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

Chapter: 7 Realizing the Opportunities: Medium- and Large-Scale Programs

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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"7 Realizing the Opportunities: Medium- and Large-Scale Programs." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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7 Realizing the Opportunities: Medium- and Large-Scale Programs The previous chapters laid out a roadmap for how multiple elements of the astrophysics enterprise need to work together to realize our scientific ambitions, and to do so in a manner that uses and supports the community’s full potential. This chapter presents the decadal survey committee’s recommendations for medium and large programs, program augmentations, and projects, as defined by their budgetary requirements. The recommendations here flow from the science and program panel reports. The science panels identified a set of compelling and inspiring science questions that are organized into three broader themes in Chapter 2: Worlds and Suns in Context, New Messengers and New Physics, and Cosmic Ecosystems. These science themes reflect that we have entered a new phase of astronomical exploration in multiple dimensions, combining detailed characterization of known classes of objects with opening up the vast discovery space of the unknown. In addition to the broad science themes, the survey committee identified three priority areas that define the scientific frontiers, and motivate the recommended new investments in large projects: Unveiling the Drivers of Galaxy Growth; New Windows on the Dynamic Universe; and Pathways to Habitable Worlds. The steering committee aggregated and balanced the panel report and agency budget guidance inputs to arrive at the program described below. We now see that the cosmos is dynamic, roiling and explosive with pulses of electromagnetic radiation, gravitational waves, and elementary particles streaming through space carrying messages of their exotic origins. Once separate lines of investigation, for example, black hole formation and large- scale structure, are now known to be inextricably intertwined. The ability to see biology’s impact on the atmosphere—signatures of life—from distant exoplanets is also now within reach. With the same capabilities, we can also characterize the gaseous halos surrounding galaxies that fuel their growth and unravel how stars live, die, and seed the universe with the elements necessary for life. This vast array of phenomena is taking place in a universe filled with the cosmic microwave background whose study can tell us about transient sources, the contents of the universe, and the production of gravitational waves from the Big Bang. In presenting the analysis of the highest-priority medium and large-scale projects to pursue, the survey emphasizes scientific breadth and balance of project scales. The interconnected science questions to be addressed require an interconnected program that recognizes and draws on program elements that vary in size, timescale, and wavelength/messenger, from radio waves to relativistic neutrinos. Astronomy is fortunate that most of its facilities are multipurpose and can simultaneously address multiple distinct science questions. Likewise, a major theme stressed by the program panels and the survey committee’s own analysis is that because different wavelengths and messengers provide such essentially different and complementary views of the universe, a diversity of observational resources is needed to tackle the questions identified by the science panels. 7.1 CONTINUING PROGRAMS AND PROJECTS IN DEVELOPMENT The new recommended medium- and large-scale activities build on missions and projects from prior surveys that have yet to begin scientific operation (Table 7.1). This survey assumes that these PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-1

compelling programs will be completed and sustained through their scientifically productive lifetimes. Ambitious and transformative large-scale efforts often take multiple decades to realize, and all of those scheduled for completion in the coming decade will provide important capabilities upon which the survey’s scientific goals rely. Further, programs resulting from decadal recommendations, such as NASA’s Explorers and NSF’s Mid-Scale Innovations Program play an essential role in sustaining scientific breadth and ensuring timely response to new opportunities. These continued and future capabilities are essential underpinnings of the survey’s new recommendations. On the ground, the Vera Rubin Observatory, with science commencing in late 2023 or early 2024, will “conduct a deep survey over an enormous area of sky, and do it with a frequency that enables images of every part of the visible sky to be obtained every few nights.”1 Several of the survey’s priority programs are designed to support follow-up of the Rubin Observatory’s static and dynamic observations. The Daniel K. Inouye Solar Telescope (DKIST) will complete commissioning this year, and will begin to observe the Sun’s fundamental magnetic and plasma processes to elucidate the role that magnetic fields and their interactions play in driving solar activity. The Mid-Scale Innovations Program, established as a result of Astro2010, is essential to the scientific balance of the Nation’s ground-based investments and has proven to be extremely cost-effective. While it has yet to ramp up to its envisioned level, it is already providing diverse scientific capabilities and community access to private facilities strongly emphasized by this survey. All three programs are essential to the scientific future, and further augmentations to the NSF mid-scale programs is one of the Sustaining Program recommendations (Section 7.6.2). In space, the James Webb Space Telescope (JWST) is a powerful strategic mission expected to launch by the end of this year, that, among many other things, will reach back in time to observe the first stages in galaxy formation, complementing the survey’s focus on unveiling the drivers of galaxy growth more locally. JWST will also characterize the inner parts of other solar systems and the potentially habitable worlds orbiting small M stars, laying the foundation for the Astro2020 program that will extend this to further distances and Sun-like stars. In the middle part of the decade the Nancy Grace Roman Space Telescope will begin its cosmology and exoplanet microlensing surveys, and with a field of view more than one hundred times greater than Hubble, will provide powerful new capabilities for General Observers (GO). NASA is also a partner in The European Space Agency (ESA)’s M-class Euclid mission, as well as the L-class Athena and Laser Interferometer Space Antenna (LISA) missions. Euclid will complement the cosmological surveys from Roman and Rubin. Athena realizes some of the capabilities of the International X-ray Observatory (IXO) recommended by Astro2010, and will probe the hot, energetic universe, and will make important and unique contributions to the Cosmic Ecosystems theme. LISA will expand gravitational wave sensitivity to low frequencies, and will be an important foundational component of the New Windows on the Dynamic Universe priority science area. These missions will provide unique and powerful observational capabilities and science reach not duplicated by any other elements of the program advanced by this survey. Finally, the Explorer program has recently reached the enhanced selection rates envisioned by Astro2010, and is providing high value scientific returns responsive to the emphasis on scientific breadth and balance of mission scales. Conclusion: The decadal survey committee’s recommendations for advancing the new programs or augmentations are predicated on the assumption that the major astrophysics facilities and missions in NASA, NSF, and DOE’s current plans are completed and fully supported for baseline operations and science. New recommendations for space are additionally predicated on the assumption that NASA’s Explorer program maintains the current, healthy selection rate. Section 7.7 provides some advice and recommendations for NASA regarding Roman, Athena and LISA intended to ensure optimal scientific return to the U.S. community from these important missions. 1 See https://www.lsst.org/about PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-2

