Pathways to Discovery in Astronomy and Astrophysics for the 2020s (2021)

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Prepublication Copy – Subject to Further Editorial Correction Pathways to Discovery in Astronomy and Astrophysics for the 2020s Committee for a Decadal Survey on Astronomy and Astrophysics 2020 (Astro2020) Space Studies Board Board on Physics and Astronomy Division on Engineering and Physical Sciences A Consensus Study Report of PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 

THE NATIONAL ACADEMIES PRESS 500 Fifth Street, NW Washington, DC 20001 This study is based on work supported by Contract NNH17CB02B with the National Aeronautics and Space Administration, Grant AST-1852611 with the National Science Foundation, Grant DE-SC0019513 with the Department of Energy, and Grant FA9550-19-1-0330 with the Air Force Office of Scientific Research. Any opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of any agency or organization that provided support for the project. International Standard Book Number-13: XXX-X-XXX-XXXXX-X International Standard Book Number-10: X-XXX-XXXXX-X Digital Object Identifier: https://doi.org/10.17226/26141 Cover design Copies of this publication are available free of charge from Space Studies Board National Academies of Sciences, Engineering, and Medicine Keck Center of the National Academies 500 Fifth Street, NW Washington, DC 20001 Additional copies of this publication are available from the National Academies Press, 500 Fifth Street, NW, Keck 360, Washington, DC 20001; (800) 624-6242 or (202) 334-3313; http://www.nap.edu. Copyright 2021 by the National Academy of Sciences. All rights reserved. Printed in the United States of America Suggested citation: National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. https://doi.org/10.17226/26141. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 

The National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, nongovernmental institution to advise the nation on issues related to science and technology. Members are elected by their peers for outstanding contributions to research. Dr. Marcia McNutt is president. The National Academy of Engineering was established in 1964 under the charter of the National Academy of Sciences to bring the practices of engineering to advising the nation. Members are elected by their peers for extraordinary contributions to engineering. Dr. John L. Anderson is president. The National Academy of Medicine (formerly the Institute of Medicine) was established in 1970 under the charter of the National Academy of Sciences to advise the nation on medical and health issues. Members are elected by their peers for distinguished contributions to medicine and health. Dr. Victor J. Dzau is president. The three Academies work together as the National Academies of Sciences, Engineering, and Medicine to provide independent, objective analysis and advice to the nation and conduct other activities to solve complex problems and inform public policy decisions. The National Academies also encourage education and research, recognize outstanding contributions to knowledge, and increase public understanding in matters of science, engineering, and medicine. Learn more about the National Academies of Sciences, Engineering, and Medicine at www.nationalacademies.org. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 

Consensus Study Reports published by the National Academies of Sciences, Engineering, and Medicine document the evidence-based consensus on the study’s statement of task by an authoring committee of experts. Reports typically include findings, conclusions, and recommendations based on information gathered by the committee and the committee’s deliberations. Each report has been subjected to a rigorous and independent peer-review process and it represents the position of the National Academies on the statement of task. Proceedings published by the National Academies of Sciences, Engineering, and Medicine chronicle the presentations and discussions at a workshop, symposium, or other event convened by the National Academies. The statements and opinions contained in proceedings are those of the participants and are not endorsed by other participants, the planning committee, or the National Academies. For information about other products and activities of the National Academies, please visit www.nationalacademies.org/about/whatwedo. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 

COMMITTEE FOR A DECADAL SURVEY ON ASTRONOMY AND ASTROPHYSICS 2020 (ASTRO2020): STEERING COMMITTEE FIONA A. HARRISON, NAS,1 California Institute of Technology, Co-Chair ROBERT C. KENNICUTT JR., NAS, University of Arizona and Texas A&M University, Co-Chair JULIANNE DALCANTON, University of Washington TIM DE ZEEUW, Leiden University ANDREW S. DRIESMAN, Johns Hopkins University Applied Physics Laboratory JONATHAN J. FORTNEY, University of California, Santa Cruz GABRIELA GONZÁLEZ, NAS, Louisiana State University JORDAN A. GOODMAN, University of Maryland MARC P. KAMIONKOWSKI, NAS, Johns Hopkins University BRUCE MACINTOSH, Stanford University JACOBUS M. OSCHMANN, Independent Consultant RACHEL A. OSTEN, Space Telescope Science Institute, Executive Officer LYMAN A. PAGE, JR., NAS, Princeton University ELIOT QUATAERT, NAS, Princeton University WANDA A. SIGUR, NAE,2 Independent Consultant RACHEL SOMERVILLE, Flatiron Institute KEIVAN G. STASSUN, Vanderbilt University JEAN L. TURNER, University of California, Los Angeles PIETER VAN DOKKUM, Yale University ELLEN G. ZWEIBEL, NAS, University of Wisconsin, Madison SCIENCE PANELS PANEL ON COMPACT OBJECTS AND ENERGETIC PHENOMENA DEEPTO CHAKRABARTY, Massachusetts Institute of Technology, Chair LAURA B. CHOMIUK, Michigan State University DANIEL E. HOLZ, University of Chicago RAFFAELLA MARGUTTI, Northwestern University JULIE McENERY, NASA Goddard Space Flight Center PETER I. MÉSZÁROS, Pennsylvania State University RAMESH NARAYAN, NAS,3 Center for Astrophysics, Harvard and Smithsonian ELIOT QUATAERT, NAS, Princeton University, Steering Committee Liaison SCOTT M. RANSOM, National Radio Astronomy Observatory TODD A. THOMPSON, Ohio State University PANEL ON COSMOLOGY DANIEL EISENSTEIN, NAS, Harvard University and Sloan Digital Sky Survey III, Chair LINDSEY E. BLEEM, Argonne National Laboratory MARC P. KAMIONKOWSKI, NAS, Johns Hopkins University, Steering Committee Liaison RACHEL MANDELBAUM, Carnegie Mellon University 1 Member, National Academy of Sciences. 2 Member, National Academy of Engineering. 3 Member, National Academy of Sciences. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION v 

