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

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Summary We live in an extraordinary period of discovery in astronomy and astrophysics. Six Nobel Prizes have been awarded over the past decade alone for discoveries based on astronomical data (dark energy, gravitational waves, neutrino oscillations, the discovery of exoplanets, cosmology, supermassive black holes). Many of the ambitious scientific visions of the 2010 New Worlds New Horizons1 (NWNH) decadal survey are being fulfilled, but momentum has only grown. We stand on the threshold of new endeavors that will transform not only our understanding of the universe and the processes and physical paradigms that govern it, but also humanity’s place in it. This report of the Committee for a Decadal Survey on Astronomy and Astrophysics 2020 (Astro2020) proposes a broad, integrated plan for space- and ground-based astronomy and astrophysics for the decade 2023-2032.2 It also lays the foundations for further advances in the following decade. This is the seventh in a sequence of decadal survey studies in this field from the National Academies of Sciences, Engineering, and Medicine. This survey examines the program of record, providing advice on the major projects from prior surveys that are yet to be completed. It also lays out priorities for future investments driven by scientific opportunities. The recommendations in this report advance foundational activities that support the people who drive innovation and discovery, and that promote the technologies and tools needed to carry out the science. The report also recommends sustaining activities on a broad range of cost and timescales, as well as activities that enable future visionary projects by maturing them scientifically and technically. Finally, the recommendations set in motion the construction of frontier facilities that will change the view and understanding of the cosmos. The survey is bounded by plausible budget scenarios based on briefings from the sponsoring agencies—the National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), and the Department of Energy (DOE). Within these bounds, the survey aims high, reflecting this time of great scientific promise and progress, with opportunities to pursue some of the most compelling scientific quests of our times. THE SCIENTIFIC OPPORTUNITIES The survey’s scientific vision is framed around three broad themes that embrace some of the most exciting new discoveries and progress since the start of the millennium, and that promise to address some of the most fundamental and profound questions in our exploration of the cosmos. The first theme, Worlds and Suns in Context builds on revolutionary advances in our observations of exoplanets and stars and aims to understand their formation, evolution, and interconnected nature, and to characterize other solar systems, including potentially habitable analogs to our own. New Messengers and New Physics will exploit the new observational tools of gravitational waves and particles, along with temporal monitoring of the sky across the electromagnetic spectrum and wide-area surveys from the ultraviolet and visible to microwave and radio to probe some of the most energetic processes in the universe and also address the nature of dark matter, dark energy, and cosmological inflation. Research in the third theme, Cosmic 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. 2 The statement of task specified a date range of 2022-2032. This has been adjusted to more accurately reflect the range that the Survey will affect. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION S-1 

