km. The implication is that JFCs smaller than this size will become dormant before they are dynamically removed, ending up as bodies that are asteroidal in appearance but cometary in orbit. Some of these dead comets are suspected to exist among the near-Earth “asteroid” population; indirect evidence for a fraction ∼10% comes from albedo measurements (Fern´andez et al. 2001) and dynamical models (Bottke et al. 2002).

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Fig. 3.3 Perspective view of the Jupiter family comets (salmon) together with the orbits of the planets out to Saturn. The Sun is at (x, y, z) = (0, 0, 0) and the distances are in AU. Plotted are 166 JFCs, selected from the JPL small-body database based on their Tisserand parameter with respect to Jupiter (2 < TJ < 3; see, e.g., Levison and Duncan 1994) and with orbital periods P < 20 yr

3.2.3.3 Main Belt Comets At the time of writing (February 2008) three objects are known to have the dynamical characteristics of asteroids but the physical appearances of comets. They show comae and particle tails indicative of on-going mass loss (Hsieh and Jewitt, 2006, Figs. 3.4 and 3.5). These are the Main-Belt comets (MBCs), most directly interpreted as ice-rich asteroids. In the modern solar system, the MBCs are dynamically isolated from the Oort cloud and Kuiper belt reservoirs (i.e. they cannot be captured from these other regions given the present-day layout of the solar system, cf. Levison et al. 2006). The MBCs should thus be regarded as a third and independent comet reservoir. Two possibilities for their origin seem plausible. The MBCs could have accreted ice if they grew in place but outside the snow-line. Alternatively, the MBCs might have been captured from elsewhere if the layout of the solar system were very different in the past, providing a dynamical paths from the Kuiper belt that do not now exist. Insufficient evidence exists at present to decide between these possibilities. These bodies escaped detection until now because their mass loss rates ( 1 kg s−1 ) are two to three orders of magnitude smaller than from typical comets.

Fig. 3.4 Perspective view of the main-belt comets (orange) together with the orbits of 100 asteroids (thin, yellow lines) and planets out to Jupiter

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Fig. 3.5 Orbital semimajor axis vs. eccentricity for objects classified as asteroids (black) and comets (blue), together with the three known main-belt comets 133P/Elst-Pizarro, P/2005 U1 (Read), and 118401 (1999 RE70 ) (plotted in red). Vertical dashed lines mark the semimajor axes of Mars and Jupiter and the 2:1 mean-motion resonance with Jupiter (commonly considered the outer boundary of the classical main belt), as labeled. Curved dashed lines show the loci of orbits with perihelia equal to Mars’s aphelion (q = Q Mar s ) and orbits with aphelia equal to Jupiter’s perihelion (Q = q J up ). Objects plotted above the q > Q Mar s line are Mars-crossers. Objects plotted to the right of the Q < q J up line are Jupiter-crossers. From Hsieh and Jewitt (2006)

The mass loss is believed to be driven by the sublimation of water ice exposed in small surface regions, with effective sublimating areas of ∼1000 m2 and less. Although sublimation at 3 AU is weak, the MBCs are small and mass loss at the observed rates cannot be sustained for the age of the solar system. Instead, a trigger for the activity (perhaps impact excavation of otherwise buried ices) is needed. This raises the possibility that the detected MBCs are a tiny fraction of the total number that will be found by dedicated surveys like PanSTARRS and LSST. Even more interesting is the possibility that many or even most outer belt asteroids are in fact ice-rich bodies which display only transient activity. Evidence from the petrology and mineralogy of meteorites (e.g. the CI and CM chondrites that are thought to originate in the outer belt) shows the past presence of liquid water in the meteorite parent bodies. The MBCs show that some of this water survived to the present day.

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Next generation facilities will offer opportunities to study the physical properties of the satellite systems of the giant planets. The regular satellites occupy prograde orbits of small inclination and eccentricity, resulting from their formation in the accretion disks through which the planets grew. As such, the individual regular satellite systems offer information about the subnebulae that must have existed around the growing planets. These sub-nebulae had their own density and temperature structures very different from the local disk of the Sun. Studies of reflected and thermal radiation with JWST and ALMA will be able to determine compositional differences in even the fainter regular satellites of the ice giants, and may provide constraints on Io-like and Enceladus-like endogenous activity. The study of comets will be greatly advanced by JWST spectroscopy of the nuclei of comets (in order to minimize the effects of scattering from the dust coma, these small objects must be observed when far from the Sun and therefore faint). Simultaneous measurements of reflected and thermal radiation will provide albedo measurements which, with high signal-to-noise ratio spectra, will help determine the nature and evolution of the refractory surface mantles of these bodies.

3.3 Kuiper Belt More than 1200 Kuiper belt objects (KBOs) have been discovered since the first example, 1992 QB1, was identified in 1992 (Jewitt and Luu 1993). The known objects have typical diameters of 100 km and larger. The total number of such objects, scaled from published surveys, is of order 70,000, showing that there is considerable remaining discovery space to be filled by future surveys. The number of objects larger than 1 km may exceed 108 . The KBO orbits occupy a thick disk or sheet outside Neptune’s orbit (Fig. 3.6). A major observational result is the finding that the KBOs can be divided on the basis of their orbital elements into several, distinct groups (Jewitt 1998). This is best seen in a (semimajor axis) vs. e (orbital eccentricity) space, shown here in Fig. 3.7. The major Kuiper belt dynamical groups in the figure are:

r

Classical Kuiper belt objects (red circles). These orbit between about a = 42 AU and the 2:1 mean-motion resonance with Neptune at a = 47 AU. Typical orbital eccentricities of the Classicals are e ∼ 0 to 0.2 while the inclination distribution appears to be bimodal with components near i = 2◦ (so-called “ColdClassicals”) and i = 20◦ (the “Hot Classicals”, Brown and Trujillo 2001, Elliot et al. 2005). Numerical integrations show that the orbits of the Classical objects are stable on timescales comparable to the age of the solar system largely because their perihelia are always so far from Neptune’s orbit that no strong scattering occurs Holman and Wisdom 1993.

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Fig. 3.6 Perspective view of the Kuiper belt. The Sun is at (x, y, z) = (0, 0, 0) and the axes are marked in AU. White ellipses show the orbits of the four giant planets. Red orbits mark the classical KBOs. Their ring-like distribution is obvious. Blue orbits show resonant KBOs, while green shows the Scattered objects. For clarity, only one fifth of the total number of objects in each dynamical type is plotted

Fig. 3.7 Orbital semimajor axis vs. eccentricity for KBOs known as of 2008 January 30. The objects are color-coded according to their dynamical type, as labeled in the Figure and discussed in the text. A vertical dashed line at a = 30 AU marks the orbit of Neptune, the nominal innerboundary of the Kuiper belt. Upper and lower solid arcs show the loci of orbits having perihelion distances q = a(1 − e) equal to 30 AU and 40 AU, respectively

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Resonant Kuiper belt objects (blue circles). A number of mean-motion resonances (MMRs) with Neptune are populated by KBOs, especially the 3:2 MMR at 39.3 AU and the 2:1 MMR at 47.6 AU (see Fig. 3.7). The 3:2 MMR objects are known as Plutinos, to recognize 134340 Pluto as the first known member of this population. The 2:1 MMR objects are sometimes called “twotinos” while those in 1:1 MMR are Neptune’s Trojans (Sheppard and Trujillo 2006). The resonant objects are dynamically stable by virtue of phase protections conferred by the resonances. For example, KBOs in 3:2 MMR can, like Pluto itself, have perihelia inside Neptune’s orbit, but their orbits librate under perturbations from that planet in such a way that the distance of closest approach to Neptune is always large. In fact, all objects above the upper solid arc in Fig. 3.7 have perihelia interior to Neptune’s orbit. The process by which KBOs became trapped in MMRs is thought to be planetary migration (Fern´andez and Ip 1984). Migration occurs as a result of angular momentum transfer during gravitational interactions between the planets and material in the disk. At late stages in the evolution of the disk (i.e. later than ∼10 Myr, after the gaseous component of the disk has dissipated) the interactions are between the planets and individual KBOs or other planetesimals. In a one-planet system, the sling-shot ejection of KBOs to the interstellar medium would result in net shrinkage of the planetary orbit. In the real solar system, however, KBOs can be scattered inwards from planet to planet, carrying energy and angular momentum with them as they go. Massive Jupiter then acts as the source of angular momentum and energy, ejecting KBOs from the system. In the process, its orbit shrinks, while those of the other giant planets expand. The timescale is the same as the timescale for planetary growth, and the distance through which a planet migrates depends on the mass ejected from the system. Outward migration of Neptune carries that planet’s MMRs outwards, leading to the sweep-up of KBOs (Malhotra 1995). Scattered Kuiper belt objects (green circles), also known as Scattered disk objects. These objects have typically eccentric and inclined orbits with perihelia in the 30 ≤ q ≤ 40 AU range (Figs. 3.7 and 3.8). The prototype is 1996 TL66, whose orbit stretches from ∼35 AU to ∼130 AU (Luu et al. 1997). The brightness of these objects varies strongly around the orbit, such that a majority are visible only when near perihelion. For example, the survey in which 1996 TL66 was discovered lacked the sensitivity to detect the object over 88% of the orbital period. Accordingly, the estimated population is large, probably rivaling the rest of the Kuiper belt (Trujillo et al. 2001). Scattered KBOs owe their extreme orbital properties to continued, weak perihelic interactions with Neptune which excite the eccentricity to larger and larger values (Duncan and Levison 1997). The current aphelion record-holder is 87269 (2000 OO67) with Q = 1123 AU. As the aphelion grows, so does the dynamical influence of external perturbations from passing stars and the asymmetric gravitational potential of the galactic disk. Detached Kuiper belt objects (black circles). The orbits of these bodies resemble those of the Scattered KBOs except that the perihelia are too far from Neptune, q > 40 AU, for planetary perturbations to have excited the eccentricities. The

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Fig. 3.8 Same as Fig. 3.7 but with a logarithmic x-axis extended to a = 1000 AU to better show the extent of the scattered KBOs

prototype is 2000 CR105, with q = 44 AU (Gladman et al. 2002) but a more extreme example is the famous Sedna with q = 74 AU (Brown et al. 2004). The mechanism by which the perihelia of the detached objects were lifted away from the influence of Neptune is unknown. The most interesting conjectures include the tidal action of external perturbers, whether they be unseen planets in our own system or unbound passing stars (Morbidelli and Levison 2004).