MIGUEL F. MORALES, University of Washington DANIEL M. SCOLNIC, Duke University MATIAS ZALDARRIAGA, NAS, Institute for Advanced Study KATHRYN M. ZUREK, California Institute of Technology PANEL ON GALAXIES DANIELA CALZETTI, NAS, University of Massachusetts, Amherst, Chair MICHAEL BOYLAN-KOLCHIN, University of Texas, Austin HSIAO-WEN CHEN, University of Chicago ANN E. HORNSCHEMEIER, NASA Goddard Space Flight Center SUSAN A. KASSIN, Space Telescope Science Institute AMANDA A. KEPLEY, National Radio Astronomy Observatory CHARLES C. STEIDEL, NAS, California Institute of Technology DANIEL K. STERN, NASA Jet Propulsion Laboratory TOMMASO TREU, University of California, Los Angeles PIETER VAN DOKKUM, Yale University, Steering Committee Liaison DAVID H. WEINBERG, Ohio State University PANEL ON EXOPLANETS, ASTROBIOLOGY, AND THE SOLAR SYSTEM VICTORIA S. MEADOWS, University of Washington, Chair DAVID A. BRAIN, University of Colorado, Boulder IAN J.M. CROSSFIELD, Massachusetts Institute of Technology COURTNEY D. DRESSING, University of California, Berkeley JONATHAN J. FORTNEY, University of California, Santa Cruz, Steering Committee Liaison TIFFANY KATARIA, Jet Propulsion Laboratory KATHLEEN E. MANDT, Johns Hopkins University Applied Physics Laboratory MARK S. MARLEY, NASA Ames Research Center BRITNEY E. SCHMIDT, Georgia Institute of Technology CHRISTOPHER C. STARK, Space Telescope Science Institute PANEL ON THE INTERSTELLAR MEDIUM AND STAR PLANET FORMATION LEE W. HARTMANN, University of Michigan, Chair SEAN M. ANDREWS, Smithsonian Astrophysical Observatory PHILIP J. ARMITAGE, Stony Brook University BRUCE T. DRAINE, NAS, Princeton University KAITLIN M. KRATTER, University of Arizona KARIN M. SANDSTROM, University of California, San Diego SNEZANA STANIMIROVIC, University of Wisconsin, Madison ELLEN G. ZWEIBEL, NAS, University of Wisconsin, Madison, Steering Committee Liaison PANEL ON STARS, THE SUN, AND STELLAR POPULATIONS SARBANI BASU, Yale University, Chair NANCY S. BRICKHOUSE, Center for Astrophysics, Harvard and Smithsonian ADAM BURGASSER, University of California, San Diego JULIANNE DALCANTON, University of Washington, Steering Committee Liaison JENNIFER A. JOHNSON, Ohio State University R.T. JAMES MCATEER, New Mexico State University PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION vi 

ELISA V. QUINTANA, NASA Goddard Space Flight Center LOUIS-GREGORY STROLGER, Space Telescope Science Institute PROGRAM PANELS PANEL ON AN ENABLING FOUNDATION FOR RESEARCH DAVID N. SPERGEL, NAS, Princeton University, Chair MICHAEL BLANTON, New York University KELLE L. CRUZ, Hunter College MARK J. DEVLIN, University of Pennsylvania MEGAN E. DONAHUE, Michigan State University KEITH A. HAWKINS, University of Texas, Austin ALINA A. KIESSLING, Jet Propulsion Laboratory KARIN ÖBERG, Harvard University ANGELA V. OLINTO, NAS, University of Chicago BERNARD J. RAUSCHER, NASA Goddard Space Flight Center RACHEL SOMERVILLE, Center for Computational Astrophysics, Flatiron Institute, Steering Committee Liaison JAMES M. STONE, Institute for Advanced Study PANEL ON ELECTROMAGNETIC OBSERVATIONS FROM SPACE 1 MARCIA J. RIEKE, NAS, University of Arizona, Chair RUSLAN BELIKOV, NASA Ames Research REBECCA A. BERNSTEIN, Carnegie Institution for Science LESTER M. COHEN, Center for Astrophysics | Harvard & Smithsonian NIKOLE K. LEWIS, Cornell University BRUCE MACINTOSH, Stanford University, Steering Committee Liaison AMY MAINZER, University of Arizona MARK P. SAUNDERS, Independent Consultant EVGENYA L. SHKOLNIK, Arizona State University GEORGE SONNEBORN, NASA Goddard Space Flight Center C. MEGAN URRY, NAS, Yale University PANEL ON ELECTROMAGNETIC OBSERVATIONS FROM SPACE 2 STEVEN M. KAHN, Stanford University, Chair LISA BARSOTTI, Massachusetts Institute of Technology ALLISON BARTO, Ball Aerospace and Technologies Corporation MICHAEL BAY, Bay Engineering Innovations MARTIN ELVIS, Center for Astrophysics | Harvard & Smithsonian CHARLES J. HAILEY, Columbia University CHRYSSA KOUVELIOTOU, NAS, George Washington University CHARLES R. LAWRENCE, Jet Propulsion Laboratory S. HARVEY MOSELEY, JR., Quantum Circuits, Inc. RESHMI MUKHERJEE, Barnard College LYMAN A. PAGE, JR., NAS, Princeton University, Steering Committee Liaison GORDON J. STACEY, Cornell University PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION vii 