Ecosystems, will link observations and modeling of the stars, galaxies, and the gas and energetic processes that couple their formation, evolution, and destinies. Within each of these broad and rich scientific themes, three priority areas motivate recommended investments over the coming decade. “Pathways to Habitable Worlds” is a step-by-step program to identify and characterize Earth-like extrasolar planets, with the ultimate goal of obtaining imaging and spectroscopy of potentially habitable worlds. “New Windows on the Dynamic Universe” is aimed at combining time-resolved multi-wavelength electromagnetic observations from space and the ground with non-electromagnetic signals to probe the nature of black holes, neutron stars, the explosive events and mergers that give rise to them, and to use signatures imprinted by gravitational waves to understand what happened in the earliest moments in the birth of the universe. “Unveiling the Drivers of Galaxy Growth” is aimed at revolutionizing our understanding of the origins and evolution of galaxies, from the nature of the tenuous cosmic webs of gas that feed them, to the nature of how this gas condenses and drives the formation of stars. THE RECOMMENDED PROGRAM   Major leaps in observational capabilities will be realized in the coming decade when new large telescopes and missions commence science operations (Table 7.1). Recommended by previous surveys, with some undertaken with international partners, these projects and programs are an essential base upon which the survey’s scientific vision is built. It is essential that these initiatives be completed, and the scientific programs be supported at levels that ensure full exploitation of their potential by the U.S. scientific community. Going forward, this survey lays out a strategy for federal investments aimed at paving a pathway from the foundations of the profession to the bold scientific frontiers. Large Programs that Forge the Frontiers These scientific visions—Pathways to Habitable Worlds, New Windows on the Dynamic Universe, and Unveiling the Drivers of Galaxy Growth— require the major recommended investments in large projects to begin design and construction in the coming 10 years (Tables S.5 and S.6; Figure S.1).3 In space, achieving the community’s most ambitious and visionary ideas in a sustainable way, and realizing the broad capabilities demanded by the richness of the science, requires a re-imagining of the ways in which large missions are planned, developed, and implemented. The Great Observatories Mission and Technology Maturation Program (Table S.5) would provide significant early investments in the co-maturation of mission concepts and technologies, with appropriate decadal survey input on scope, and with checks and course corrections along the way. Inspired by the vision of searching for signatures of life on planets outside of the solar system, and by the transformative capability such a telescope would have for a wide range of astrophysics, the survey recommends that the first mission to enter this program is a large (~6 m aperture) infrared/optical/ultraviolet (IR/O/UV) space telescope. The scientific goals of this mission, when achieved, have the potential to profoundly change the way that human beings view our place in the universe. With sufficient ambition, we are poised scientifically and technically to make this transformational step. This endeavor represents a quest that is on the technical forefront, is of an ambitious scale that only NASA can undertake, and it is one where the United States is uniquely situated to lead the world. If maturation proceeds as expected, the survey recommends that formulation and implementation begin by the end of the 2020 decade. To prepare for future large, strategic missions, 5 years after beginning the maturation program for the IR/O/UV mission, the survey 3 For space, large projects are defined as those with costs exceeding $1.5 billion. For ground-based initiatives, large projects are defined as those exceeding $130 million for the total program investment. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION S-2 

recommends commencing mission and technology maturation of both a far-IR and an X-ray large strategic mission, both scoped to have implementation costs in the $3 billion to $5 billion range. Because of the powerful potential that large (20–40 m) telescopes with diffraction-limited adaptive optics have for astronomy, and because of the readiness of the projects, the survey’s priority for a frontier ground-based observatory is a significant U.S. investment in the Giant Magellan Telescope (GMT) and Thirty Meter Telescope (TMT) projects, ideally as components of a coordinated U.S. Extremely Large Telescope Program (ELT) program. These observatories will create enormous opportunities for scientific progress over the coming decades and well beyond, and they will address nearly every important science question across all three priority science areas. After this, given technical and scientific progress over the last decades, ground-based cosmic microwave background (CMB) studies are poised in the next decade to make a major step forward, and the CMB Stage 4 (CMB-S4) observatory (with support from NSF and DOE) will have broad impact on cosmology and astrophysics. It is also essential to astronomy that the Karl Jansky Very Large Array (JVLA) and Very Long Baseline Array (VLBA), which have been the world-leading radio observatories, be replaced by an observatory that can achieve roughly an order of magnitude improvement in sensitivity compared to those facilities. The Next Generation Very Large Array (ngVLA) will achieve this, with a phased approach where design, prototyping, and cost studies are completed and reviewed in advance of commencing construction. Finally, neutrino observations are important to understanding some of the most energetic processes in the universe, and the Ice Cube-Generation 2 (IceCube-Gen2) observatory will make advances in important astrophysics questions, although it is beyond the charge of this survey to recommend it.4 Programs that Sustain and Balance the Science Turning to medium-scale missions and projects, the scientific richness of a broader set of themes—exploring New Messengers and New Physics, understanding Cosmic Ecosystems, and placing Worlds and Suns in Context—as well as the need to capitalize on major existing investments and those coming online in the next decades drive the essential sustaining projects (Tables S.5 and S.6). In space, the highest-priority sustaining activity is a space-based time-domain and multi-messenger program of small and medium-scale missions. In addition, the survey recommends a new line of probe missions to be competed in broad areas identified as important to accomplish the survey’s scientific goals. For the coming decade, a far-IR mission, or an X-ray mission designed to complement the European Space Agency (ESA’s) Athena mission, would provide powerful capabilities not possible at the Explorer scale. With science objectives that are more focused compared to a large strategic mission, and a cost cap of $1.5 billion, a cadence of one probe mission per decade is realistic. The selection of a probe mission in either area would not replace the need for a future large, strategic mission. For ground-based projects, the highest-priority sustaining activity is a significant augmentation and expansion of mid-scale programs, including the addition of strategic calls to support key survey priorities. The survey also strongly endorses investments in technology development for advanced gravitational wave interferometers, both to upgrade NSF’s Laser Interferometer Gravitational-Wave Observatory (LIGO), and to prepare for the next large facility.5 4 IceCube is supported and managed by the NSF Division of Physics, rather than the Division of Astronomical Sciences. 5 Technology development for gravitational wave detection is funded out of the NSF Division of Physics. The survey strongly endorses the importance of the science to astronomy and astrophysics. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION S-3 