The Kuiper belt is a thick disk (Fig. 3.9), with an apparent full width at half maximum, F W H M ∼ 10◦ (Jewitt et al. 1996). The apparent width is an underestimate of the true width, however, because most KBOs have been discovered in surveys aimed near the ecliptic, and the sensitivity of such surveys varies inversely with the KBO inclination. Estimates of the unbiased (i.e. true) inclination distribution give F W H M ∼ 25 to 30◦ (Jewitt et al. 1996, Brown 2001, Elliot et al. 2005 – see especially Fig. 3.20). Moreover, all four components of the Kuiper belt possess broad inclination distributions (Fig. 3.9). The inclination distribution of the Classical KBOs appears to be bimodal, with a narrow core superimposed on a broad halo (Brown and Trujillo 2001, Elliot et al. 2005).

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Fig. 3.9 Orbital semimajor axis vs. inclination for KBOs known as of 2008 January 30. The plot shows that the KBOs occupy orbits having a wide range of inclinations. Most of the plotted objects were discovered in surveys directed towards the ecliptic, meaning that an observation bias against finding high inclination bodies is imprinted on the sample

The Kuiper belt is not thin like the Sun’s original accretion disk and it is clear the the inclinations of the orbits of its members have been excited. The velocity dispersion amongst KBOs is ΔV = 1.7 km s−1 (Jewitt et al. 1996, Trujillo et al. 2001). At these velocities, impacts between all but the largest KBOs lead to shattering and the production of dust, rather than to accretion and growth. Next generation facilities should provide unprecedented survey capabilities (through LSST) that will provide a deep, all-sky survey of the solar system to 24th visual magnitude, or deeper. The number of objects for which reliable orbits exist will increase from ∼103 , at present, by one to two orders of magnitude, depending on the detailed strategies employed by the surveys. Large samples are needed to assess the relative populations of the resonances and other dynamical niches that may place limits on the formation and evolution of the OSS. Objects much larger than Pluto, perhaps in the Mars or Earth class, may also be revealed by careful work.

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3.4 Interrelation of the Populations Evidence that the small-body populations are interrelated is provided by dynamical simulations. The interrelations are of two basic types: (a) those that occur through dynamical processes operating in the current solar system and (b) those that might have operated at an earlier epoch when the architecture of the solar system may have been different from now. As examples of the first kind, it is clear that objects in the Oort cloud and Kuiper belt reservoirs can be perturbed into planet crossing orbits, and that these perturbations drive a cascade from the outer solar system through the Centaurs (bodies, asteroidal or cometary in nature) that are strongly interacting with the giant planets to the Jupiter family comets (orbits small enough for the Sun to initiate sublimation of near-surface ice and strongly interacting with Jupiter) to dead and dormant comets in the near-Earth “asteroid” population. In the current system, simulations show that it is not possible to capture the Trojans or the irregular satellites (Fig. 3.11) of the giant planets from the passing armada of small bodies, and there is no known dynamical path linking comets from the Kuiper belt, for example, to comets in the main-belt (the MBCs). The second kind of interrelation is possible because of planetary migration. The best evidence for the latter is deduced from the resonant Kuiper belt populations which, in one model, require that Neptune’s orbit expanded by roughly 10 AU, so pushing its mean-motion resonances outwards through the undisturbed Kuiper belt (Malhotra 1995). The torques driving the migration moved Jupiter inwards (by a few ×0.1 AU, since it is so massive) as the other giants migrated outwards. In one exciting model, the “Nice model”, this migration pushed Jupiter and Saturn across the 2:1 mean motion resonance (Gomes et al. 2005). The dynamical consequences of the periodic perturbations induced between the solar system’s two most massive planets would have been severe (Fig. 3.10). In published models, these perturbations excite a 30 M⊕ Kuiper belt, placing large numbers of objects into planet crossing orbits and clearing the Kuiper belt down to its current, puny mass of ∼0.1 M⊕ . During this clearing phase, numerous opportunities exist for trapping scattered KBOs in dynamically surprising locations. For example, the Trojans of the planets could have been trapped during this phase (Morbidelli et al. 2005). Some of the irregular satellites might likewise have been acquired at this time (Nesvorny et al. 2007; furthermore, any irregular satellites possessed by the planets before the mean-motion resonance crossing would have been lost). Other KBOs might have been trapped in the outer regions of the main-asteroid belt, perhaps providing a Kuiper belt source for ice in the MBCs. Whether or not Jupiter and Saturn actually crossed the 2:1 resonance is unknown. In the Nice model, the 2:1 resonance crossing is contrived to occur at about 3.8 Gyr, so as to coincide with the epoch of the late-heavy bombardment (LHB). The latter is a period of heavy cratering on the Moon, thought by some to result from a sudden shower of impactors some 0.8 Gyr after the formation epoch. However, the interpretation of the crater age data is non-unique and reasonable

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Fig. 3.10 “Nice” model in which the architecture of the solar system is set by the clearing of a massive (30 M⊕ ) Kuiper belt (stippled green region) when planets are thrown outwards by strong interactions between Jupiter (red) and Saturn (pink) at the 2:1 mean-motion resonance. (a) The initial configuration with the giant planets at 5.5, 8.2, 11.5 and 14.2 AU (b) Just before the 2:1 resonance crossing, timed to occur near 880 Myr from the start (c) 3 Myr after resonance crossing (note the large eccentricity of Uranus (purple) at this time and the placement of Neptune (blue) in the Kuiper belt) and (d) 200 Myr later, by which time the planetary orbits have assumed nearly their current properties. Adapted with permission from Gomes et al. 2005

arguments exist to interpret the LHB in other ways (Chapman et al. 2007). For example, the cratering rate could merely appear to peak at 3.8 Gyr because all earlier (older) surfaces were destroyed by an impact flux even stronger than that at the LHB. (The LHB would then be an analog to the epoch in the expanding early universe when the optical depth first fell below unity and the galaxies became visible).

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Fig. 3.11 Saturn’s irregular satellite Phoebe, which might be a captured Kuiper belt object. The effective spherical radius is 107±1 km: the largest crater, Jason (at left), has a diameter comparable to the radius. The surface contains many ices (water, carbon dioxide) and yet is dark, with geometric albedo 0.08, as a consequence of dust mixed in the ice. Image from Cassini Imaging Team/NASA/JPL/Space Science Institute

3.5 Evidence Concerning the Birth Environment Several lines of evidence suggest that the Sun was formed in a dense cluster. The existence of widespread evidence for the decay products of short-lived isotopes in mineral inclusions in meteorites suggests that one or more supernovae exploded in the vicinity of the Sun, shortly before its formation. 26 Al (half-life 0.7 Myr) was the first such unstable isotope to be identified (Lee et al. 1977) but others, like 60 Fe (half-life 1.5 Myr) are known (and, unlike 26 Al, cannot be produced by nuclear spallation reactions; Mostefaoui et al. 2004). It is possible, although not required, that the collapse formation of the protoplanetary nebula was triggered by shock compression from the supernova that supplied the unstable nuclei (Ouellette et al. 2007). The sharp outer edge to the classical Kuiper belt could be produced by a stellar encounter having an impact parameter ∼150 AU to 200 AU (Ida et al. 2000), although this is only one of several possible causes. Such close encounters are highly improbable in the modern epoch but would be more likely in the dense environment of a young cluster. At the same time, the survival and regularity of the orbits of the planets suggests that no very close encounter occurred (Gaidos 1995). As noted above, the existence of a dense inner Oort cloud, required in some models to explain the source of the Halley family comets (long-period comets which include retrograde examples but which are not isotropically distributed), can only be populated via the stronger mean perturbations exerted between stars in a dense cluster. Adams and Laughlin (2001) conclude from these and related considerations that the Sun formed in a cluster of 2000 ± 1000 stars.

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3.6 Colors and Physical Properties It has long been recognized that Kuiper belt objects exhibit a diversity in surface colors unparalleled among solar system populations (Luu and Jewitt 1996). The distribution is relatively smooth from neutrally colored to extremely red objects. Indeed, a large fraction of KBOs (and Centaurs) is covered in “ultrared matter” (Jewitt 2002), the reddest material observed on small bodies. This material is absent in other populations, and is thought to be due to irradiated complex organics (Cruikshank et al. 2007). Further, the UBVRIJ colors are mutually correlated (Jewitt and Luu 1998, Jewitt et al. 2007), which seems to indicate that the spread is caused by a single reddening agent. The wide range of colors suggests a broad range of surface compositions. However, such extreme non-uniformity in composition is unlikely to be intrinsic given the uniform and low temperatures across the disk of the Kuiper belt; the compositional spread is probably the result of some evolutionary process. Early theories to explain the color scatter invoke a competition between (the reddening) longterm exposure to cosmic radiation and (the de-reddening) impact resurfacing (Luu and Jewitt 1996, Delsanti et al. 2004), but the implied rotational color variability and correlation between color and collision likelihood are not observed (Jewitt and Luu 2001, Th´ebault and Doressoundiram 2003). More recently it has been suggested that the diversity was emplaced when the small body populations were scattered by the outward migration of the ice giant planets (Gomes 2003), as a consequence of the mutual 2:1 resonance crossing by Jupiter and Saturn (see Nice model above). In other words, some of the objects now in the Kuiper belt may have formed much closer to the Sun, in the 10–20 AU region, where the chemistry would have been different and perhaps more diverse. Although appealing, the theory that the KB region was sprinkled with bodies from various heliocentric distances remains a non-unique explanation and the implicit assumption that KBOs formed closer to the Sun should be less red remains adhoc. The relative importances of this dynamical mixing and the reddening effect by cosmic irradiation are still poorly understood. Cosmic-ray reddening seems to be important as the Classical KBOs, supposedly formed locally at 40 AU and having passively evolving surfaces only subject to cosmic radiation (their circular orbits protect them from mutual collisions), are on average the reddest KBOs (Tegler and Romanishin 2000). A powerful way to investigate the physical properties of KBOs is by the analysis of their rotational properties, usually inferred from their lightcurves (Sheppard and Jewitt 2002, Lacerda and Luu 2006). Lightcurves are periodic brightness variations due to rotation: as a non-spherical (and non-azimuthally symmetric) KBO rotates in space, its sky-projected cross-section will vary periodically, and thus modulate the amount of sunlight reflected back to the observer. The period P and range Δm of a KBO’s lightcurve provide information on its rotation period and shape, respectively. Multi-wavelength lightcurves may also reveal surface features such as albedo or color patchiness (Buie et al. 1992, Lacerda et al. 2008). These features are usually seen as second order effects superimposed on the principal,