TABLE 7.1 Medium and Large Programs and Projects in Development GROUND Mid-scale Innovations Program Competed projects in the Scientifically broad, includes new range $4 million– $100 observing capabilities, million instrumentation, archiving, and data accessibility. Daniel K. Inouye Solar Telescope 4 m optical telescope Stellar magnetic fields, Solar activity Vera Rubin Observatory 8.4 m aperture optical Nature of dark matter, dark energy, wide field telescope cataloging the Solar system, exploring the dynamic sky SPACE Explorer Program Includes completing Scientifically broad, including recent current selections and missions and MoOs emphasizing X- maintaining a cadence of ray spectroscopy, polarimetry, all-sky two MIDEX, two SMEX infrared spectroscopy, and time and four MoOs/decade. domain astrophysics. James Webb Space Telescope 6.5 m IR telescope First galaxies, star and planet formation, cosmic feedback Roman Space Telescope 2.4 m wide field of view Cosmology, exoplanet microlensing, O/IR telescope GO program U.S. contribution to Euclid (ESA 1.2 m aperture telescope, Dark matter, dark energy, expansion led M-class mission) optical imager/near-IR history of the universe. spectrometer/photometer U.S. contribution to Athena (ESA Large area X-ray The hot and energetic universe: black led L-class mission) spectroscopy and imaging holes, galaxy halos, neutron stars U.S. contribution to LISA (ESA Low-frequency massive black hole mergers, white led L-class mission) gravitational wave dwarf binaries, stochastic background interferometer 7.2 BUDGETARY GUIDANCE FOR NEW PROGRAMS AND PROJECTS A primary objective of the survey is to develop an integrated roadmap for astrophysics that is both aspirational and foundational, while also conforming to reasonable expectations regarding budgets available from the federal government. NASA and NSF presented budget guidance, which provided the framework within which the survey committee established its recommended program.2 NASA provided information on its Astrophysics budget in July 2019, which was then updated in August 2020. The budget was broken into two parts, NASA budgetary requests for FY2021 through FY2025, and a budget extrapolation from FY2026 through FY2040. This budget extrapolation started at $1.9 billion in FY2025. The most optimistic scenario provided by NASA, adopted as the guidance for this survey, has the budget growing after FY2025 at approximately 2 percent per year to $2.5 billion in FY2040 and beyond. For inflating project budget profiles, the analysis uses an inflation rate of 2.7% per year, as derived from NASA’s new start inflation index for FY2020.3 NASA’s guidance for the most optimistic budget growth, shown by the solid blue line in Figure 7.10, is less than projected for inflation. Approximately $1 billion is held aside to fund existing NASA Astrophysics Programs; the remainder is available for strategic initiatives associated with the survey recommendations. 2 See Section 7.8 for additional details of the NASA and NSF budgetary analysis. 3 https://www.nasa.gov/sites/default/files/atoms/files/2019_nasa_new_start_inflation_index_for_fy20_final2.xls x). PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-3

The NSF budget projections were presented by the agency in July 2019 and were updated in August 2020. The budget guidance was divided into two lines: the Astronomy Division (AST) budget, which covers ongoing research grants, education, facility operations, and administration; and the agency- wide Major Research Equipment and Facilities Construction (MREFC) line, which funds construction of medium to large facilities with costs exceeding approximately $135 million. The MREFC guidance ended in 2030, and it is extrapolated forward from this assuming 2.7 percent inflation, the same as that used for inflating project budget requirements.4 Figure 7.8 (solid blue line) shows the MREFC budget profile consistent with the agency guidance. The NSF AST budget was given to the survey for 2019 only. Figure 7.9 shows the starting point, at approximately $290 million, consistent with the current AST budget and programs. The future growth required to accommodate inflation, as well as the initiatives recommended by the survey is shown in Figure 7.9, broken down by major constituent components. The survey did not receive specific budgetary guidance from the DOE, however DOE communicated programmatic support for initiatives aligned with its scientific objectives, including investigation and refinement of fundamental cosmological parameters and the nature of dark matter, as aligned with the objectives of its High Energy Physics (HEP) program. 7.3 PRIORITIZATION PRINCIPLES Multifaceted considerations are involved in identifying the highest priority programs and projects for investment of federal funding. Major investments must advance a bold and broad scientific vision, while at the same time ensuring a balanced program that responds to scientific opportunity. Astronomy and astrophysics advances in a global context, and the survey committee recognized and responded to the need for synergy with, and complementarity to, activities world-wide. Especially for ground-based observatories, private institutions and philanthropic entities have been, and continue to be central to some of the most ambitious endeavors. The survey committee carefully considered how to best leverage these private-public partnerships in a way that achieves ambitious science and also advances the aspirations of the entire community. There is also the challenging issue of balancing scientific ambition with feasibility and timeliness. All of these factors shaped the recommended programs, and their phasing. Maintaining a program that is balanced along many axes was a driving objective in the design of the portfolio of projects and programs advanced here. Of primary importance is that this portfolio must be scientifically balanced and broad. Supporting a range of project scales is not only essential for achieving this, but also for balancing science return in the near term with longer-term, more ambitious goals. It also maintains the crucial flexibility to respond rapidly to new discoveries. The survey committee also strived to prioritize both efforts that would maximize return on new facilities that will come online in this decade, as well as projects that will bring about wholly new capabilities in the future. A final consideration was the need to build foundations for future large strategic missions so that their scientific capabilities and scope can be better understood in the next decade, while at the same time advancing the highest priority project now. In prioritizing the new, large projects, the survey committee adopted a set of guiding principles. Primary among these is that large strategic missions and MREFC-scale observatories must each advance a broad set of Astro2020’s priority science questions. The survey committee was also guided by the judgement that the estimated time from inception to science for any recommended project is an important consideration, and it must be based on a schedule analysis and assume optimal, but realistically achievable budget profiles. The rationale for this is to balance timeliness and scope in a rapidly evolving scientific landscape. These principles shaped not only the elements in the program advanced here, but also their scale. These guidelines also led to the identification of key decision points, designed to enable rescoping to adapt to project uncertainties and risks, as well as to uncertainties in the future budgetary landscape. These same guidelines can also be used to take advantage of opportunities if the budgetary 4 Ibid. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-4