PANEL ON OPTICAL AND INFRARED OBSERVATIONS FROM THE GROUND TIMOTHY M. HECKMAN, NAS, Johns Hopkins University, Chair DAVID A. BEARDEN, Jet Propulsion Laboratory DAVID CHARBONNEAU, NAS, Harvard University SUVI GEZARI, University of Maryland ANDREA M. GHEZ, NAS, University of California, Los Angeles JENNY E. GREENE, Princeton University J. TODD HOEKSEMA, Stanford University JACOBUS M. OSCHMANN, Independent Consultant, Steering Committee Liaison RICHARD W. POGGE, Ohio State University MASSIMO ROBBERTO, Space Telescope Science Institute NATASCHA M. FÖRSTER SCHREIBER, Max Planck Institute for Extraterrestrial Physics DAVID R. SILVA, University of Texas, San Antonio PANEL ON PARTICLE ASTROPHYSICS AND GRAVITATION JOHN F. BEACOM, Ohio State University, Co-Chair LAURA CADONATI, Georgia Institute of Technology, Co-Chair DAVID Z. BESSON, University of Kansas GABRIELA GONZÁLEZ, NAS, Louisiana State University, Steering Committee Liaison JORDAN A. GOODMAN, University of Maryland, Steering Committee Liaison ELIZABETH A. HAYS, NASA Goddard Space Flight Center N. JEREMY KASDIN, University of San Francisco DAVID B. KIEDA, University of Utah ANDREA N. LOMMEN, Haverford College BRIAN D. METZGER, Columbia University JAMES H. YECK, University of Wisconsin, Madison NICOLAS YUNES, University of Illinois, Urbana-Champaign PANEL ON RADIO, MILLIMETER, AND SUBMILLIMETER OBSERVATIONS FROM THE GROUND ANDREW J. BAKER, Rutgers, The State University of New Jersey, Chair JACQUELINE N. HEWITT,4 Massachusetts Institute of Technology, Vice Chair HÉCTOR G. ARCE, Yale University RAVINDER S. BHATIA, Thirty Meter Telescope TRACY E. CLARKE, Naval Research Laboratory MATT A. DOBBS, McGill University DAVID L. KAPLAN, University of Wisconsin, Milwaukee DANIEL P. MARRONE, University of Arizona LYNN D. MATTHEWS, Massachusetts Institute of Technology Haystack Observatory JOAN R. NAJITA, NSF National Optical-Infrared Astronomy Research Laboratory (NOIRLab) RICHARD L. PLAMBECK, University of California, Berkeley JEAN TURNER, University of California, Los Angeles, Steering Committee Liaison 4 Resigned from the panel on April 16, 2020, and did not participate in the panel’s deliberations or the writing of its report. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION viii 

PANEL ON STATE OF THE PROFESSION AND SOCIETAL IMPACTS MARGARET M. HANSON, University of Cincinnati, Co-Chair ENRICO RAMIREZ-RUIZ, University of California, Santa Cruz, Co-Chair GURTINA BESLA, University of Arizona PATRICIA T. BOYD, NASA Goddard Space Flight Center KATHRYNE J. DANIEL, Bryn Mawr College MARTHA P. HAYNES, NAS, Cornell University JEDIDAH C. ISLER, Dartmouth University RACHEL L. IVIE, American Institute of Physics KATHRYN V. JOHNSTON, Columbia University CASEY W. MILLER, Rochester Institute of Technology JESÚS PANDO, DePaul University JULIE R. POSSELT, University of Southern California JANE R. RIGBY, NASA Goddard Space Flight Center WILLIE S. ROCKWARD, Morgan State University KEIVAN G. STASSUN, Vanderbilt University, Steering Committee Liaison PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION ix 