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shape-regulated lightcurve. By combining the period and the range of a KBO lightcurve it is possible to constrain its density under the assumption that the object’s shape is mainly controlled by its self-gravity (Jewitt and Sheppard 2002, Lacerda and Jewitt 2007). Lightcurves can also uncover unresolved, close binary objects (Sheppard and Jewitt 2004, Lacerda and Jewitt 2007). Interesting KBOs, whose lightcurves have been particularly informative include: (1) 134340 Pluto, whose albedo-controlled light variations have been used to map the distribution of ices of different albedos across the surface (Buie et al. 1992, Young et al. 1999), which is likely controlled by the surface deposition of frosts from Pluto’s thin atmosphere (Trafton 1989), (2) 20000 Varuna, whose rapid rotation (P = 6.34 ± 0.01 hr) and elongated shape (Δm = 0.42 ± 0.03 mag) indicate a bulk density ρ∼1000 kg m−3 and hence require an internally porous structure (Jewitt and Sheppard 2002), (3) 139775 2001 QG298, whose extreme lightcurve (large range Δm = 1.14 ± 0.04 mag and slow period P∼13.8 hr) suggests an extreme interpretation as a contact or near-contact binary (Sheppard and Jewitt 2004, Takahashi et al. 2004, Lacerda and Jewitt 2007), and finally (4) 136198 2003 EL61, exhibiting super-fast rotation (P=3.9 hr) that requires a density ∼2500 kg m−3 , and a recently identified surface feature both redder and darker than the average surface (Lacerda et al. 2008). Statistically, the rotational properties of KBOs can be used to constrain the distributions of spin periods (Lacerda and Luu 2006) and shapes (Lacerda and Luu 2003), which in turn can be used to infer the importance of collisions in their evolution. For instance, most main-belt asteroids have been significantly affected by mutual collisions, as shown by their quasi-Maxwellian spin rate distribution (Harris 1979, Farinella et al. 1981) and a shape distribution consistent with fragmentation experiments carried out in the laboratory (Catullo et al. 1984). KBOs spin on average more slowly ( PK B O  ∼ 8.4 hr vs. Past  ∼ 6.0 hr; Lacerda and Luu 2006) and are more spherical (as derived from the Δm distribution; Luu and Lacerda 2003, Sheppard et al. 2008) than asteroids of the same size, both indicative of a milder collisional history. Typical impact speeds in the current Kuiper belt and main asteroid belt are respectively 2 and 5 km s−1 . However, three of the four KBOs listed in the previous paragraph show extreme rotations and shapes, likely the result of collision events. Because the current number density of KBOs is too low for these events to occur on relevant timescales, their rotations were probably acquired at an early epoch when the Kuiper belt was more massive and collisions were more frequent (Davis and Farinella 1997, Jewitt and Sheppard 2002). Next generation telescopes will provide revolutionary new data on the physical properties of KBOs. Their colors and rotational properties can potentially reveal much about these objects’ surface and physical natures. JWST, in particular, will provide high quality near-infrared spectra that will place the best constraints on the (probably) organic mantles of the KBOs, thought to be some of the most primitive matter in the solar system. Separate measurements

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of the albedos and diameters, obtained from optical/thermal measurements using ALMA and JWST, will give the albedos and accurate diameters, needed to fully understand the surface materials. Survey data will permit the identification of >100 wide binaries, while the high angular resolution afforded by JWST will reveal a much larger number (thousands?) of close binaries. Orbital elements for each will lead to the computation of system masses through Kepler’s law. Diameters from optical/thermal measurements (using ALMA and JWST) will then permit the determination of system densities for KBOs over a wide range of diameters and orbital characteristics in the Kuiper belt. Density, as the “first geophysical parameter”, provides our best handle on accretion models of the Kuiper belt objects.

3.7 Solar System Dust Collisions between solids in the early solar system generally resulted in agglomeration due to the prevailing low impact energies. However, at present times, high impact energies dominate and collisions result in fragmentation and the generation of dust from asteroids, comets and KBOs. Due to the effect of radiation forces (see Section 3.7.2) these dust particles spread throughout the solar system forming a dust disk.

3.7.1 Inner Solar System: Asteroidal and Cometary Dust The existence of dust in the inner solar system (a.k.a. zodiacal cloud) has long been known since the first scientific observations of the zodiacal light by Cassini in 1683, correctly interpreted by de Duiliers in 1684 as produced by sunlight reflected from small particles orbiting the Sun. Other dust-related phenomena that can be observed naked eye are dust cometary tails and “shooting stars”. The sources of dust in the inner solar system are the asteroids, as evident from the observation of dust bands associated with the recent formation of asteroidal families, and comets, as evident from the presence of dust trails and tails. Their relative contribution can be studied from the He content of collected interplanetary dust particles, which is strongly dependent on the velocity of atmospheric entry, expected to be low for asteroidal dust and high for cometary dust. Their present contributions are thought to differ by less than a factor of 10 (Brownlee et al. 1994), but they have likely changed with time. It is thought that due to the depletion of asteroids, the asteroidal dust surface area has slowly declined by a factor of 10 (Grogan et al. 2001) with excursions in the dust production rate by up to an order of magnitude associated with breakup events like those giving rise to the Hirayama

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asteroid families that resulted in the formation of the dust bands observed by IRAS (Sykes and Greenberg 1986). The formation of the Veritas family 8.3 Myr ago still accounts for ∼25% of the zodiacal thermal emission today (Dermott et al. 2002). A major peak of dust production in the inner solar system is expected to have occurred at the time of the LHB (Section 3.4), as a consequence of an increased rate of asteroidal collisions and to the collisions of numerous impactors originating in the main asteroid belt (Strom et al. 2005) with the terrestrial planets. The thermal emission of the zodiacal cloud dominates the night sky between 5–500 ␮m and has a fractional luminosity of Ldust /LSun ∼ 10−8 –10−7 (Dermott et al. 2002). Studied by IRAS, COBE and ISO space telescopes, it shows a featureless spectrum produced by a dominant population of low albedo (<0.08) rapidlyrotating amorphous forsterite/olivine grains that are 10–100 ␮m in size and are located near 1 AU. The presence a weak (6% over the continuum) 10 ␮m silicate emission feature indicates the presence of a small population of ∼1 ␮m grains of dirty crystalline olivine and hydrous silicate composition (Reach et al. 2003). Interplanetary dust particles (IDPs) have been best characterized at around 1 AU by in situ satellite measurements, observations of micro-meteorite impact craters on lunar samples, ground radar observations of the ionized trails created as the particles pass through the atmosphere and laboratory analysis of dust particles collected from the Earth’s stratosphere, polar ice and deep sea sediments. Laboratory analysis of collected IDPs show that the particles are 1–1000 ␮m in size, typically black, porous (∼40%) and composed of mineral assemblages of a large number of submicron-size grains with chondritic composition and bulk densities of 1–3 g/cm3 . Their individual origin, whether asteroidal or cometary, is difficult to establish. The cumulative mass distribution of the particles at 1 AU follows a broken power-law such that the dominant contribution to the cross sectional area (and therefore to the zodiacal emission) comes from 10−10 kg grains (∼30 ␮m in radius), while the dominant contribution to the total dust mass comes from ∼10−8 kg grains (Leinert and Gr¨un 1990). In situ spacecraft detections of zodiacal dust out to 3 AU, carried out by Pioneer 8–11, Helios, Galileo and Ulysses, showed that the particles typically have i < 30◦ and e > 0.6 with a spatial density falling as r −1.3 for r < 1 AU and r −1.5 for r > 1 AU, and that there is a population of grains on hyperbolic orbits (a.k.a β-meteoroids), as well as stream of small grains origina jovian system (see review by Gr¨un et al. 2001).

3.7.2 Outer Solar System: Kuiper Belt Dust As remarked above, collisions in the modern-day Kuiper belt are erosive, not agglomerative, and result in the production of dust. In fact, two components to Kuiper belt dust production are expected: (1) erosion of KBO surfaces by the flux of interstellar meteoroids (e.g. Grun et al. 1994), leading to the steady production of dust at about 103 to 104 kg s−1 (Yamamoto and Mukai 1998); and (2) mutual collisions

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between KBOs, with estimated dust production of about (0.01–3)×108 kg s−1 (Stern 1996). For comparison, the dust production rate in the zodiacal cloud, from comets and asteroids combined, is about 103 kg s−1 (Leinert et al. 1983). The fractional luminosity of the KB dust is expected to be around Ldust /LSun ∼ 10−7 –10−6 (Stern 1996), compared to Ldust /LSun ∼ 10−8 –10−7 for the zodiacal cloud (Dermott et al. 2002). Observationally, detection of Kuiper belt dust at optical wavelengths is confounded by the foreground presence of cometary and asteroidal dust in the zodiacal cloud. Thermally, these near and far dust populations might be distinguished on the basis of their different temperatures (∼200 K in the Zodical cloud vs. ∼40 K in the Kuiper belt) but, although sought, Kuiper belt dust has not been detected this way (Backman et al. 1995). At infrared thermal wavelengths both foreground zodiacal cloud dust and background galactic dust contaminate any possible emission from Kuiper belt dust. The cosmic microwave background radiation provides a very uniform source against which emission from the Kuiper belt might potentially be detected but, again, no detection has been reported (Babich et al. 2007). Whereas remote detections have yet to be achieved, the circumstances for insitu detection are much more favorable (Gurnett et al. 1997). The Voyager 1 and 2 plasma wave instruments detected dust particles via the pulses of plasma created by high velocity impacts with the spacecraft. Because the plasma wave detector was not built with impact detection as its primary purpose, the properties and flux of the impacting dust are known only approximately. Still, several important results are available from the Voyager spacecraft. Impacts were recorded continuously as the Voyagers crossed the (then unknown) Kuiper belt region of the solar system. The smallest dust particles capable of generating measurable plasma are thought to be a0 ∼ 2␮m in radius. Measured in the 30 AU to 60 AU region along the Voyager flight paths, the number density of such particles is N1 ∼ 2 × 10−8 m−3 . Taking the thickness of the Kuiper belt (measured perpendicular to the midplane) as H ∼ 10 AU, this corresponds to an optical depth τ ∼ πa02 H N1 ∼ 4 × 10−7 , roughly 103 times smaller than the optical depth of a β-Pictoris class dust circumstellar disk. An upper limit on the density of gravel (cm-sized) particles in the Kuiper belt is provided by the survival of a 20-cm propellant tank on the Pioneer 10 spacecraft (Anderson et al. 1998).