climate improves due to changes in federal funding, private philanthropy, or increased international participation. 7.4 SUMMARY OF RECOMMENDED NEW MEDIUM AND LARGE PROJECTS AND ACTIVITIES (2023 – 2033) The survey committee has developed a vision that capitalizes on the tremendous momentum and scientific opportunities before us this decade. Laid out below is an ambitious roadmap for high-priority space- and ground-based large and medium scale initiatives that are both compelling and ready to begin implementation in the decade 2023–2033 (Table 7.2). The program includes new, large-scale projects, as well as initiatives that sustain and build on past investments, that harness and advance the creativity of the entire talent base, and that prepare groundwork for future decades. The initiatives in this roadmap are given priority, based on criteria described above, over the many exciting projects that were presented by the community. This program necessarily extends beyond the decade. At the large scale, the ambitious, transformative projects that the survey advances will take more than a decade to bring to fruition. In developing the program, key milestones are established where evaluation of these projects and guidance on their scope and direction will be required. These future assessments are needed to assure that the appropriate balance between ambition and timeliness is maintained considering evolving budgetary and technical realities. Cognizant that new opportunities will certainly arise in the coming decade in response to scientific discovery and technical advances, the survey balances present ambition with leaving room for future initiatives to start in the 2033–2043 timeframe. The decadal survey roadmap advances the Astro2020 scientific agenda through a balance of major programs, projects and medium-scale observatories and missions. The priority activities are organized separately for ground and space, and for each they are binned into three categories: 1) sustaining programs; projects and programs that optimize science return from facilities coming on line in the decade, and that maintain scientific and program balance, 2) enabling programs that prepare for future large observatories, and 3) frontier projects; large strategic missions in space and MREFC-scale observatories on the ground. Specifically, the third Frontier Projects category includes space missions in excess of $1.5 billion, and those ground-based projects individually exceeding $135 million. Previous surveys have chosen to organize activities based on small, medium and large cost bins. In FY2020 dollars, medium scale projects for NSF would range from $30 million to $135 million, and for NASA, from $300 million to $1.5 billion. For this survey, the organization of the medium and large scale projects emphasizes the role the activity plays. The categories enumerated above are equally important to advancing the survey’s scientific goals. While they roughly correspond to cost scale, this mapping is not exact. Previous surveys have not prioritized projects in one category against projects in a different category, but rather emphasize the need for a balance programs at all cost scales. Similarly, this survey emphasizes scientific and program balance, and does not prioritize one category over the others. Within each category, there are instances where one activity has priority over others in the same category, due either to its scientific importance, or due to technical readiness and/or programmatic urgency. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-5

TABLE 7.2 New Medium and Large Projects, Activities and Augmentations (2023–2033) The Frontiers: Major New Projects (Space) The Frontiers: Major New Projects (Ground) IR/O/UV Large Strategic Mission Extremely Large Telescope (ELT) Program (highest priority)  IR/O/UV telescope for exoplanet characterization and general astronomy.  Federal investment in the U.S.-ELT program for Mission-specific funding to begin mid-late the U.S. community decade after mission and technology  $1.7 billion NSF share of $5.1 billion project maturation program  Total implementation and operations cost (5 CMB-S4 years) estimated at $11 billiona  Stage 4 Cosmic Microwave Background Enabling Programs (Space) Observatory  NSF share $273 million, DOE share $387 million Great Observatories Mission and Technology Maturation Program The ngVLA Program to co-mature large strategic missions  Design, cost trade studies and prototyping to and technologies. First entrant: IR/O/UV prepare for construction, which could begin by observatory, Far-IR and high resolution X-ray the end of the decade observatories recommended to enter in second  $2.5 billion NSF share of $3.2 billion project half of the decade Sustaining Projects (Ground) Sustaining Programs (Space) Mid-scale Augmentation: Open, Strategic and Time-Domain Program (highest priority) Sustaining Instrumentation Calls  A program of competed missions and missions Augmentation to mid-scale programs; inclusion of of opportunity to realize and sustain the suite open calls that tap into the creativity of the entire of capabilities required to study transient community, strategic calls that maintain scientific phenomena and follow-up multi-messenger breadth and foundational instrumentation capacity, events. and sustaining instrumentation calls to optimize the  Notional cost: $500 million–$800 million over return on the investment in existing facilities. the decade Strategic calls for the coming decade are in time domain astrophysics (highest priority), radio Probe Line instrumentation, and highly-multiplexed optical spectroscopy.  Competed line of cost-capped probe missions to bridge the gap between Explorers and strategic missions; focused on gaps in science Programs and Facilities Funded and wavelength capabilities– this decade Far- through NSF Physics IR and an X-ray complement to Athena Technology Development for Future Gravitational  $1.5 billion/mission, cadence of approx. Wave Detectors one/decade Technology development for LIGO upgrades, such as Voyager, and for next generation observatories such as Cosmic Explorer. IceCube-Gen2 Neutrino Detector Upgrades to the IceCube high-energy neutrino detector a Project costs are in FY20$. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-6