DECADAL SURVEY ON ASTRONOMY AND ASTROPHYSICS 2020 (ASTRO2020) STAFF COLLEEN N. HARTMAN, Director, Space Studies Board and Board on Physics and Astronomy1 JAMES LANCASTER,2 Director, Board on Physics and Astronomy GREGORY MACK, Senior Program Officer, Board on Physics and Astronomy, Steering Committee Co- Study Director and Panel on Compact Objects and Energetic Phenomena ABIGAIL A. SHEFFER, Senior Program Officer, Space Studies Board, Steering Committee Co-Study Director and Panel on Electromagnetic Observations from Space 2 DIONNA WISE, Program Coordinator, Space Studies Board, Steering Committee and Panel on State of the Profession and Societal Impacts MIA BROWN, Research Associate, Space Studies Board, Steering Committee ALAN ANGLEMAN, Senior Program Officer, Aeronautics and Space Engineering Board, Panel on Particle Astrophysics and Gravitation SHENAE BRADLEY, Administrative Assistant, Computer Science and Telecommunications Board, Panel on Galaxies MEGAN CHAMBERLAIN, Senior Program Assistant, Space Studies Board, Panel on Electromagnetic Observations from Space 2, Panel on Optical and Infrared Observations from the Ground, and Panel on Particle Astrophysics and Gravitation ARTHUR CHARO, Senior Program Officer, Space Studies Board, Panel on Radio, Millimeter, and Submillimeter Observations from the Ground DWAYNE DAY, Senior Program Officer, Aeronautics and Space Engineering Board, Panel on Electromagnetic Observations from Space 1 GREG EYRING, Senior Program Officer, National Materials and Manufacturing Board, Panel on the Interstellar Medium and Star Planet Formation, Panel on an Enabling Foundation for Research, and Panel on State of the Profession and Societal Impacts SANDRA GRAHAM, Senior Program Officer, Space Studies Board, Panel on Stars, the Sun, and Stellar Populations GAYBRIELLE HOLBERT, Program Assistant, Space Studies Board, Panel on Stars, the Sun, and Stellar Populations AMISHA JINANDRA, Associate Program Officer, Board on Physics and Astronomy Panel on Exoplanets, Astrobiology, and the Solar System CHRISTOPHER JONES, Program Officer, Board on Physics and Astronomy, Panel on Exoplanets, Astrobiology, and the Solar System JAMES MYSKA, Program Officer, National Materials and Manufacturing Board, Panel on Cosmology DANIEL NAGASAWA, Program Officer, Space Studies Board, Panel on State of the Profession and Societal Impacts ERIK SAARI, Administrative Assistant, Board on Army Research Development (until November 30, 2019), Panel on Compact Objects and Energetic Phenomena, and Panel on Exoplanets, Astrobiology, and the Solar System ERIK SVEDBERG, Senior Program Officer, National Materials and Manufacturing Board, Panel on Galaxies, and Panel on Optical and Infrared Observations from the Ground LINDA WALKER, Program Coordinator, Board on Physics and Astronomy Panel on Cosmology, Panel On The Interstellar Medium And Star Planet Formation, Panel on an Enabling Foundation for Research, Panel on Electromagnetic Observations from Space 1, and Panel on Radio, Millimeter, and Submillimeter Observations from the Ground 1 Since May 2021. 2 Retired May 2021. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION x 

*We want to thank the following—Senior Scholar and former BPA Board Director, Donald C. Shapero; Christine Mirzayan Science and Technology Policy Fellows Sara Crandall and Emily Moravec; and Lloyd V. Berkner Space Policy Interns Robert Bullard, Benjamin Cassese, Katherine Dzurilla, Tarini Konchady, Sarah Moran, Osase Omoruyi, Genevieve Payne, Cindy Vo. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION xi 

BOARD ON PHYSICS AND ASTRONOMY ANDREW J. LANKFORD, University of California, Irvine, Chair MEIGAN ARONSON, University of British Columbia WILLIAM BIALEK, NAS, Princeton University JILL DAHLBURG, Naval Research Laboratory, ret. SALLY DAWSON, Brookhaven National Laboratory LOUIS F. DIMAURO, The Ohio State University WENDELL T. HILL III, University of Maryland ALAN J. HURD, Los Alamos National Laboratory CHUNG-PEI MA, University of California, Berkeley ANGELA VILLELA OLINTO, The University of Chicago DAVID H. REITZE, California Institute of Technology SUNIL SINHA, University of California, San Diego RISA H. WECHSLER, Stanford University WILLIAM A. ZAJC, Columbia University STAFF JAMES C. LANCASTER,3 Director COLLEEN N. HARTMAN,4 Director NEERAJ P. GORKHALY, Associate Program Officer AMISHA JINANDRA, Associate Program Officer CHRISTOPHER J. JONES, Program Officer RADAKA LIGHTFOOT, Finance Business Partner GREGORY MACK, Senior Program Officer LINDA WALKER, Program Coordinator 3 Retired May 2021. 4 Since May 2021. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION xii 

SPACE STUDIES BOARD MARGARET G. KIVELSON, NAS, University of California, Los Angeles, Chair GREGORY P. ASNER, NAS, Carnegie Institution for Science ADAM S. BURROWS, NAS, Princeton University JAMES H. CROCKER, NAE, Lockheed Martin Space Systems Company MARY LYNNE DITTMAR, Coalition for Deep Space Exploration JEFF DOZIER, University of California, Santa Barbara MELINDA DARBY DYAR, Mount Holyoke College ANTONIO L. ELIAS, NAE, Orbital ATK, Inc. VICTORIA E. HAMILTON, Southwest Research Institute CHRYSSA KOUVELIOTOU, NAS, George Washington University DENNIS P. LETTENMAIER, NAE, University of California, Los Angeles ROSALY M. LOPES, Jet Propulsion Laboratory STEPHEN J. MACKWELL, American Institute of Physics DAVID J. MCCOMAS, Princeton University LARRY J. PAXTON, The Johns Hopkins University ELIOT QUATAERT, NAS, Princeton University MARK SAUNDERS, Independent Consultant BARBARA SHERWOOD LOLLAR, University of Toronto HOWARD SINGER, National Oceanographic and Atmospheric Administration HARLAN E. SPENCE, University of New Hampshire ERIKA B WAGNER, Blue Origin, LLC PAUL D. WOOSTER, Space Exploration Technologies EDWARD L. WRIGHT, NAS, University of California, Los Angeles STAFF COLLEEN N. HARTMAN, Director ALAN ANGLEMAN, Associate Director ALEXIS BHADHA, Financial Assistant MIA BROWN, Research Associate MEGAN CHAMBERLAIN, Senior Program Assistant ARTHUR CHARO, Senior Program Officer DWAYNE DAY, Senior Program Officer SANDRA GRAHAM, Senior Program Officer GAYBRIELLE HOLBERT, Senior Program Assistant MEG KNEMEYER, Senior Finance Business Partner DANIEL NAGASAWA, Program Officer CELESTE A. NAYLOR, Information Management Associate TANJA PILZAK, Manager, Program Operations ANDREA REBHOLZ, Program Coordinator ABIGAIL SHEFFER, Senior Program Officer DAVID SMITH, Senior Program Officer DIONNA WISE, Program Coordinator PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION xiii 