3.7.3 Dust Dynamics and Dust Disk Structure After the dust particles are released from their parent bodies (asteroids, comets and KBOs) they experience the effects of radiation and stellar wind forces. Due to radiation pressure, their orbital elements and specific orbital energy change immediately upon release. If their orbital energy becomes positive (β > 0.5), the dust particles escape on hyperbolic orbits (known as β-meteoroids – Zook and Berg 1975). If their orbital energy remains negative (β < 0.5), their semi-major axis increases but they remain on bound orbits. Their new semimajor axis and

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1−β eccentricity (a , e ) in terms of that of their parent bodies (a and e) are a = a 1−2aβ/r 2

1/2

)(1−e ) and e = | 1 − (1−2aβ/r | (their inclination does not change), where r is the (1−β 2 ) particle location at release and β is the ratio of the radiation pressure force to the gravitational force. For spherical grains orbiting the Sun, β = 5.7Q pr /ρb, where ρ and b are the density and radius of the grain in MKS units and Q pr is the radiation pressure coefficient, a measure of the fractional amount of energy scattered and/or absorbed by the grain and a function of the physical properties of the grain and the wavelength of the incoming radiation (Burns et al. 1979). With time, Poynting–Rorbertson (P–R) and solar wind corpuscular drag (which result from the interaction of the dust grains with the stellar photons and solar wind particles, respectively) tend to circularize and decrease the semimajor axis of the orbits, forcing the particles to slowly drift in towards the they are   staruntil  central  R 2  L   ρ b destroyed by sublimation in a time given by t PR = 0.7 μm AU L∗ kg/m 3 1 1+albedo

yr , where R is the starting heliocentric distance of the dust particle and b and ρ are the particle radius and density, respectively (Burns et al. 1979, Backman and Paresce 1993). If the dust is constantly being produced from a planetesimal belt, and because the dust particles inclinations are not affected by radiation forces, this inward drift creates a dust disk of wide radial extent and uniform density. Grains can also be destroyed by mutual grain collisions, with a collisional lifetime of tcol =  R 3/2  M 1/2  10−5  1.26 × 104 AU yr (Backman and Paresce 1993). M∗ L dust /L ∗

For dust disks with Mdust > 10−3 M⊕ , tcol < t PR , i.e. the grains are destroyed by multiple mutual collisions before they migrate far from their parent bodies (in this context, “destruction” means that the collisions break the grains into smaller and smaller pieces until they are sufficiently small to be blown away by radiation pressure). This regime is referred to as collision-dominated. The present solar system, however, is radiation-dominated because it does not contain large quantities of dust and tcol > t PR , i.e. the grains can migrate far from the location of their parent bodies. This is particularly interesting in systems with planets and outer dust-producing planetesimal belts because in their journey toward the central star the orbits of the dust particles are affected by gravitational perturbations with the planets via the trapping of particles in mean motion resonances (MMRs), the effect of secular resonances and the gravitational scattering of dust. This results in the formation of structure in the dust disk (Fig. 3.12). Dust particles drifting inward can become entrapped in exterior MMRs because at these locations the particle receives energy from the perturbing planet that can balance the energy loss due to P-R drag, halting the migration. This makes the lifetime of particles trapped in outer MMRs longer than in inner MMRs (Liou and Zook 1997), with the former dominating the disk structure. This results in the formation of resonant rings outside the planet’s orbit, as the vast majority of the particles spend most of their lifetimes trapped in exterior MMRs. In some cases, due to the geometry of the resonance, a clumpy structure is created. Figure 3.12 (from Moro-Mart´ın and Malhotra 2002) shows the effect of resonant trapping expected in the (yet to be observed) KB dust disk, where the ring-like structure, the asymmetric

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clumps along the orbit of Neptune, and the clearing of dust at Neptune’s location are all due to the trapping of particles in MMRs with Neptune (as seen in the histogram of semimajor axis). Neptune plays the leading role in the trapping of dust particles because of its mass and because it is the outermost planet and its exterior resonances are not affected by the interior resonances from the other planets. Trapping is more efficient for larger particles (i.e. smaller β values) because the drag force is weaker and the particles cross the resonance at a slower rate increasing their probability of being captured. The effects of resonance trapping in hypothetical planetary systems each consisting of a single planet on a circular orbit and an outer planetesimal belt similar to the KB are shown in Fig. 3.13. More eccentric planets (e < 0.6) can also create clumpy eccentric rings and offset rings with a pair of clumps (Kuchner and Holman 2003). Even though, as mentioned in Section 3.7.2, the KB dust disk has yet to be observed, this is not the case for the Zodical cloud, for which IRAS and COBE thermal observations show that there is a ring of asteroidal dust particles trapped in exterior resonances with the Earth at around 1 AU, with a 10% number density enhancement on the Earth’s wake that results from the resonance geometry (Dermott et al. 1994). Secular perturbations, the long-term average of the perturbing forces, act on timescales >0.1 Myr. If the planet and the planetesimal disk are not coplanar, the secular perturbations can create a warp in the dust disk as their tendency to align the orbits operates on shorter timescales closer to the star. A warp can also be created in systems with two non-coplanar planets. If the planet is in an eccentric orbit, the secular resonances can force an eccentricity on the dust particles and this creates an offset in the disk center with respect to the star that can result in a brightness asymmetry. Other effects of secular perturbations are spirals and inner gaps (Wyatt et al. 1999). The effect of secular perturbations can be seen in IRAS and COBE observations on the zodiacal cloud and account for the presence of an inner edge around 2 AU due to a secular resonance with Saturn (that also explains the inner edge of the main asteroid belt), the offset of the cloud center with respect to the 

Fig. 3.12 (continued) Expected number density distribution of the KB dust disk for nine different particle sizes (or β values). β is a dimensionless constant equal to the ratio between the radiation pressure force and the gravitational force and depends on the density, radius and optical properties of the dust grains. If we assume that the grains are composed of spherical astronomical silicates (ρ = 2.5, Weingartner and Draine 2001), β values of 0.4, 0.2, 0.1, 0.05, 0.025, 0.0125, 0.00625, 0.00312, 0.00156 correspond to grain radii of 0.7, 1.3, 2.3, 4.5, 8.8, 17.0, 33.3, 65.9, 134.7 ␮m, respectively. The trapping of particles in MMRs with Neptune is responsible for the ring-like structure, the asymmetric clumps along the orbit of Neptune, and the clearing of dust at Neptune’s location (indicated with a black dot). The disk structure is more prominent for larger particles (smaller β values) because the P-R drift rate is slower and the trapping is more efficient. The disk is more extended in the case of small grains (large β values) because small particles are more strongly affected by radiation pressure. The histogram shows the relative occurrence of the different MMRs for different sized grains, where the large majority of the peaks correspond to MMRs with Neptune. The inner depleted region inside ∼10 AU is created by gravitational scattering of dust grains with Jupiter and Saturn. More details on these models can be found in Moro-Mart´ın and Malhotra (2002, 2003)

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Fig. 3.13 Same as Fig. 3.12 but for nine hypothetical planetary systems around a solar type star consisting of a single planet with a mass of 1, 3 or 10M J up in a circular orbit at 1, 5 or 30 AU, and a coplanar outer planetesimal belt similar to the KB. The models with 1 M J up planet at 1 and 5 AU show that the dust particles are preferentially trapped in the 2:1 and 3:1 resonances, but when the mass of the planet is increased to 10 M J up , and consequently the hill radius of the planet increases, the 3:1 becomes dominant. The resonance structure becomes richer when the planet is further away from the star, and when the mass of the planet decreases (compare the model for a 1M J up planet at 30 AU with the solar system model in Fig. 3.12, where the structure is dominated by Neptune). From Moro-Mart´ın, Wolf & Malhotra (in preparation)

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Fig. 3.14 Expected number density distribution of a KB dust disk composed of particles with β = 0.2 with the trajectories of the particles ejected by Jupiter in white. The black dot indicates the position of Neptune and the circles correspond to the orbits of the Giant planets. In addition to the population of small grains with β > 0.5 blown-out by radiation pressure, the gravitational scattering by the giant planets (Jupiter and Saturn in the case of the solar system) produces an outflow of large grains (β < 0.5) that is largely confined to the ecliptic (Moro-Mart´ın and Malhotra 2005b). Interestingly, a stream of dust particles arriving from the direction of β Pictoris has been reported by Baggaley (2000)

Sun, the inclination of the cloud with respect to the ecliptic, and the cloud warp (see review in Dermott et al. 2001). The efficient ejection of dust grains by gravitational scattering with massive planets as the particles drift inward from an outer belt of planetesimals (Figs. 3.14 and 3.15) can result in the formation of a dust depleted region inside the orbit of the planet (Figs. 3.12 and 3.13), such as the one expected inside 10 AU in the KB dust disk models (due to gravitational scattering by Jupiter and Saturn).