7.5 RECOMMENDED NEW ACTIVITIES FOR SPACE The sections below describe the new space-based activities that the survey recommends NASA undertake in the next decade. In considering large strategic missions required to address the Pathways to Habitable Worlds, New Windows on the Dynamic Universe, and Unveiling the Drivers of Galaxy Growth priority areas, this survey takes a new approach. Rather than recommending missions in a rank-ordered list as they were presented in concept studies and white papers, the survey recommends a new strategy for rephasing mission and technology maturation and decadal survey recommendations (Section 7.5.1). This strategy is aimed at balancing the scope of large strategic missions to better address priority science in a timely way. Combined with this, a cost capped ($1.5 billion per mission) Probe line (Section 7.5.3.2), competed in strategic areas, will broaden the range of observational capabilities targeted at decadal science questions in the Ecosystems and Suns and Worlds in Context themes. Finally, a time domain program of competed small missions responds to the diverse observational needs required to address questions in the broad New Messenger, New Physics theme. FIGURE 7.1 (top panel) Understanding the nature of the complex ecosystems operating inside galaxies requires observations across the electromagnetic spectrum. Many phenomena are only observable from space platforms (indicated by the blue shaded regions), and require large aperture, sensitive telescopes only achievable on a probe or large strategic mission scale. (bottom panel) The need for a nearly panchromatic suite of facilities is illustrated by the composite image of our own Milky Way galaxy made by NASA’s Great Observatories: Chandra (X-ray), Spitzer (IR) and Hubble (UV/O). SOURCES: NASA, https://arxiv.org/pdf/2104.00023.pdf. NASA/Great Observatories Science Analysis Group. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-7

7.5.1 Advancing NASA’s Large Strategic Missions—the Great Observatories Mission and Technology Maturation Program 7.5.1.1 Background The richness of the Astro2020 science calls for a broad range of observational capabilities spanning the electromagnetic spectrum. The power of broad wavelength coverage was demonstrated by NASA’s Great Observatories, a panchromatic suite of four missions, launched over the course of three decades, that operated with contemporaneous overlap (Figure 7.1, Table 7.3).5 These missions spanned a range of cost scales, from Hubble at approximately $10 billion in today’s dollars, to Spitzer and the Compton Gamma-ray Observatory that today would be considered Probe scale. This was an extremely successful model that propelled major scientific advances on broad fronts over the course of two decades or more. The survey committee believes that it is scientifically compelling to replicate this approach today. TABLE 7.3 NASA Flagship Mission Cost at Launch and Timescales Observatory Waveband Launch date Development (year) 2020 Cost (B$) Hubble UV/O 1990 18 9.4a Compton GRO Gamma-ray 1991 14 1.2 Chandra X-ray 1999 17 3.1 Spitzer IR 2003 11 1.0 JWST IR/O 2021 (expected) 21 11 Roman IR/O 2026 (expected) 14 3.5 a Cost at launch. Does not include servicing missions NOTE: Shaded boxes indicate NASA’s Great Observatory suite. Development timescale indicates time from survey recommendation to launch. SOURCE: Data from https://arxiv.org/pdf/2104.00023.pdf. While NASA’s strategic missions must be driven by transformative scientific visions, they must at the same time advance a broad range of scientific objectives. If the scientific balance and wavelength breadth of the Great Observatories is to be replicated, these requirements must be balanced by the need for missions to be achievable on acceptable timescales. Balance and scientific breadth cannot be maintained if implementation costs are so large, and technology development so challenging, that projects can only be developed serially, with multiple decades required between inception of a mission concept by the community, adoption by a decadal survey, and the ultimate launch. These considerations are important in assessing how to best advance future large strategic astrophysics missions. In preparation for Astro2020, NASA sponsored four Large Mission Concept Studies aimed at developing reference missions for consideration by the survey: Habitable Exoplanet Observatory (HabEx), Large UV/Optical/IR Surveyor (LUVOIR), Origins Space Telescope (Origins), and Lynx. These mission concepts were chosen by NASA as a result of broad community engagement. Over the course of several years, four Science and Technology Definition Teams (STDTs), working with a designated NASA center, developed the scientific case and possible mission implementation architectures, including required instrument capabilities. NASA then assembled a Large Mission Concept Independent Assessment Team (LCIT) to conduct technical, risk, and cost assessments that were 5 Great Observatories the Past and Future of Panchromatic Astrophysics, 2020, https://arxiv.org/pdf/2104.00023.pdf. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-8