Preface The summary charge for the Committee for a Decadal Survey of Astronomy and Astrophysics (Astro2020; hereafter the “steering committee”) reads as follows: The National Academies of Sciences, Engineering, and Medicine shall convene an ad hoc survey committee and supporting study panels to carry out a decadal survey in astronomy and astrophysics. The study will generate consensus recommendations to implement a comprehensive strategy and vision for a decade of transformative science at the frontiers of astronomy and astrophysics. The complete statement of task and its scope is provided in Appendix A of this report. The steering committee, with inputs from 13 expert panels encompassing the breadth of astronomy and astrophysics, was specifically asked to (1) provide an overview of the current state of astronomy and astrophysics science, and technology research in support of that science, with connections to other scientific areas where appropriate; (2) identify the most compelling science challenges and frontiers in astronomy and astrophysics, which shall motivate the committee’s strategy for the future; (3) develop a comprehensive research strategy to advance the frontiers of astronomy and astrophysics for the period 2022-2032 that will include identifying, recommending, and ranking the highest-priority research activities; (4) utilize and recommend decision rules, where appropriate, that can accommodate significant but reasonable deviations in the projected budget or changes in urgency precipitated by new discoveries or unanticipated competitive activities; (5) assess the state of the profession, including workforce and demographic issues in the field, identify areas of concern and importance to the community, and where possible, provide specific, actionable, and practical recommendations to the agencies and community to address these areas. Astro2020 was sponsored by the National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), the Department of Energy (DOE) Office of High Energy Physics, and the Air Force Office of Space Research (AFOSR). These federal agencies all participate in different aspects of the U.S. space- and ground-based astronomy and astrophysics program. Internally, the decadal survey effort at the National Academies was a joint project of the Board on Physics and Astronomy and the Space Studies Board. This is the seventh decadal plan for astronomy and astrophysics conducted over the history of the National Academies. The scope of the science assessed by the decadal survey is broad, encompassing all aspects of observational, theoretical, and computational astronomy including ground-based solar observations, but specific activity recommendations were limited to those administered by NSF Division of Astronomical Sciences and the NASA Astrophysics Division. Scientific areas in astronomy and astrophysics pursued by the DOE Office of High Energy Physics were also included in the study, but activity recommendations were limited to NSF and NASA as described above. The committee was also tasked with assessing three space projects, WFIRST (since renamed the Nancy Grace Roman Space Telescope), Athena (Advanced Telescope for High-Energy Astrophysics), and LISA (Laser Interferometer Space Antenna)—the latter two being European-led missions with significant NASA participation. These three projects were highly ranked priorities for the 2010 New Worlds New Horizons (NWNH) decadal survey, also called Astro2010, and are under development but not yet launched.1 The committee was invited to comment on the status and future direction of NASA 1 National Research Council, 2010, New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C., https://doi.org/10.17226/12951. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION xiv 

support for these projects, but they were not considered together with nor ranked against new projects proposed for the coming decade. Likewise the survey was invited to consider the status and evolution of ongoing programs within NASA’s program of record. STUDY PROCESS AND PARTICIPANTS Organization of the Astro2020 decadal survey, including the steering committee and 13 expert panels, began with the appointment of the co-chairs in late 2018. The steering committee consisted of 20 members, including the co-chairs. Members were selected to cover as fully as possible the scientific scope of the survey, the range of observational (ground, space, and particle/gravitational astrophysics) and theoretical disciplines, as well as technical and managerial background in space and/or ground-based facilities, and to comprise as representative a group of experts as possible in terms of individual, institutional, and geographical demographics. National Academies policies governing potential conflicts of interest by steering committee and panel members were strictly enforced. In particular, broad and open-minded thinkers were sought out as opposed to advocates for individual missions or subfields. The role of the Executive Officer was also redefined for this survey, to that of a full voting member of the steering committee, with additional administrative and coordination responsibilities as part of the leadership team. All meetings and deliberations for the survey operated under a code of conduct to guide discussions. This code of conduct was developed in addition to the National Academies policy on sexual harassment and bullying for committee members, panelists, and meeting attendees, and in addition to a code of conduct for National Academy of Sciences members. The statement includes a recognition of strongly held, possibly differing opinions; a dedication to open dialogue and open exchange of ideas; and professional, civil, and collegial treatment of colleagues so that an open exchange of ideas can occur. During the course of the panel and steering committee meetings, a number of public information- gathering sessions were held. These included briefings by the agencies (NASA, NSF, DOE), invited presentations by selected projects to the program panels, and other informational sessions sponsored by the other panels or the steering committee. Throughout the survey, the representatives of all three agencies urged the committee to be “ambitious”2 and “aspirational,”3 and the committee hopes that it has risen to this challenge. The charges of the expert panels were similar to that of the Astro2010 survey, but with a few important changes. The number of science panels was increased from five to six panels, to accommodate the very rapid growth over the previous decade of exoplanetary astronomy and multi-messenger astrophysics, while preserving balance across all subject areas. The basic programmatic panel structure from Astro2010 was also retained, except that the Panel on Electromagnetic Observations from Space was divided into two panels, in order to accommodate the very large number of project proposals and community white papers in that area. Finally, two new panels for Astro2020 were appointed—the Panel on an Enabling Foundation for Research (program panel) was charged with evaluating cross-cutting supporting programs (e.g., grants programs, laboratory astrophysics, data archiving and data science, computation, theory), and the Panel on State of the Profession and Societal Impacts (SoPSI) was charged to advise the steering committee on this specific area in its statement of task. These 13 appointed panels (six science, six program, and one state of the profession) comprised 127 members. In addition, a steering committee “liaison” member was appointed to each panel to facilitate the flow of information and communication between panels and the main survey committee. The liaisons participated in the panel discussions but did not hold formal voting rights. In all, 141 individuals participated in the panel deliberations with a purpose of providing input to the steering committee. Each panel drafted its own report, with suggestions for the steering committee to consider, as it held its own deliberations to reach its 2 Presentation to Astro2020 committee by Paul Hertz, NASA Astrophysics Division, July 2019. 3 Presentation to Astro2020 committee by Ralph Gaume, NSF Division of Astronomical Sciences, July 2019. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION xv 