Next generation facilities will probably be unable to detect diffuse emission from Kuiper dust because the optical depth is so low, and the effects of foreground and background confusion so large. However, small Kuiper belt objects are sufficiently numerous that there is a non-negligible chance that the collision clouds of recent impacts will be detected (Stern 1996). Such clouds can potentially be very bright, and evolve on timescales (days and weeks) that are amenable to direct observational investigation using LSST and JWST. Collision cloud measurements provide our best chance to understand

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Fig. 3.15 Percentage of dust particles ejected from the system by gravitational scattering with the planet for the single planet models in Fig. 3.13. The particle size is fixed, corresponding to grains with β = 0.044. Because gravitational scattering is independent of the particle size, the efficiency of ejection is fairly independent of β. Left: Dependency of the efficiency of ejection on the planet’s mass (x-axis) and the planet’s semimajor axis (indicated by the different colors). Right: Dependency of the efficiency of ejection on the planet semimajor axis (x-axis) and eccentricity (corresponding to the different colors). The models in the right panel correspond to a 1 M J up mass planet on a circular orbit around a solar type star. Planets with masses of 3–10 M J up at 1 AU–30 AU in a circular orbit eject >90% of the dust grains that go past their orbits under P-R drag; a 1 M J up planet at 30 AU ejects >80% of the grains, and about 50–90% if located at 1 AU, while a 0.3 M J up planet is not able to open a gap, ejecting <10% of the grains. These results are valid for dust grains sizes in the range 0.7 ␮m–135 ␮m. From Moro-Mart´ın and Malhotra 2005

the sub-kilometer population in the Kuiper belt. These objects are too small to be directly detected but are of special relevance as the precursors to the Centaurs and Jupiter family comet nuclei. Measurement of their number is of central importance in understanding the role of the Kuiper belt as the JFC source, and as the source of Kuiper dust.

3.8 Kuiper Belts of Other Stars Radial velocity studies have revealed that >7% of solar-type stars harbor giant planets with masses <13 M J up and semimajor axis <5 AU (Marcy et al. 2005). This is a lower limit because the duration of the surveys (6–8 years) limits the ability to detect long-period planets; the expected frequency extrapolated to 20 AU is ∼12% (Marcy et al. 2005). As of February 2008, 276 extra-solar planets have been detected with a mass distribution that follows dN /dM ∝ M −1.05 from 0.3M Jup to 10 M Jup (the surveys are incomplete at smaller masses). A natural question arises whether these planetary systems, some of them harboring multiple planets, also contain planetesimals like the asteroids, comets and KBOs in the solar system. Long before extra-solar planets were discovered we inferred that the answer to this question was

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yes: colliding planetesimals had to be responsible for the dust disks observed around mature stars. In Section 3.8.1 we will discuss how these dust disks, known as debris disks, can help us study indirectly Kuiper belts around other stars; other methods by which extra-solar Kuiper belts might be found and characterized in the future will be discussed in Sections 3.8.2 and 3.8.3.

3.8.1 Debris Disks 3.8.1.1 Evidence of Planetesimals Theory and observations show that stars form in circumstellar disks composed of gas and dust that had previously collapsed from the densest regions of molecular clouds. For solar type stars, the masses of these disks are ∼0.01–0.10 M and extend to 100s of AU, comparable to the minimum mass solar nebula (∼0.015 M , which is the mass required to account for the condensed material in the solar system planets). Over time, these primordial or proto-planetary disks, with dust grain properties similar to those found in the interstellar medium, dissipate as the disk material accretes onto the star, is blown away by stellar wind ablation, photo-evaporation or high-energy stellar photons, or is stripped away by passing stars. The primordial gas and dust in these disks dissipate in less than 107 years (see e.g. Hartmann 2000 and references therein). However, it is found that some main sequence stars older than ∼107 years show evidence of dust emission. In most cases, this evidence comes from the detection of an infrared flux in excess of that expected from the stellar photosphere, thought to arise from the thermal emission of circumstellar dust. In some nearby stars, like the ones shown in Figs. 3.16, 3.17 and 3.18, direct imaging has confirmed that the emission comes from a dust disk. In Section 3.7.3 we discussed the lifetimes of the dust grains due to radiation pressure, P-R drag and mutual grain collisions. It is found that in most cases these lifetimes are much shorter than the age of the star,1 and therefore the observed dust cannot be primordial but is more likely produced by a reservoir of undetected dust-producing planetesimals, like the KBOs, asteroids and comets in the solar system (see e.g. Backman and Paresce 1993). This is why these dust disks observed around mature main sequence stars are known as debris disks. Debris disks are evidence of the presence of planetesimals around other main sequence stars. In the core accretion model, these planetesimals formed in the earlier protoplanetary disk phase described above, as the ISM-like dust grains sedimented into the mid-plane of the disk and aggregated into larger and larger bodies (perhaps helped by turbulence) until they became planetesimals, the largest of which could

1 One needs to be cautious with this argument because the ages of main sequence stars are difficult to determine. For example, the prototype (and best studied) of debris disk β-Pictoris, was dated at ∼100 to 200 Myr (Paresce 1991) but later become only ∼20 Myr old (Barrado y Navascu´es et al. 1999).

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Fig. 3.16 Spatially resolved images of nearby debris disks showing dust emission from 10s to 100s of AU with a wide diversity of complex features including inner gaps, warps, brightness asymmetries, offsets and clumply rings, some of which may be due to the presence of massive planets (7.3). Left (from top to bottom): AU Mic (Keck AO at 1.63 ␮m; Liu 2004), β-Pic (STIS CCD coronography at 0.2–1 ␮m; Heap et al. 2000), -Eri (JCMT/SCUBA at 850 ␮m; Greaves et al. 2005) and HD 32297 (HST/NICMOS coronography at 1.1 ␮m; Schneider, Silverstone and Hines 2005). Right: All images correspond to Fomalhaut (7.7 pc away) and are on the same scale. From top to bottom: Spitzer/MIPS at 24 ␮m (Stapelfeldt et al. 2004); Spitzer/MIPS at 70 ␮m (Stapelfeldt et al. 2004); JCMT/SCUBA at 450 ␮m (Holland et al. 2003) and HST/ACS at 0.69– 0.97 ␮m (Kalas et al. 2005). For this last panel, the annular disk in the scattered light image has an inner radius of ∼133 AU and a radial thickness of ∼25 AU and its center is offset from the star by about 15 ± 1 AU in the plane, possibly induced by an unseen planet. Its sharp inner edge has also been interpreted as a signature of a planet (Kalas et al. 2005). The dust mass of the Fomalhaut debris disk in millimeter-sized particles is about 1023 kg (∼0.02 M⊕ ; Holland et al. 1998) but, if larger bodies are present, the mass could be 50 to 100 M⊕ , or about 1000 times the mass of our Kuiper belt

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Fig. 3.17 Keck Telescope image of the AU Mic debris disk, showing a 100 AU diameter region with central obscuration by a coronagraphic mask (Liu 2004). AU Mic is a 12(+8, −4) Myr old M dwarf (about 0.5 M ) about 9.9 pc from Earth (Zuckerman et al. 2001) and with a dust mass near 0.01 M⊕ (Liu 2004). In scattered light, it shows a nearly edge-on disk about 200 AU in diameter with evidence for sub-structure (Kalas et al. 2004, Liu 2004)

potentially become the seeds out of which the giant planets form from the accretion of gas onto these planetary cores. Even though these extra-solar planetesimals remain undetected, the dust they produce has a much larger cumulative surface area that makes the dust detectable in scattered light and in thermal emission. The study of these debris disks can help us learn indirectly about their parent planetesimals, roughly characterizing their frequencies, location and composition, and even the presence of massive planets. 3.8.1.2 Spatially Resolved Observations Most debris disk observations are spatially unresolved and the debris disks are identified from the excess thermal emission contributed by dust in their spectral energy distributions (SEDs). In a few cases (about two dozen so far), the disks are close enough and the images are spatially resolved. Figures 3.16, 3.17 and 3.18 show the most spectacular examples. These high resolution observations show a rich diversity of morphological features including warps (AU Mic & β-Pic), offsets of the disk center with respect to the central star (ε-Eri and Fomalhaut), brightness asymmetries (HD 32297 and Fomalhaut), clumpy rings (AU Mic, β-Pic, ε-Eri and Fomalhaut) and sharp inner edges (Fomalhaut), features that, as discussed in Section 3.7.3 could be due to gravitational perturbations of massive planets, and that in some cases have been observed in the zodiacal cloud, while in other cases used to belong to the realm of KB disk models. Even though the origin of individual features is still under discussion and the models require further refinements (e.g. in the dust collisional processes and the effects of gas drag), the complexity of these features, in particular the azimuthal asymmetries, indicate that planets likely play a role in the creation of structure in the debris disks. This is of interest because the structure, in particular that created by the trapping of particles in MMRs, is sensitive to the presence of moderately massive planets at large distances (recall the KB dust disk models and the structure created by Neptune). This is a parameter space that cannot be explored with the present planet detection techniques, like the radial velocity and

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Vega 70 µm

Vega 160 µm

Vega 850 µm

200 AU

Fig. 3.18 Spatially resolved images of Vega from Spitzer/MIPS at 24, 70, 160 ␮m (Su et al. 2005) and from JCMT/SCUBA at 850 ␮m (Holland et al. 1998). All images are in the same scale. The instrument beam sizes (shown in white circles) indicate that the wide radial extent of the MIPS disk images compared to the SCUBA disk image is not a consequence of the instrumental PSF but due to a different spatial location of the particles traced by the two instruments. The sub-mm emission is thought to arise from large dust particles originating from a planetesimal belt analogous to the KB, while the MIPS emission is though to correspond to smaller particles with β < 0.5 (may be due to porosity), produced by collisions in the planetesimal belt traced by the sub-mm observations, and that are blown away by radiation pressure to distances much larger than the location of the parent bodies (Su et al. 2005). This scenario would explain not only the wider extent of the MIPS disk but also its uniform distribution, in contrast with the clumply and more compact sub-mm disk

transient studies, and therefore the study of debris disk structure can help us learn about the diversity of planetary systems. In this context, high resolution debris disk observations over a wide wavelength range are of critical importance, like those to be obtained with Herschel, JWST and ALMA.