independent of the STDTs.6 This preparation and the mission definition studies were significantly more coordinated and uniform than has been done for previous decadal surveys. This advanced preparation provided a basis for the survey to evaluate the designs, performance, and likely budget scenarios. TABLE 7.4 Large Mission Cost Estimates and Development Timescales Large Strategic LCIT Cost Bin TRACE Cost Estimate Development Timea Mission Waveband (FY2020, B$) (FY2020, B$) (years, TRACE est.) LUVOIR-B UV/O/IR 8-10 17 20 HabEx 4-H UV/O/IR 8-10 10.5 18 HabEx 3.2S UV/O/IR -- 7.8 17 Lynx X-ray 6–8 9 18.5 Origins Far-IR 6-8 10.6 15.5 a Minimum, assuming immediate start and optimum budget profile. The large mission concepts were studied in detail by the Panels on Electromagnetic Observations from Space (EOS-1 and EOS-2), which considered the mission science and evaluated its relevance to the key science questions and discovery areas identified by the Science Panels. In addition, the EOS Program Panels performed an assessment of the mission technology readiness, risk, and costs weighed against the scientific opportunities. Independent Technical Risk and Cost Evaluation (TRACE) studies performed by the Aerospace Corporation for five of the implementations (HabEx and LUVOIR both presented more than one architecture for consideration) were additional important inputs to the panels and steering committee. Table 7.4 summarizes the cost bins as determined by NASA’s LCIT study, along with the TRACE estimates.7 The LCIT binned missions into cost categories, and although lacking the extensive model-based analysis of TRACE, serves to bound the expected costs at the low end. As noted above, establishing missions with complementary, panchromatic coverage operating near-contemporaneously, as done with the Great Observatories, can only be achieved if the missions in the suite can be launched in rapid enough succession, and have sufficiently long operating lifetimes to have overlap. While the mission concepts in Table 7.4 are all long-lived, their 15 to 20-year development times and large costs preclude more than one mission operating simultaneously, if they are built in a sequential development-then-launch cycle. The likely development times are in fact even longer than in Table 7.4, given that these assume the most optimistic funding profiles, which are rarely achieved in practice. The survey committee and EOS panels agree on this challenge, and take a two-pronged approach to addressing it by considering descopes to speed the development time, combined with a change in the development paradigm to allow some degree of parallel development to shrink the interval between launches. Conclusion: Establishing a panchromatic suite of observatories over the next 30 years is essential to address key questions in all three of the survey’s priority science themes. The large strategic mission implementations presented to the survey cannot all be built and launched in an optimal timeframe given the current designs, available budgets, and approaches to mission development. The universally long development times for the missions in Table 7.4 indicate that general purpose observatories with the full capabilities envisioned by the community will each take 15-20 years 6 Large Mission Concept Independent Assessment Team (LCIT), 2019, https://science.nasa.gov/science- pink/s3fs-public/atoms/files/LCIT_FinalRpt-2019-Nov8.pdf 7 See Appendix O for a discussion of the TRACE process. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-9

to be developed and launched. The survey’s recommendation to implement a probe mission line (Section 7.5.3.2) can partially address the need for scientific balance, however, to be consistent with the cost cap, probe missions must be significantly more focused than strategic missions. Accelerating the cadence of missions will require a combination of limiting their scope through careful selection of capabilities, and developing a new approach to the phasing of mission and technology maturation. This was done to some extent with the Great Observatories, in that both Spitzer and Chandra were significantly rescoped relative to the original concepts presented to the decadal surveys, yet both provided transformative observational advances.8 A rephasing of the mission and technology maturation process, with more significant and coordinated investment prior to a decadal survey recommendation to proceed with mission development, would provide multiple important benefits. This rephasing would recognize the multi-decadal timescales associated with large strategic mission and their associated technology maturation, and would better avoid the negative consequences associated with commencing missions prior to this maturation.9 By investing more in the maturation process, NASA could develop missions to a level where there is significantly more confidence in the costs and requisite cost profiles before seeking Congressional approval for the final implementation. This would build a higher degree of confidence with the stakeholders. It would also make it more likely that the optimal cost profile could be obtained, speeding up development times and reducing the eventual total mission cost. Finding: For a decadal survey to confidently recommend implementation of a strategic mission as its highest priority, the mission’s technology and architecture needs to be developed to a level of maturity that allows a reasonable assessment of budget profile, scientific performance, and technology risk. The mission’s cost range and development timescale must be deemed appropriate for the scientific scope. This survey suggests a restructured process for strategic mission maturation and decadal approval, shown schematically in Figure 7.2. The first stage is an initial mission concept study, similar to those done in preparation for this survey. However, rather than develop a small number of design reference missions, these studies would emphasize identifying key science break points, architecture options and trades, and would provide a description of key technologies and their maturation requirements and risks.10 The decadal survey would then evaluate the missions’ science promise and importance, and also, where possible, define the cost box and associated timescale deemed appropriate for the given mission. Based on the relative priorities, the survey would then recommend those candidate missions it finds to be compelling for significant investment in co-maturation of the mission design, science definition, and technologies through the Great Observatories Mission and Technology Maturation Program. With direction from an associated Program Office, these investments would take place in a phased way that represents the survey’s priorities, and this phasing could be reevaluated by the mid-decadal to reflect circumstances (international landscape, technology progress, etc.). An External Review, either by a mid-decadal or decadal survey, or some other process external to NASA’s usual program reviews, would decide whether the mission science capabilities and programmatic implementation is consistent with the decadal evaluation. A process such as the one described above would address a number of important issues. If the initial concept studies clearly identify scientific, implementation and cost breakpoints, as well as technology development needs, the survey can better evaluate and recommend the appropriate scope for 8 https://arxiv.org/pdf/2104.00023.pdf 9 R. E. Bitten, S. A. Shinn, and D. L. Emmons, 2019, Challenges and Potential Solutions to Develop and Fund NASA Flagship Missions, NASA Technical Reports, https://ntrs.nasa.gov/citations/20190001455. 10 Note: this was in essence what was done for HabEx and LUVOIR, which combined presented a number of point designs of differing scale all aimed at the goal of high contrast imaging and spectroscopy of extrasolar planets, with varying capabilities for additional astrophysics. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-10