recommendations for the main report. To underscore the importance of the panel reports, they have been published together with this main report as appendixes. The science panels were asked to provide a brief review of the current state of the science in their topic areas and determine four important science questions to be addressed in the next decade and one area that shows great promise for discovery. The program panels were charged to assess the ability of current and proposed projects under consideration to address the science panels’ questions and discovery areas, to comment on the Technical, Risk, and Cost Evaluations (TRACE) of the proposed projects, to identify key areas of technical development or precursor research activities, and to discuss the balance of small, competitively selected activities versus larger strategic investments needed to address the science questions. The program panels were not asked to prioritize or rank projects, but rather to suggest to the steering committee the projects with the best potential to realize the capabilities needed to address the science panels’ questions and discovery areas. The Panel on the State of the Profession and Societal Impacts was asked to gather information on the health and demographics of the astronomy and astrophysics community and make actionable suggestions to the steering committee on the topics of demographics, diversity and inclusion, workplace climate, workforce development, education, public outreach, and relevant areas of astronomy and public policy. Further information about the charges to the panels is found in Appendix A. The information-gathering and deliberative phases of Astro2020 were carefully coordinated. Members of the astronomical community were invited to submit white papers to the survey, and these papers formed the foundation and starting point for all of the panel deliberations. In the first phase, 572 science white papers were received in early 2019. A second call for “activity, project, and state of profession consideration” (APC) white papers in July 2019 elicited 294 responses. Every white paper was assigned to and read by one or more of the panels. The panel meetings themselves were phased. The science panels each held two formal meetings, the program panels each held three formal meetings, and all held several additional teleconferences. Meetings of the science panels took place during the second half of 2019, so that the priorities emerging from the reports of those panels could be incorporated into the program panel deliberations. The science panel chairs presented their findings to the steering committee and the program panels at a face-to-face meeting in December 2019 and delivered their written reports in early 2020. The program panels’ meetings began in November 2019, and they presented their results to the steering committee in May 2020 and delivered initial written reports in June 2020. The SoPSI panel met and deliberated on an independent schedule, including holding a public listening session at the American Astronomical Society meeting on January 6, 2020. The SoPSI report was fully incorporated into the overall deliberations and prioritization phases of the steering committee activities. During the course of the panel deliberations, a number of other inputs were received, and these were especially important for the program panels. After an initial review of all projects proposed for a given panel area, the panels issued requests for information (RFIs) from selected projects to obtain more detailed information that was initially provided in the respective APCs. These included all of the large space and ground “flagship” proposals and selected examples of smaller projects. Selected projects were also invited to present summaries to their respective program panels in public sessions. Many of these projects then underwent a detailed TRACE study, conducted by an independent contractor (The Aerospace Corporation). This independent analysis was mandated by the 2008 NASA Authorization Act, which “directs the Administrator to enter into agreements periodically with the National Academies for decadal surveys to take stock of the status and opportunities for Earth and space science discipline fields and aeronautics research and to recommend priorities for research and programmatic areas over the next decade.”4 Additionally, the act “requires that such agreements include independent estimates of life cycle costs and technical readiness of missions assessed in the surveys whenever possible.” In-house analyses of technology readiness, risk, and cost estimates provided by the project teams themselves supplemented 4 National Aeronautics and Space Administration Authorization Act of 2008, P.L. 110-422, Section 1104 (October 15, 2008). PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION xvi 

this analysis. Details of the TRACE process are provided in Appendix O of this report. This process was formerly labeled “Cost and Technical Evaluation” (CATE) and was conducted for recent National Academies surveys in planetary science and solar and space physics, as well as Astro2010. The schedule for this review was impacted by two outside events—a 35-day government shutdown from December 2018 to January 2019, and the COVID-19 pandemic of 2020 to 2021. The shutdown happened just as science white papers were being solicited, so the deadline for submissions was delayed by a month. The impacts of the COVID-19 pandemic were much more severe. The initial disruptions in March and April 2020 occurred when the program panels were completing their final meetings. Final panel deliberations were held virtually, and delivery of the panel reports to the steering committee were delayed by up to 2 months as everyone adjusted to the new reality of working, caring for children, teaching, and performing service to the community, all while under a stay-at-home order. The greatest impact was on the deliberations of the steering committee, which needed to replace its remaining schedule of four 3- to 4-day face-to-face meetings (out of six total) with more than 20 all-day Zoom meetings. Early into the pandemic, the survey co-chairs and National Academies’ staff decided not to allow the disruptions to compromise the quality or integrity of the survey, and the inevitable result was a several month delay from the original schedule. Included in these virtual meetings were presentations of preliminary results by the program and SoPSI panels during the summer of 2020. After the panel reports were received and assembled, the steering committee proceeded with the main prioritization discussions, fully informed by the panel reports. The steering committee addressed a few additional topics that were not taken up in full by a program panel (e.g., satellite constellations and radio frequency interference). In such cases, working groups were appointed within the steering committee or by committee and cross-panel working groups. The steering committee’s deliberations were aided by the introduction of innovative strategies to assist in reaching consensus in the virtual environment necessitated by COVID-19, such as online voting tools, collaborative online document editing, the utilization of various videoconferencing features, and asynchronous deliberations (Figure P.1). FIGURE P.1 Steering committee members and staff met virtually on May 27, 2021. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION xvii 