3.8.1.3 Spectral Energy Distributions As we mentioned above, most of the debris disks observations are spatially unresolved and are limited to the study of the SED of the star+disk system. Even

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assuming that the dust is distributed in a disk (and not, for example, in a spherical shell) there are degeneracies in the SED analysis and the dust distribution cannot be unambiguously determined (e.g. Moro-Moro-Mart´ın, Wolf and Malhotra 2005a). Nevertheless, a wealth of information can be extracted from the SED. IRAS and ISO made critical discoveries on this front, but the number of known debris disks remained too small for statistical studies. This changed recently with the unprecedented sensitivity of the Spitzer instruments, that allowed the detection of hundreds of debris disks in large stellar surveys that searched for dust around 328 single FGK stars (Hillenbrand et al. 2008, Meyer et al. 2008, Carpenter et al. 2008), a different sample of 293 FGK stars (Trilling et al. 2008, Beichman et al. 2006a,b, Bryden et al. 2006), 160 A single stars (Su et al. 2006, Rieke et al. 2005); 69 A3–F8 binary stars (Trilling et al. 2007), and in young stellar clusters (Gorlova et al. 2007, Siegler et al. 2007). As a result, we now possess information concerning their frequencies, their dependency with stellar type and stellar environment, their temporal evolution and the composition of the dust grains (see e.g. Moro-Mart´ın et al. 2008 for a recent review).

Debris Disk Frequencies The Spitzer FEPS2 survey of 328 FGK stars found that the frequency of 24 ␮m excess is 14.7% for stars younger than 300 Myr and 2% for older stars, while at 70 ␮m, the excess rates are 6–10% (Hilllenbrand et al. 2008; Carpenter et al. 2008). These disks show characteristic temperatures of 60–180 K with evidence of a population of colder grains to account for the 70 ␮m excesses; the implied disk inner radii are > 10 AU and extend over tens of AU (see Fig. 3.19 – Carpenter et al. 2008). Figure 3.20 shows the debris disks incidence rates derived from a combined sample of 350 AFGKM stars from Trilling et al. (2008); for the 225 Sun-like (FG) stars in the sample older than 600 Myr, the frequency of the debris disks are 4.2+2 −1.1 % at +2.8 24 ␮m and 16.4−2..9 % at 70 ␮m. The above debris disks incidence rates compare to ∼20% of solar-type stars that harbor giant planets inside 20 AU (Marcy et al. 2005). Even though the frequencies seem similar, one should keep in mind that the sensitivity of the Spitzer observations is limited to fractional luminosities of Ldust /L∗ > 10−5 , i.e. >100 times the expected luminosity from the KB dust in our solar system. Assuming a gaussian distribution of debris disk luminosities and extrapolating from Spitzer observations (showing that the frequency of dust detection increases steeply with decreasing fractional luminosity), Bryden et al. (2006) found that the luminosity of the solar system dust is consistent with being 10 × brighter or fainter than an average solar-type star, i.e. debris disks at the solar system level could be common. Observations therefore indicate that planetary systems harboring dust-producing KBOs are more common than those with giant planets, which would be in agreement with the core accretion 2

http://feps.as.arizona.edu/

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Fig. 3.19 Probability distribution for disk inner radii based on the analysis of the spectra (12– 35 ␮m) of 44 debris disks around FGK stars from the FEPS survey. The dashed and grey histograms correspond to sources with and without 70 ␮m excess, respectively (with best fit parameters are shown as open and grey circles). Typical disk inner radius are ∼ 40 AU and ∼ 10 AU for disks with and without 70 ␮m excess, respectively, indicating that most of the debris disks observed are KB-like. Figure adapted with permission from Carpenter et al. 2008

models of planet formation where the planetesimals are the building blocks of planets and the conditions required for to form planetesimals are less restricted than those to form gas giants. Indeed, there is no apparent difference between the incidence rate of debris disks around stars with and without known planetary companions (Moro-Moro-Mart´ın et al. 2007; Bryden et al. in preparation), although planet-bearing stars tend to harbor more dusty disks (Bryden et al. in preparation), which could result from the excitation of the planetesimals’ orbits by gravitational perturbations with the planet. Figure 3.20 shows that there is no dependency on stellar type, neither in the frequency of debris disks, nor on the dust mass and location, indicating that planetesimal formation can take place under a wide range of conditions (Trilling et al. 2008). Debris Disk Evolution The FEPS survey of 328 FGK stars found that at 24 ␮m, the frequency of excess (> 10.2% over the stellar photosphere) decreases from 14.7% at ages < 300 Myr to 2% for older stars; at 70 ␮m, there is no apparent dependency of the excess frequency with stellar age, however, the amplitude of the 70 ␮m excess emission seems to decline from stars 30–200 Myr in age to older stars (Hillenbrand et al. 2008; Carpenter et al. 2008). Figures 3.21 and 3.22 from Trilling et al. (2008) show that for FGK type stars the debris disks incidence and fractional luminosity do not have a strong dependency with stellar age in the 1–10 Gyr time frame, in contrast with the 100– 400 Myr evolution timescale of young (0.01–1 Gyr) stars seen in Fig. 3.23. Trilling

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Fig. 3.20 The percentage of stars showing excess dust emission, i.e. with indirect evidence of the presence of dust-producing planetesimal belts, as a function of stellar type for ages >600 Myr (the mean ages within each type are shown at the top). The vertical bars correspond to binomial errors that include 68% of the probability (1 σ for Gaussian errors). Black is for 24 μm excess emission (tracing warmer dust) and red is for 70 μm (tracing colder dust). The data are consistent with no dependence on spectral type. There seems to be a weak decrease with spectral type at 70 μm but so far this is statistically not significant and could be due to an effect of age. Further analysis indicates that percentage of stars showing excess is different in old A stars and in M stars than in FGK stars. The excess rate for old M stars is 0% with upper limits (binomial errors) of 2.9% at 24 ␮m and 12% at 70 ␮m (Gautier et al. 2007). The lack of distant disks around K stars may be an observational bias because their peak emission would be at λ >70 ␮m and therefore remain undetected by Spitzer. The upcoming Herschel space telescope will provide the sensitivity to explore more distant and fainter debris disks. Figure adapted from Trilling et al. (2008) with data from Su et al. (2006), Trilling et al. (2007), Beichman et al. (2006b) and Gautier et al. (2007)

et al. (2007) argues that this data suggests that the dominant physical processes driving the evolution of the dust disks in young stars might be different from those in more mature stars, and operate on different timescales: while the former might be dominated by the production of dust during transient events like the LHB in the solar system or by individual collisions of large planetesimals (like the one giving rise to the formation of the Moon), the later might be the result of a more steady collisional evolution of a large population of planetesimals. The debris disks evolution observed by Spitzer for solar-type (Fig. 3.23 – Siegler et al. 2007) and A-type stars (Rieke et al. 2005, Su et al. 2006) indicate that both transient and more steady state dust production processes play a role; however, their relative importance and the question of how the dust production could be maintained in the oldest disks for billions of years is still under discussion. An interesting example is HD 69830, one of the outliers in Fig. 3.24, a KOV star (0.8 M , 0.45 L ) known to harbor three Neptune-like planets inside 0.63 AU. It shows a strong excess at 24 ␮m but no emission at 70 ␮m, indicating that the dust is warm and is located close to the star. The spectrum of the dust excess

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Fig. 3.21 The percentage of stars showing excess dust emission, i.e. with indirect evidence of the presence of dust-producing planetesimal belts, as a function of age for the F0–K5 stars. The horizontal error bars are the age bins (not the age uncertainties). The vertical bars correspond to binomial errors that include 68% of the probability (1 ␴ for Gaussian errors). The data are consistent with no dependency with age with a rate of ∼20%. The data seems to suggest an overall decrease but so far is statistically not significant, and if present may be due to an observational bias (because of the deficiency of excesses around the K stars in the oldest age bin). The number of stars in the bins are (from young to old): 24, 57, 60, 52, 33, and 7 (the high value of the oldest bin may be a small number statistical anomaly). For comparison, A-type stars evolve on timescales of 400 Myr (Su et al. 2006). Figure adapted from Trilling et al. (2008) with data from Beichman et al. (2006b)

Fig. 3.22 The fractional luminosity of the debris disks, Ldust /Lstar , as a function of age for FGK stars. The open symbols show the means within each age bin; the horizontal and vertical error bars show the bin widths and 1 σ errors, respectively. The data are consistent with no trend of Ldust /Lstar with age, but there seems to be a deficiency of disks with high Ldust /Lstar older than 6 Gyr. For comparison, the solar system is 4.5 Gyr old and is expected to have a dust disk with Ldust /Lstar ∼ 10−7 –10−6 . Figure adapted from Trilling et al. (2008) with data from Beichman et al. (2006b)

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Fig. 3.23 Ratio of the 24 ␮m Excess Emission over the expected stellar value for FGK stars as a function of stellar age (triangles for F0–F4 stars and circles for F5–K7 stars). The vertical alignments correspond to stars in clusters or associations. The data agree broadly with collisional cascade models of dust evolution (resulting in a 1/t decay for the dust mass) punctuated by peaks of dust production due to individual collisional events. A-type stars show a similar behavior. Figure from Siegler et al. (2007) using data from Gorlova et al. (2004), Hines et al. (2006) and Song et al. (2005)