the mission in light of the survey’s priority science. By recommending one or more missions enter into the maturation program in the coming decade, significant investments can immediately begin to refine them, develop their technologies and define the best mission possible for the recommended level of investment. Subsequent decisions about whether to start implementation of a given project can be based on the success and speed at which each project develops, and on the available budget. The survey committee expects that this process will result in decreased cost and risk and enable more frequent launches of flagship missions, even if it does require significantly more upfront investment prior to a decadal recommendation regarding implementation. This investment will not be wasted, even if a decision is made not to advance a particular mission in the subsequent decade. Many aspects of the technology investments, particularly in the areas of optics and sensors, will also be relevant for Probe and Explorer scale missions, and will benefit these programs. Conclusion: Enabling subsequent decadal surveys to recommend mission implementations with sufficient knowledge of the feasibility, overall budgetary needs, and timescale requires significant investment towards maturing large strategic mission science, technologies, and architecture in an integrated way. FIGURE 7.2 Flow diagram showing the concept for maturation, recommendation and implementation of NASA Large Strategic Missions. This does not represent the actual recommendations in this report, instead it represents how the program would be structured in general. If implemented, this survey would be the first to adopt this process by recommending the first entrant into the GO Mission and Technology Maturation Program. SOURCE: Fiona Harrison. Recommendation: The NASA Astrophysics Division should establish a Great Observatories Mission and Technology Maturation Program, the purpose of which is to co-develop the science, mission architecture, and technologies for NASA large strategic missions identified as high priority by decadal surveys. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-11

For recommended missions, decadal surveys would provide guidance, where possible, on the mission cost target, and scientific scope. Key elements of the program, summarized in Figure 7.3, include: ENABLER • Project Teams  • Funding  PROCESS: Great Observatories Mission & Technology Maturation  INPUT  OUTPUT  Flagship Proposals  Burndown GO Program Risk:  • Mission  • LCIT: Cost and  • Iterate Science Goals using teams to  Architecture  schedule risk  trade science and performance.  with supporting  • Concepts: Design  • Perform scientific simulations to  technologies,  options,  define key objectives  science  capabilities  • Develop converging, mission‐specific  objectives,  • Technology  technical capabilities using  flagship  requirements &  development roadmaps, subsystem‐ capabilities that  plans  level demonstrations, and  fit into decadal  • Targeted science  demonstration of production  science per cost    processes of sufficient scale, with  box  Decadal Survey  multi‐functional teams of scientists,  • Trades strategy  technologists & industrial partners  to drive  • Science  • Define Mission Architecture. Trade  technology  prioritization  achievable capabilities   development to  • Programmatic  • Transition to design by supporting  support Decadal  cost & timing  the Flagship Program Office  constraints  box    boxes  • Execution risk  Leverage Technology & Build  • Transition  box  Foundations  processes     • Technology  Wavelength  • Enable smaller projects (Explorers to  Probes) to address wavelength gaps,  roadmaps,  Coverage  timelines and  vet new technologies, mature  • Capabilities:  Technology Readiness Level or  mature  existing,  Manufacturing Readiness Level  technologies  planned, sun‐ • Manage GO Grants Program to build  • Wavelength gap  setting  science base for long‐cycle  forecasts &  capabilities  coverage  strategy, incl.  program size  CONTROL • Decadal priorities and decision rules  • Flagship Program Office  • SMD Oversight & Senior Program Review  PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-12