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Acknowledgments The completion of an undertaking of this scale and importance relies on the contributions of many, and we conclude this introduction by thanking some of the key people. The members of the steering committee, the panel chairs, and the members of the panels contributed tirelessly to the effort, even after the onset of the pandemic extended the timetables and the complexity of their activities. One individual in particular, Executive Officer Rachel Osten, provided extraordinary service as a logistical and scientific coordinator, valued committee member, and advisor to the co-chairs, and is amply deserving of special mention. We likewise received unflagging support from the National Academics of Sciences, Engineering, and Medicine, including staff from the Board on Physics and Astronomy and the Space Studies Board who supported the work of the panels and steering committee and facilitated the smooth flow of work, meetings, reports, and other activities. We also gratefully acknowledge the contributions of the National Academies staff, interns, and fellows who participated in the survey at various stages——panel staff Alan Angleman, Shenae Bradley, Megan Chamberlain, Arthur Charo, Dwayne Day, Greg Eyring, Sandra Graham, Gaybrielle Holbert, Amisha Jinandra, Christopher Jones, James Myska, Daniel Nagasawa, Erik Saari, Erik Svedberg, and Linda Walker; Christine Mirzayan Science and Technology Policy Fellows Sara Crandall and Emily Moravec; and Lloyd V. Berkner Space Policy Interns Robert Bullard, Benjamin Cassese, Katherine Dzurilla, Tarini Konchady, Sarah Moran, Osase Omoruyi, Genevieve Payne, and Cindy Vo. Above all we wish to thank the survey study co-directors, Abigail Sheffer and Gregory Mack, and program coordinator, Dionna Wise, for their steadfast efforts in overseeing and managing the survey process. Senior advice and leadership was provided throughout the survey by Colleen N. Hartman, director of the Space Studies Board, and James Lancaster, director of the Board on Physics and Astronomy, who are also deserving of special thanks. The foundation materials for Astro2020 were the 867 science and activity, project, and state of profession consideration white papers, which were contributed by thousands of authors from around the astronomical community. The quality of those papers testified to the effort and thought that went into their preparation, and we express our heartfelt thanks to everyone who participated in this effort and made this a truly community-based survey. We are also grateful to the members of our sponsoring agencies, especially Ralph Gaume at the National Science Foundation, Paul Hertz at NASA, and Kathleen Turner at the Department of Energy, and their staffs for sharing their time during the early phases of the survey to provide comprehensive background information and guidance, and for responding to our many requests for further background and statistical information. The Technical, Risk, and Cost Evaluations assessments of the large projects considered by Astro2020 were carried out by the Aerospace Corporation, and we thankfully acknowledge the contributions of their team members, including in particular Randy Persinger and Matthew Marshall. We also thank the many experts who generously contributed their help to the steering committee or its panels as consultants. These include Steven Battel (Battel Engineering), who provided early advice on the setup of the survey processes, and Justin Jonas (South African Radio Astronomy Observatory), Joseph McMullin (Square Kilometre Array Organisation), James Moran (Harvard-Smithsonian Center for Astrophysics), and Antony Schinckel (Commonwealth Scientific and Industrial Research Organisation), who provided expert technical advice to the Panel on Radio, Millimeter, and Submillimeter Observations from the Ground. Sherard Robbins (University of Arizona) kindly served as the facilitator of the Panel on State of the Profession and Societal Impacts listening session at the American Astronomical Society meeting. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION xix 

The scientific vision and recommended strategic program arising from this process are presented in the remainder of this report and will not be summarized here. As with all of the preceding astronomy and astrophysics surveys, difficult choices were necessary, but that is the main reason for conducting these decadal reviews. We hope that we have provided not only an ambitious, inspirational, and aspirational vision and roadmap for the coming decade, but also a pathway towards realizing even greater objectives in the future. Fiona Harrison and Robert Kennicutt, Co-Chairs Committee for a Decadal Survey on Astronomy and Astrophysics 2020 PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION xx 