(Fig. 3.24) shows strong silicate emission lines thought to arise from small grains of highly processed material similar to that of a disrupted P- or D-type asteroid plus small icy grains, likely located outside the outermost planet (Lisse et al. 2007). The observed levels of dust production are too high to be sustained for the entire age of the star, indicating that the dust production processes are transient (Wyatt et al. 2007). Whether the disks are transient or the result of the steady erosion of planetesimals is of critical importance for the interpretation of the statistics of the incidence rate of 24 ␮m excesses. For solar type stars, the 24 ␮m emission traces the 4–6 AU region. Terrestrial planet formation is expected to result in the production of large quantities of dust in this region, due to gravitational perturbations produced by large 1000 km-sized planetesimals that excite the orbits of a swarm of 1–10 km-size planetesimals, increasing their rate of mutual collisions and producing dust (Kenyon and Bromley 2005). This warm dust can therefore serve as a proxy of terrestrial planet formation. Figure 3.25 show the frequency of 24 ␮m emission for solar type (FGK) stars as a function of stellar age. This rate is <20% inside each age bin and

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Fig. 3.24 Spectrum of the dust excess emission from HD 69830 (Beichman et al. 2005 – top) compared to the spectrum of comet Hale-Bopp normalized to a blackbody temperature of 400 K (Crovisier et al. 1996 – bottom). HD 69830 is one of the outliers in Fig. 3.23

decreases with age. If the dust-producing events are very long-lived, the stars that show dust excesses in one age bin will also show dust excesses at later times. In this case the frequency of warm dust (which indirectly traces the frequency of terrestrial planet formation) is <20%. However, if the dust-producing events are short-lived, shorter than the age bins, the stars showing excesses in one age bin are not the same as the stars showing excesses at other age bins, i.e. they can produce dust at different epochs, and in this case the overall frequency of warm dust is obtained from adding all the frequencies in all age bins, which results in >60% (assuming that each star only has one epoch of high dust production). If this is the case, the frequency of terrestrial planet formation would be high (Meyer et al. 2008). However, the interpretation of the data would change if the observed 24 ␮m excesses arise from the steady erosion of cold-KB-like disks (Carpenter et al. 2008). Spatially resolved observations able to directly locate the dust would help resolve this issue.

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Fig. 3.25 Fraction of FGK type stars with 24 ␮m excess emission as a function of stellar age from a sample of 328 stars. The data points correspond to average values within a given age bin: (5/30) for stars 3–10 Myr, (9/48) for 10–30 Myr, (5/59) for 30–100 Myr, (9/62) for 100–300 Myr, (2/53) 300–1000 Myr. The widths of the age bins are shown by the horizontal bars, while the vertical bars show Poisson errors. Figure from Meyer et al. (2008)

Next generation facilities will offer high-sensitivity, high-resolution, multiwavelength observations that should result in major breakthroughs in the study of debris disks. Debris disks are proxies for the presence of planetesimals around mature stars having a wide diversity of stellar types (A–K), suggesting that planetesimal formation is a robust process. The study of the warm dust can tell us about the frequency of terrestrial planet formation and the presence of asteroid-like bodies, while the study of the cold dust sheds light on the population of small bodies in KB-like regions. In addition, the study of debris disks around stars having a wide diversity of ages can help us learn about the evolution of planetary systems. However, the statistics so far are limited to dust disks 100–1000 × more luminous than that of our solar system and the observations are generally spatially unresolved. High-sensitivity observations with future telescopes like Herschel, JWST and ALMA will be able to detect dust at the solar system level, will help us improve our understanding of the frequency of planetesimals and, together with the result from planet searches, will show the diversity of planetary systems. Multi-wavelength observations are critical to help locate the dust in spatially unresolved disks and fundamental to interpret the debris disks statistics. High-resolution imaging

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observations are very important to directly locate the dust (circumventing the SED degeneracy), and to study the structure of the debris disks, perhaps serving as a planet-detection method sensitive to long-period Neptune-like planets that otherwise may be undetectable in the foreseeable future. Multiwavelength observations also play a critical role in the interpretation of the structure because different wavelengths trace different particle sizes which have distinct dynamical characters that affect the disk morphology.

3.8.2 Photospheric Pollution Dust produced collisionally in a Kuiper belt may spiral to the central star under the action of radiation and/or plasma drag, contaminating the photosphere with metalenriched material. Separately, gravitational interactions and dynamical instabilities in a Kuiper belt may eject large objects (comets), causing some to impact the central star. Both processes operate in our solar system but neither produces a spectrally distinctive signature, for example a metal enrichment, on the Sun. This is simply because the photosphere of the Sun already contains a large mass of metals and the addition of dust or macroscopic bodies makes only a tiny, fractional contribution. However, the atmospheres of some white dwarf stars offer much more favorable opportunities for the detection of a photospheric pollution signal. First, many white dwarfs are naturally depleted in metals as a result of sedimentation of heavy elements driven by their strong gravitational fields. Since their atmospheres should be very clean, quite modest masses of heavy-element pollutants can be detected. About one fifth of white dwarfs expected to have pure hydrogen or pure helium atmospheres in fact show evidence for heavier elements, most likely due to pollution from external sources. Second, stellar evolution leading to the white dwarf stage includes the loss of stellar mass through an enhanced wind. As the central star mass decreases, the semimajor axes of orbiting bodies should increase, leading to dynamical instabilities caused by resonance sweeping and other effects (Debes and Sigurdsson 2002). Separately, white dwarf stars with Oort clouds should experience a steady flux of impacts from comets deflected inwards by stellar and galactic perturbations (Alcock et al. 1986). An observed depletion of carbon relative to iron may suggest that the infalling material is cometary rather than of interstellar origin (Jura 2006).

3.8.3 Thermal Activation The blackbody temperature, in Kelvin, of an object located at distance, R AU , from −1/2 a star of luminosity, L /L  , is TB B = 278 R AU (L /L  )1/4 . In the Kuiper belt today, at R AU = 40, the blackbody temperature is TB B = 44 K. Water sublimation

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at this low temperature is negligible. However, the sublimation rate is an exponential function of temperature and, by TB B = 200 K, water sublimates rapidly (with a mass flux ∼10−4 kg m−2 s−1 , corresponding to ice recession at about 3 m yr−1 for density 1000 kg m−3 ). A 1 km scale body would sublimate away in just a few centuries. By the above relation, temperatures of 200 K are reached when L /L  = 400, for the same 40 AU distance. Stellar evolution into the red giant phase will drive the Sun’s luminosity to exceed this value after about 10 Gyr on the main-sequence, with an increase in the luminosity (by up to a factor of ∼104 ), and in the loss of mass through an enhanced stellar wind (from the current value, ∼10−11 M yr−1 , to ∼10−7 M yr−1 , or more). When this happens, the entire Kuiper belt will light up as surface ices sublimate and dust particles, previously embedded in the KBOs, are ejected into space. Not all KBOs will be destroyed by roasting in the heat of the giant Sun: observations of comets near the Sun show that these bodies can insulate themselves from the heat by the development of refractory mantles, consisting of silicate and organic-rich debris particles that are too large to be ejected by gas drag. Still, the impact of the red giant phase should be dramatic and suggests that the sublimated Kuiper belts of other stars might be detected around red-giants. The key observational signatures would be the thermal excess itself, at temperatures appropriate to Kuiper belt-like distances, and ring-like morphology. Sensitivity at thermal wavelengths combined with high angular resolution will lend JWST to this type of observation, although the overwhelming signal from the star itself will present a formidable observational limitation to any imaging studies. Water vapor has been reported around carbon stars that are not expected to show water and interpreted as produced by sublimated comets (Ford and Neufeld 2001). In IRC +10216, the mass of water is estimated as 3×10−5 M (Melnick et al. 2001). This is about 10 M⊕ , or 100 times the mass of the modern Kuiper belt but perhaps comparable to the mass of the Kuiper belt when formed. A possible explanation is that the water derives from sublimated comets in an unseen Kuiper belt, but chemical explanations for this large water mass may also be possible (Willacy 2004). On the other hand, a search for (unresolved) thermal emission from dust around 66 first-ascent red giants proved negative, with limits on the Kuiper belt masses of these stars near 0.1 M⊕ , the current mass of the Kuiper belt (Jura 2004). Acknowledgments DJ was supported by a grant from NASA’s Origins program, PL by an NSF Planetary Astronomy grant to DJ. A.M.M. is under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Michelson Fellowship Program. A.M.M. is also supported by the Lyman Spitzer Fellowship at Princeton University.

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Th´ebault, P., & Doressoundiram, A. 2003: Colors and Collision Rates Within the Kuiper belt: Problems with the Collisional Resurfacing Scenario. Icarus 162, 27 Trafton, L. M. 1989: Pluto’s Atmosphere Near Perihelion. Geophy. Res. Lett., 16, 1213 Trilling, D. E., et al. 2007: Debris Disks in Main-Sequence Binary Systems.. Ap. J., 658, 1289 Trilling, D. E., et al. 2008: Debris Disks around Sun-like Stars. Ap. J., 674, 1086 Trujillo, C. A., Jewitt, D. C., & Luu, J. X. 2001: Properties of the Trans-Neptunian Belt. Astron. J. 122, 457 Weingartner, J., & Draine, B. 2001. Dust Grain-Size Distributions and Extinction in the Milky Way and Magellanic Clouds. Ap. J., 548, 296. Willacy, K. 2004: A Chemical Route to the Formation of Water in the Circumstellar Envelopes around Carbon-rich Asymptotic Giant Branch Stars: Fischer-Tropsch Catalysis. Ap. J. Lett., 600, L87 Wyatt, M. C., Dermott, S. F., Telesco, C. M., Fisher, R. S., Grogan, K., Holmes, E. K., & Pi˜na, R. K. 1999: How Observations of Circumstellar Disk Asymmetries Can Reveal Hidden Planets: Pericenter Glow and Its Application to the HR 4796 Disk. Ap. J., 527, 918 Wyatt, M. C., Smith, R., Greaves, J. S., Beichman, C. A., Bryden, G., & Lisse, C. M. 2007: Transience of Hot Dust around Sun-like Stars. Ap. J., 658, 569 Yamamoto, S., & Mukai, T. 1998: Dust Production by Impacts of Interstellar Dust on EdgeworthKuiper Belt Objects. Astron. Ap., 329, 785 Young, E. F., Galdamez, K., Buie, M. W., Binzel, R. P., & Tholen, D. J. 1999: Mapping the Variegated Surface of Pluto. Astron. J., 117, 1063 Zook, H. A., & Berg, O. E. 1975: A Source for Hyperbolic Cosmic Dust Particles. Planetary Space Sci., 23, 183 Zuckerman, B., Song, I., Bessell, M. S., & Webb, R. A. 2001. The Beta Pictoris Moving Group. Ap. J., 562, L87

Yvonne Pendleton graciously allowed a birthday party to be thrown in her honor during the meeting

Chapter 4

The Future of Ultracool Dwarf Science with JWST Mark S. Marley and S.K. Leggett

Abstract Ultracool dwarfs exhibit a remarkably varied set of characteristics which hint at the complex physical processes acting in their atmospheres and interiors. Spectra of these objects not only depend upon their mass and effective temperature, but also their atmospheric chemistry, weather, and dynamics. As a consequence divining their mass, metallicity and age solely from their spectra has been a challenge. JWST, by illuminating spectral blind spots and observing objects with constrained masses and ages should finally unearth a sufficient number of ultracool dwarf Rosetta Stones to allow us to decipher the processes underlying the complex brown dwarf cooling sequence. In addition the spectra of objects invisible from the ground, including very low mass objects in clusters and nearby cold dwarfs from the disk population, will be seen for the first time. In combination with other groundand space-based assets and programs, JWST will usher in a new golden era of brown dwarf science and discovery.