FIGURE 7.3 Flow diagram showing the key functions and scope of the Great Observatories (GO) Mission and Technology Maturation Program. SOURCE: Sigur/Harrison.  Mitigating the program risk associated with implementation of new large observatories by iterating and defining the science using broad community engagement, including using teams and open grants competitions, to evaluate science and performance trades and undertake scientific simulations for key objectives.  Developing the mission architecture best suited to accommodate programmatic constraints, while also supporting optimized science objectives. This would be accomplished using vetted and reviewed development roadmaps, subsystem-level demonstrations, and prototyping of production and manufacturing processes at a sufficient scale, with multi-functional teams of scientists, technologists and industrial partners.  Transitioning collaboratively from The Great Observatories Maturation Program Office which oversees feasibility studies and technology demonstrations, to design and production, as recommended by a decadal survey or mid-term decadal assessment. At this review it could be decided not to advance a mission due to cost or other factors. For this decade, the following missions are advanced for inclusion in this program: The survey’s top priority for this program is an IR/O/UV telescope optimized for observing habitable exoplanets and general astrophysics. As described in more detail in Section 7.5.2, the mission is recommended for implementation later in the decade, but only after the successful completion of the associated Great Observatories Mission and Technology Maturation program. Based on TRACE analysis and program panel input, the Survey estimates that 6 years will be required for this maturation, starting as soon as possible, before the mission is ready to be considered for implementation. The estimated cost of this mission and technology maturation program is ~$800 million, based on the cost and schedule analyses from the TRACE for the LUVOIR-B technology maturation program. These costs are carried within the Great Observatory Mission and Technology Maturation Program for approximately 6 years, at which point any residual technology development and the associated costs are transferred to the IR/O/UV mission development line. If this schedule and funding level can be achieved, by late decade it will be possible to assess the mission design, scientific reach, technology readiness at both the component and system level, feasibility of manufacturing processes, and cost for consistency with the survey’s recommendation and NASA’s budget guidance prior to transitioning to formulation and implementation. After an IR/O/UV exoplanet and astrophysics mission enters formulation, the survey assigns equal priority to commencing mission maturation and technology development for a far-IR spectroscopy and imaging strategic mission, and a high spatial and spectral resolution X-ray strategic mission. The survey committee believes an appropriate cost target for implementing these missions is $3 billion– $5 billion (FY2020) each. The motivation for this cost target is to balance the scientific scope with timeliness. Unlike the IR/O/UV mission, which has a target aperture based on a requisite number of Earth-like exoplanet detections, which in large part drives the cost, the Far-IR and X-ray missions are more easily scalable. In the survey committee’s judgement, missions with simplified design and selected instrument capabilities relative to Origins and Lynx can make substantial progress in addressing the priority science themes. A lower cost target relative to the TRACE estimates in Table 7.4 will enable these missions to advance more rapidly to implementation and realization, so that they can potentially have some overlap in operational lifetime. The survey committee views this as more important than achieving the full scope of Origins or Lynx. Finally, the scientific focus on the co-evolution of black holes and galaxies suggested by the EOS-2 panel is not necessarily the correct one. Rather, the mission maturation program would include trade studies to determine the scientific foci that are consistent with the broad set of the survey’s identified science priorities as well as the suggested cost target. An appropriate funding level for each of these programs is $40 million a year beginning in the second half of the decade. This is based on the level of mission and technology development funds PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-13

described in the concept studies (~$600 million for Lynx, ~$350 million for Origins), and the expectation that a significant fraction of at least 20-30 percent of this will be required to mature the missions and technologies to the appropriate level to enable a recommendation for mission implementation in the 2030 decade. If available budget levels require a choice to be made about which mission enters the program first, the survey committee suggests that the mid-decadal review evaluate the international scientific landscape, outcome of the probe selection, and that this review provide advice on which should commence maturation first. 7.5.2 Frontier Projects: A Future Large IR/Optical/UV Telescope Optimized for Observing Habitable Exoplanets and General Astrophysics Exploring terrestrial planets outside our Solar System through direct imaging and spectroscopy will advance one of humanity’s greatest quests - the search for habitable environments and life outside of the Solar System. This transformative goal is at the scientific forefront, connecting astronomy, astrobiology, planetary and Earth Science, and is one that captures the imagination of all humankind. Searching for signatures of life on potentially habitable planets around stars like the Sun, and exploring entire solar systems beyond our own (Figure 7.4) can only be achieved with a large aperture space telescope optimized for this purpose. FIGURE 7.4 Simulated space-telescope image of a complete planetary system including a life-bearing Earth-like planet. Simulated planets are shown clockwise from bottom left: e) close-in Neptune, b) sub-Neptune, d) Saturn, c) Jupiter, and a) exo-Earth. The system also contains inner and outer dust belts. Spectroscopic studies would allow the mission to distinguish between the planets and explore the complete evolutionary history of the system. SOURCE: NASA, HabEx report. The Habitable Exoplanet Observatory Study Team. A mission reaching this goal is challenging technically, scientifically and programmatically, yet, given rapid progress over the past 10 years, commencing its design and development is now possible. In the last decade the uncertainty in the number of Earth-sized potentially habitable planets has been reduced PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-14

by Kepler and other missions, and it is now known that such planets are common. Improved understanding of the complexities of planetary atmospheres lets us identify the spectroscopic measurements needed to assess the signatures of life. The technological feasibility of blocking starlight to see planets ten billion times fainter than their host stars has been demonstrated in laboratory testbeds, and although not at the level required by the IR/O/UV observatory envisioned here, several key capabilities will be tested by the Roman Space Telescope. We are on the threshold of a transformational leap in capability that will enable not just discovery but also exploration of planets beyond our Solar System. The key pathway to finding new, habitable worlds leads directly through this IR/O/UV space observatory. The same large aperture telescope that can identify Earth analogs would be equally transformative for general astrophysics. Its broad wavelength coverage, extending from the ultraviolet through the visible into the near-infrared, would inherit the scientific power of the Hubble Space Telescope, but with a light collecting area several times larger, 2-3 times sharper image quality, and instruments and detectors significantly more sensitive, providing 1-2 order-of-magnitude leaps in sensitivity and performance over HST. This telescope will be capable of achieving breakthrough discoveries across nearly all of astrophysics. Prime examples include ultraviolet and visible spectroscopy of the circumgalactic halos and the intergalactic medium and of mass flows within and out of galaxies to reveal the workings of cosmic ecosystems in detail and depth for the first time; high-resolution observations of supermassive black holes and their host galaxies locally and over cosmic time; and the construction of stellar fossil histories of the galaxies in the neighborhood of the Milky Way. The nature and effects of dark matter can be addressed by measuring the joint 3-dimensional kinematic and dark matter density profiles of dwarf galaxies. These examples all constitute major components on the Dynamic Universe and Unveiling the Drivers of Galaxy Growth priority science, and they only represent the tip of the iceberg of the impact such a telescope would have. This observatory will become one of the most scientifically versatile astronomical telescopes ever flown, and its observations will directly address two-thirds of the 24 key science questions identified in Chapter 2 and will contribute towards answering many of the others. FIGURE 7.5 Simulated spectrum of an Earth-twin planet observed from the UV to near-IR by a space coronagraph. Spectral features from oxygen, water, ozone and CO2 show the presence of a biosphere. SOURCE: NASA LUVOIR report. J. Lustig-Yaeger (University of Washington). The mission the survey puts forward will combine a large, stable telescope with an advanced coronagraph intended to block the light of bright stars. It will be capable of surveying a hundred or more nearby Sun-like stars to discover their planetary systems and determine their orbits and basic properties. Then for the most exciting ~25 planets, astronomers will use spectroscopy at ultraviolet, visible, and near- infrared wavelengths to identify multiple atmospheric components that could serve as biomarkers (see Figure 7.5). It will also have high-resolution imaging and multi-object spectroscopic capability (particularly at ultraviolet wavelengths) to address a broad range of astrophysical science selected through guest investigator programs. After considering the analysis from the EOS-1 panel regarding technology readiness, cost, and science capability, and weighing the need for program balance and timeliness, the survey committee PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-15