Foundational Activities A successful decadal survey strategy requires serious attention to the smaller but vital investments in the foundations of the research. The people who make up the profession are the most fundamental component of the research enterprise, without whom the ambitious facilities, instruments, and experiments, as well as the promised transformative discoveries, would lie unfulfilled. Recognizing that diversity is a driver of innovation, and that the astronomy and astrophysics enterprise can be at its most innovative only when it maximizes and fully utilizes the broadest range of human talent, the survey forwards several crucial programs (Table S.1) to support early-career entrants, with a strong emphasis on broadening access, removing barriers to participation, and creating an environment that eschews harassment and discrimination of all kinds. The future of the field also requires that greater attention be paid to issues of sustainability and accountability, and several recommendations address these issues. Among the recommendations regarding the state of the profession, the most urgent need is maintenance of accurate data on funding outcomes, because it is sufficiently critical to the other recommendations. (Table S.1). Science cannot progress without the essential support to individual investigators who take the data and transform them into scientific understanding and discovery. Accordingly, augmenting the NSF Astronomy and Astrophysics Grants program is the highest priority among the foundational recommendations. Science also cannot progress without the necessary tools, such as archives, data pipelines, laboratory work, and theoretical tools that provide the essential, cross-cutting foundations. The computational revolution continues to transform the conduct and culture of astronomy through the growing roles of large surveys and shared public data sets, big-team research, applications of machine learning, and numerical simulations, among others, and research investments will need to evolve to adapt to this changing landscape. Several critical areas require a healthier balance in order to optimize the scientific returns on past and future major investments (Table S.2). The currently operating facilities on the ground and in space, along with the scientists who use them, are the primary engines of scientific discovery and progress in astronomy and astrophysics. In this regard, it is essential to adequately support the costs of operating facilities in space and on the ground, review them regularly during their productive lives, and for ground-based observatories, maintain them as premier facilities with modern, state-of-the-art instrumentation. Table S.3 summarizes this report’s recommendations relative to the agencies’ operational portfolios. A balanced portfolio that includes a healthy investment in small- and medium-scale projects that are competed, draws from the ingenuity and breadth of the community, and enables science on a broad range of costs and timescales is essential for sustaining a vibrant astronomy and astrophysics program. These activities sustain scientific progress, amplify and enhance return from operating missions and observatories, and respond nimbly to new discovery. The survey recognizes the foundational need for supporting basic technology development and the crucial role small- and medium-scale projects play in broadening science and as a means of developing the next generation of technologists and instrumentalists. Table S.4 summarizes recommendations aimed at strengthening these. Enabling Future Visions The community’s most ambitious and visionary ideas now require timelines that are pan-decadal and even multi-generational. This is particularly true for NASA’s large strategic missions and NSF’s premier observatories that are driven by transformative scientific visions but are technically challenging. They also represent large investments of resources. Optimizing the cadence of major facilities and developing them in a sustainable way that ensures the appropriate level of maturity prior to a decadal or agency commitment and tighter control on ultimate project costs requires new, enabling programs and approaches. The Great Observatories Mission and Technology Maturation Program would provide a new approach for developing large space strategic missions. In addition, for all large projects, the survey PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION S-4 