4.1 Introduction The explosive growth of brown dwarf and ultracool dwarf discoveries over the past dozen years has been so extraordinary that it is a rare paper in the field that does not open by remarking upon it. The first undisputed brown dwarf, Gl 229 B, was discovered as a companion to an M dwarf in 1995 (Nakajima et al. 1995). The TwoMicron All Sky Survey (2MASS, Skrutskie et al. 2006) and the Sloan Digital Sky Survey (SDSS, York et al. 2000) subsequently revealed large numbers of ultracool low-mass field dwarfs. These surveys first led to the discovery of the isolated field late-L dwarfs (Kirkpatrick et al. 1999), then the mid-T dwarfs (Burgasser et al. 1999, Strauss et al. 1999) and finally the early-T dwarfs (Leggett et al. 2000). Today over 600 warm (Teff ∼ 2400 to 1400 K) L and cool (600 to 1400 K) T dwarfs are known1 and the quest for the elusive, even cooler “Y” dwarfs is ongoing. M.S. Marley (B) NASA Ames Research Center, Mail Stop 245-3, Moffett Field CA 94035, USA e-mail: [email protected] 1

See http://www.DwarfArchives.org

H.A. Thronson et al. (eds.), Astrophysics in the Next Decade, Astrophysics and Space Science Proceedings, DOI 10.1007/978-1-4020-9457-6 4,  C Springer Science+Business Media B.V. 2009

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Note that collectively late M and later type dwarfs are often termed ‘Ultracool Dwarfs’ (or UCDs) to avoid having to distinguish whether particular warm objects in this group are above or below the hydrogen burning minimum mass, the requirement for bestowing the term ‘brown dwarf’. Brown dwarfs will continuously cool over time. The more massive UCDs will eventually arrive on the bottom of the hydrogen burning main sequence (Burrows et al. 1997). Ultracool dwarf science is exciting not only for the rapid pace of discovery, but for a host of other reasons as well. First, since brown dwarfs lack an internal energy source (beyond a brief period of deuterium burning), they cool off over time; they thus reach effective temperatures below those found in stars and enter the realm where chemical equilibrium favors such decidedly ‘unstellar’ atmospheric species as CH4 and NH3 . Along with these more typical ‘planetary’ gasses, silicate and iron clouds are found in their atmospheres, leading to interesting, complex interactions between atmospheric chemical, radiative-transfer, dynamical, and meteorological processes. Ultracool dwarfs thus bridge the domain between the bottom of the stellar main sequence and giant planets. They are a laboratory for understanding processes that will also be important in the characterization of extrasolar giant planets. Second, they occupy the low mass end of the stellar initial mass function. Understanding the IMF requires that we understand the masses of individual field objects, which ultimately requires an understanding of their luminosity evolution as well as the dependence of their spectra on mass, gravity, effective temperature and metallicity. Finally, as terra incognita, brown dwarfs (some of our nearest stellar neighbors) have offered a series of surprises that test our ability to understand the universe around us. The most distinctive features of the UCD spectral sequence are highlighted in Fig. 4.1. At effective temperatures below those of late-M dwarfs, several chemical changes (illustrated in Fig. 4.2) occur that strongly impact the spectral energy distribution. First, major diatomic metal species (particularly TiO and FeH) become incorporated into grains, leading to the gradual departure of hallmarks of the M spectral sequence (Kirkpatrick et al. 1999). Second, the formation of iron and silicate grains produces optically thick clouds that veil gaseous absorption bands and redden the near-IR JHK colors of L dwarfs. The atmospheric temperature domain where these clouds are most important is ∼1500–2000 K (e.g. Ackerman & Marley 2001). At lower Teff , the clouds lie near or below the base of the wavelengthdependent photosphere, and only marginally affect the SEDs of T dwarfs. Finally, CH4 supplants CO as the dominant carbon-bearing molecule. This transition is first noted in the 3–4 ␮m spectra of mid-L dwarfs (Noll et al. 2000) and appears in both the H and K bands of T0 dwarfs (Geballe et al. 2002). Together, increasing CH4 absorption and sinking cloud decks cause progressively bluer near-IR colors of T dwarfs. For types T5 and later, significant collision-induced H2 opacity in the K band enhances the trend toward bluer near-infrared colors. These changes in spectral features are used to assign spectral types to UCDs as briefly explained in Section 4.2. Given assigned spectral types, measurement of the bolometric luminosity of individual objects along with their parallaxes connect the spectral sequence to effective temperature. Figure 4.3 illustrates the effective

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Fig. 4.1 The most prominent signatures of the ultra cool dwarf spectral sequence are seen in these 0.65 to 14.5 ␮m spectra of a mid-M, L, and T dwarfs as well as Jupiter (adapted from Cushing et al. (2006)). The spectra have been normalized to unity at 1.3 ␮m and multiplied by constants. Major absorption bands are marked. The collision-induced opacity of H2 is indicated as a dashed line because it shows no distinct spectral features but rather a broad, smooth absorption. Jupiter’s flux shortward of ∼ 4 ␮m is predominantly scattered solar light; thermal emission dominates at longer wavelengths (near- and mid-infrared Jovian spectra from Rayner, Cushing & Vacca (in preparation) and Kunde et al. (2004), respectively)

temperature as a function of spectral type from late M through late T. While the general correlation of increasing spectral type with falling effective temperature is unmistakable, a remarkably rapid set of spectral changes (as expressed in the variation in spectral type) happens over a relatively small span of Teff near 1400 K. As we will discuss, understanding this variation in expressed spectral signatures, the ‘L to T transition’, is a key subject of current brown dwarf research. Most of the scientific inquiry into these ultracool dwarfs has focused on their formation and youth, on the resultant initial mass function, and on the determination of their global properties, particularly mass, effective temperature, metallicity, and cloudiness. This review will focus primarily on the latter areas. Burrows et al. (2001) provide a much more in depth review and background to brown dwarf science and is an excellent starting point for those new to the subject. Kirkpatrick (2008) provides a more current look at outstanding issues in the field from an observational perspective. Here we first present a very brief review of the ultracool dwarf spectral types and the nature of the current datasets. We then move on to discuss the role that clouds and atmospheric mixing play in controlling the emitted spectra of these objects and the enigmatic L- to T-type transition. Because clouds control the spectral energy distribution of the L and early T dwarfs, and since clouds are inherently difficult to model, constraining gravity solely by comparison of observations to spectral data is particularly challenging. Finally we will close with a look forward to some of the ultracool dwarf science opportunities that will be enabled by JWST. Because

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Fig. 4.2 Important chemical equilibrium boundaries for substellar objects (modified from Lodders & Fegley 2006). Green, red, and blue lines denote various condensation boundaries for a solar abundance mixture of gasses in a substellar atmosphere. Light purple lines denote equilibrium boundaries between important gaseous species. Grey dashed lines show model atmospheric temperature-pressure profiles for M, L and T dwarfs (the latter specifically for Gl 229 B) as well as for Jupiter. As one moves upwards in the diagram along the model (T, P) curves, the labeled species will condense at the intersection with the condensation curves and would be expected to be absent from the gas at lower temperatures further up along the model profiles. This figure can be compared with Fig. 4.1 to understand why spectral features for various compounds are present or absent in each observed spectrum

of space limitations we neglect several other important avenues of UCD research, including studies of the IMF, very young objects, and of objects with unusual colors.

4.2 Spectral Type The currently known ultracool dwarfs span spectral types from late M, L0 through L9, and T0 through T9. The TiO and VO bands, which dominate the optical portions of late-M dwarf spectra, disappear in the L dwarfs (Kirkpatrick et al. 1999), where metallic oxides are replaced by metallic hydrides and where features due to neutral alkali metals are strong. To systematize such objects Kirkpatrick et al. established spectral indices and defined an optical classification scheme for L dwarfs, which is commonly used. The indices measure the strengths of TiO, VO, CrH, Rb and Cs features as well as a red color term, at wavelengths between 0.71 and 0.99 ␮m.

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Fig. 4.3 Effective temperature as a function of infrared spectral type for ultracool dwarfs with known parallax (data from Golimowski et al. 2004, Vrba et al. 2004, and Luhman et al. 2007). Note the roughly constant effective temperature for dwarfs of spectral types from late L to early T. See Kirkpatrick (2008) for further discussion

With the discovery of the T dwarfs, which have very little flux in the optical, Geballe et al. (2002) defined a near-infrared classification scheme that encompassed both the L and T dwarfs. The indices measure the strength of the H2 O and CH4 absorption features between 1.1 and 2 ␮m, and for the early L dwarfs a red color term is also used which is slightly modified from Kirkpatrick et al. (1999). Burgasser et al. (2002a) introduced a near-infrared classification scheme for T dwarfs that was very similar to that of Geballe et al. The two schemes were unified in Burgasser et al. (2006) and this near-infrared scheme is the commonly used scheme for typing