Acknowledgment of Reviewers This Consensus Study Report was reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise. The purpose of this independent review is to provide candid and critical comments that will assist the National Academies of Sciences, Engineering, and Medicine in making each published report as sound as possible and to ensure that it meets the institutional standards for quality, objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We thank the following individuals for their review of this report: Main Report Stephon Alexander, Brown University Beverly K. Berger, Retired Jeremiah K. Darling, University of Colorado, Boulder Catherine Espaillat, Boston University Lennard Fisk, NAS,1 University of Michigan Kathryn Flanagan, Space Telescope Science Institute Thomas Greene, NASA Ames Research Center Jeffrey Hall, Lowell Observatory Victoria Kaspi, NAS, McGill University Lisa Kewley, Australian National University Christopher Kochanek, Ohio State University Roy Maizel, NASA Headquarters (retired) Christopher F. McKee, NAS, University of California, Berkeley Sally Oey, University of Michigan Scott Tremaine, NAS, Canadian Institute for Theoretical Astrophysics and the Institute for Advanced Study Michael S. Turner, NAS, Kavli Foundation Rainer Weiss, NAS, Massachusetts Institute of Technology Science Panel Appendixes Bangalore Sathyaprakash, University of Pennsylvania Catherine Espaillat, Boston University Katherine Freese, NAS, University of Texas, Austin Lisa Kaltenegger, Cornell University Chip Kobulnicky, University of Wyoming Christopher F. McKee, NAS, University of California, Berkeley M. Claudia Scarlatta, University of Minnesota Risa Wechsler, Stanford University 1 Member, National Academy of Sciences. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION xxii 

Program Panel Appendixes Panel on An Enabling Foundation for Research Marcel Agüeros, Columbia University Beverly K. Berger (retired) Christopher F. McKee, NAS, University of California, Berkeley Arfon Smith, Journal of Open Source Software and GitHub Panel on Electromagnetic Observations from Space 1 Orlando Figueroa, Orlando Leadership Enterprise, LLC Christopher F. McKee, NAS, University of California, Berkeley Lisa A. Poyneer, Lawrence Livermore National Laboratory Dmitry Savransky, Cornell University Alycia J. Weinberger, Carnegie Institution of Washington Panel on Electromagnetic Observations from Space 2 Orlando Figueroa, Orlando Leadership Enterprise, LLC Robert Gehrz, University of Minnesota Dan McCammon, University of Wisconsin, Madison Christopher F. McKee, NAS, University of California, Berkeley Panel on Optical and Infrared Observations from the Ground Lisa Kewley, Australian National University K.D. Leka, NorthWest Research Associates Christopher F. McKee, NAS, University of California, Berkeley Christy Tremonti, University of Wisconsin, Madison Robert E. Williams, University of California, Santa Cruz Panel on Particle Astrophysics and Gravitation Beverly K. Berger (retired) Christopher F. McKee, NAS, University of California, Berkeley Kohta Murase, Pennsylvania State University Steven M. Ritz, University of California, Santa Cruz Panel on Radio Millimeter and Submillimeter Observations from the Ground Jeremiah K. Darling, University of Colorado, Boulder Neal Evans, University of Texas, Austin Christopher F. McKee, NAS, University of California, Berkeley Anna Scaife, University of Manchester State of the Profession and Societal Impacts Appendix Marcel Agüeros, Columbia University Rosanna Alegado, University of Hawaii, Manoa Neta A. Bahcall, NAS, Princeton University Edmund Bertschinger, Massachusetts Institute of Technology Joan Centrella, West Virginia University Alison Coil, University of California, San Diego Kate Follette, Amherst College Felicia Jones, KBR, Inc. Philip (Bo) W. Hammer, Institute for Mathematical and Statistical Innovation Roberta Marinelli, Oregon State University PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION xxiii 

Although the reviewers listed above provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations of this report nor did they see the final draft before its release. The review of this report was overseen by Charles F. Kennel, NAS, University of California, San Diego, and Rosalba Perna, State University of New York at Stony Brook. They were responsible for making certain that an independent examination of this report was carried out in accordance with the standards of the National Academies and that all review comments were carefully considered. Responsibility for the final content rests entirely with the authoring committee and the National Academies. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION xxiv 

Contents SUMMARY 1 PATHWAYS FROM FOUNDATIONS TO FRONTIERS 1-1 2 A NEW COSMIC PERSPECTIVE 2-1 3 THE PROFESSION AND ITS SOCIETAL IMPACTS: GATEWAYS TO SCIENCE, PATHWAYS TO DIVERSITY, EQUITY, AND SUSTAINABILITY 3-1 4 OPTIMIZING THE SCIENCE: FOUNDATIONS 4-1 5 EVALUATING AND BALANCING THE OPERATIONAL PORTFOLIO 5-1 6 TECHNOLOGY FOUNDATIONS AND SMALL AND MEDIUM-SCALE SUSTAINING PROGRAMS 6-1 7 REALIZING THE OPPORTUNITIES: MEDIUM- AND LARGE-SCALE PROGRAMS 7-1 APPENDIXES A Statement of Task and Panel Descriptions A-1 B Report of the Panel on Compact Objects and Energetic Phenomena B-1 C Report of the Panel on Cosmology C-1 D Report of the Panel on Galaxies D-1 E Report of the Panel on Exoplanets, Astrobiology, and the Solar System E-1 F Report of the Panel on the Interstellar Medium and Star and Planet Formation F-1 G Report of the Panel on Stars, the Sun, and Stellar Populations G-1 H Report of the Panel on an Enabling Foundation for Research H-1 I Report of the Panel on Electromagnetic Observations from Space 1 I-1 J Report of the Panel on Electromagnetic Observations from Space 2 J-1 K Report of the Panel on Optical and Infrared Observations from the Ground K-1 L Report of the Panel on Particle Astrophysics and Gravitation L-1 M Report of the Panel on Radio, Millimeter, and Submillimeter Observations from the Ground M-1 N Report of the Panel on State of the Profession and Societal Impacts N-1 O Independent Technical, Risk, and Cost Evaluation O-1 P Acronyms P-1 Q Committee and Staff Bios Q-1 PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION xxv