provides decision rules and recommends reviews, where required, to ensure technical, scientific, and cost- readiness prior to commitment of major resources. The survey also identifies a few future projects that are targets for significant investment in maturation for consideration by future decadal surveys, as summarized in Tables S.5 and S.6, column 2. A very large fraction of the astronomical community contributed to this survey through the almost 900 excellent science, activity, program, and state of the profession white papers and through active engagement in town hall meetings. The program laid out in this report represents a collective vision for the future and will require the engagement of a broad community to advance. FIGURE S.1 Timeline for the recommended medium and large programs and projects. The starting point of each, indicated by the logos, shows the projected start of science operations for missions and observatories, or the start date of the program. The boxes on the right show the survey’s three broad science themes, and the placement of the logos to the left of the boxes indicate which activities address the indicated theme. Tables S.1 to S.6 below summarize the survey’s recommended program, divided into tables that follow the chapter structure of the report. These tables are only intended to provide a capsule summary of the recommendations. The survey’s report provides detailed guidance on the implementation of major programs and emphasizes the range of scales and capabilities necessary for a healthy, balanced, and visionary program. The ordering of projects in the tables below does not indicate priority ranking. The body of the report provides guidance on which programs or projects are the most urgent and have highest PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION S-5 

priority within their programmatic category, emphasizing that even within a given cost scale, a balance of programmatic function is required. TABLE S.1 Foundations of the Profession Per Year Budget Increases Cross- Relative to FY2019 Agency Reference in Recommendation Topic Agency Budget Allocations (FY2020$) Chapter 3 Faculty diversity, and early-career NSF (augmentation of) $2.5M: p. 3-14 faculty awards NASA $1M NSF; $1M NASA; $0.5M DOE DOE Workforce development/diversity, NSF (augmentation of) $4.5M: p. 3-22 bridge programs and minority serving NASA $1.5M NSF; $3M NASA institutions partnerships Undergraduate and graduate NSF (augmentation of) $3M: p. 3-23 “traineeship” funding NASA $1M NSF; $1M NASA; $1M DOE DOE Independent postdoc fellowships NSF (augmentation of) $1M: p. 3-23 NASA $0.5M NSF; $0.5M NASA Treat discrimination and harassment as NSF N/A p. 3-27 professional misconduct NASA DOE Collecting, evaluating, and reporting NSF (augmentation of) $1M p. 3-29 demographic data and indicators NASA Split NSF/NASA pertaining to equitable outcomes Include diversity in evaluation of NSF N/A p. 3-30 funding awards NASA DOE Establish community astronomy model Community N/A p. 3-35 for observatory sites Mitigation of radio-frequency and NSF TBD after evaluation pp. 3-38 and optical interference from sources NASA 3-40 including satellite constellations Climate change mitigation actions Community N/A p. 3-42 PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION S-6 

TABLE S.2 The Research Foundation Per Year Budget Increases Relative to FY2019 Agency Cross-Reference Budget Allocations Page Number, Recommendation Topic Agency (FY2020$) Ch. 4 Compile and regularly report data on NSF N/A 4-3 proposal submissions and success rates NASA DOE Augmentation to NSF Astronomy and NSF (augmentation) ramps up to 4-8 Astrophysics Grants program additional $16.5M/yr (FY20) by 2028 Augmentation and restoration of annual NASA (augmentation) ramps up to 4-10 proposal calls for Astrophysics Theory additional $2.5M/yr (FY20) Program by 2028 Support for large key projects on NSF N/A 4-11 MREFC facilities Improve coordination among U.S. data NSF TBD depending on outcome 4-20 centers supported by NSF and NASA NASA of study Data pipeline development, archiving NSF TBD depending on plan 4-21 for ground-based telescopes Augmentation and improved NSF (augmentation of) ~$2 M/yr, 4-28 coordination of laboratory astrophysics NASA TBD after plan is developed funding TABLE S.3 Sustaining the Operating Portfolio Cross-Reference Budget Page Number, Recommendation Topic Agency (FY2020$) Ch. 5 New MREFC facilities contingent on NSF N/A 5-5 development of plan for supporting operations and maintenance costs NSF to establish regular cadence of NSF N/A 5-6 portfolio reviews of operating facilities End SOFIA operations by 2023, NASA No impact if adopted 5-12 consistent with current NASA plan PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION S-7 

TABLE S.4 The Technological Foundations Per Year Budget Increases Relative to FY2019 Agency Cross-Reference Page Budget Allocations Number, Recommendation Topic Agency (FY2020$) Ch. 6 Augmentation to NASA Astrophysics NASA (augmentation of) $4M/yr 6-4 Research and Analysis program Continue NASA Strategic NASA N/A 6-5 Astrophysics Technology (SAT) program, expand eligibility to Explorer and Probe mission development Augmentation to NSF Advanced NSF (augmentation of) $8M/yr 6-6 Technologies and Instrumentation starting 2023, ramp up to (ATI) program $14M additional by 2028 (assumes current budget is $6M/yr) Review NASA’s balloon program for NASA TBD depending on outcome 6-8- optimal balance of review TABLE S.5 New Medium and Large Initiatives: Space Cross-Reference Programmatic Cost Appraisal Page Number Recommendation Topic Function (FY2020$) Ch. 7 Great Observatories Mission Enabling future $1.2B this decade 7-11 and Technology Maturation frontier projects Program for IR/O/UV (first half of decade), far-IR and X- ray (second half of decade) missions Near- Frontier project, to $11B (estimated) 7-17 Infrared/Optical/Ultraviolet begin after maturation telescope with high-contrast program imaging capability Time Domain and Multi- Sustaining scientific TBD ($500-800M 7-19 messenger Follow-Up balance and scale this decade est.) Program Astrophysics Probe Mission Sustaining scientific $1.5B cost cap 7-20 Program balance and scale PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION S-8 

TABLE S.6 New Medium and Large Initiatives: Ground Capital Cost Cross- Recommendation Programmatic (FY2020$) Operations Cost Reference Page Topic Function (TRACE) (FY2020$) Number Ch.7 Extremely Large Frontier Project $1.6B (NSF share of $32M/yr 7-24, 7-25 Telescope Program $5.1B total project (NSF share of the (ELT) cost) $98M total) Stage 4 Cosmic Frontier Project $660M DOE+NSF; $17M/yr (NSF 7-26 Microwave NSF share $273M share of $40M/yr) Background Observatory (CMB- S4; joint NSF/DOE) Next Generation Enabling $2.5B (NSF share of $98M/yr; NSF 7-28 Very Large Array development $3.2B project cost) Share $73M/yr (ngVLA) program, followed by construction if possible Augmentation of Sustaining Ramps up to Operations in total 7-29, 7-30 Mid-scale Program: $50M/yr total for program funding open and strategic Mid-scale calls Innovations Program and Mid-scale Research Infrastructure Technology Enabling N/A N/A (not NSF 7-31 development for development AST) gravitational wave program for LIGO upgrades and future frontier for future GW observatories observatories IceCube-Gen2 Frontier project N/A N/A (not NSF 7-32 AST) PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION S-9