concludes that a high-contrast direct imaging mission with a target off-axis aperture of approximately 6 meters provides an appropriate balance between scale and feasibility. Such a mission would yield a robust sample of ~25 atmospheric spectra of potentially habitable exoplanets, and it could launch by the first half of the 2040 decade. A sample this size provides robustness against the uncertainties in the occurrence rate of Earth-sized worlds, and against the vagaries associated with the particular systems near Earth. Analysis by the EOS-1 panel finds that, given the budget requirements and realistically achievable yearly funding levels, an 8 m aperture telescope of the scale of LUVOIR-B would be unlikely to launch before the late 2040’s or early 2050’s. On the other hand, a smaller telescope such as the HabEx 4H design may fall short of providing a robust exoplanet census, and was judged by EOS-1 not to advance general astrophysics capabilities sufficiently to justify the high cost and long timescale for completion. FIGURE 7.6 Potentially habitable exoplanet yield vs telescope diameter for different telescope architectures. Right axis shows the number of habitable zones surveyed (weighted by completeness); left axis shows the expected number of planets discovered assuming the occurrence rate of rocky planets in the optimistic habitable zones of different stars, eta_earth=0.24 (Bryson et al. 2021). The red dot shows the expected yield for the target 6-m inscribed diameter. NOTE: Habitable zone is defined as 0.95-1.67 AU for planets of 0.8-1.4 Earth radii. SOURCE: Adapted from C. Stark (Space Telescope Science Institute), D. Mawet (California Institute of Technology), and B. Macintosh (Stanford University). Conclusion: A high-contrast direct imaging mission with a target off-axis inscribed diameter of approximately 6 meters provides an appropriate balance between scale and feasibility. Such a mission will provide a robust sample of ~25 atmospheric spectra of potentially habitable exoplanets, will be a transformative observatory for general astrophysics, and given optimal budget profiles it could launch by the first half of the 2040 decade. Realizing this mission requires significant technology development and maturation of the design and implementation. The best path forward is to have NASA immediately commence aggressive technology development aimed at achieving the goal described above as part of the Great Observatories Mission and Technology Maturation Program. This program would consider and optimize configurations targeted at performance consistent with the target 6-m off-axis aperture as indicated in Figure 7.6. These studies would combine scientific and technical ideas and talent from the entire community to develop a PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-16

single mission architecture and associated technology roadmap. Broad participation of astronomers, exoplanet scientists, and Earth scientists would be beneficial in refining the exoplanet observation strategy. As noted above, even if begun immediately, the technology development phase is likely to take 6 years, and possibly more, and could require an investment of $800 million prior to the Phase A start of the resulting mission. Given current uncertainties in how key technologies will advance, and what the budget climate will be for implementing an ambitious strategic mission, the survey recommends that the trade between launch date and mission scale be reviewed again after the crucial technologies have reached maturity (technology readiness level [TRL] 5-6), and the mission implementation, descopes and trades are better understood. At this time key scientific inputs, such as the frequency of Earth-sized planets in habitable zones of stars, will also be better constrained by ongoing observational work, and the mission capabilities, cost, and schedule will be significantly refined. Discussions about key international partnerships on this ambitious program can also be established at this time. Prior to commencing formulation, the proposed implementation would be subject to an independent review which could assess plans in light of budgetary needs and realities, and subsequently NASA could move forward with implementing a mission that accomplishes transformative exoplanet and general astrophysics goals while being realizable by the first half of the 2040 decade. Recommendation: After a successful mission and technology maturation program, NASA should embark on a program to realize a mission to search for biosignatures from a robust number of about ~25 habitable zone planets and to be a transformative facility for general astrophysics. If mission and technology maturation are successful, as determined by an independent review, implementation should start in the latter part of the decade, with a target launch in the first half of the 2040s. This is an ambitious strategic mission, and while not at the cost scale of LUVOIR, it will still require an investment comparable to Hubble or JWST. To assess the budget scale and profile requirements for the recommended direct imaging mission, the survey committee performed an analysis assuming the cost profile and schedule from the LUVOIR-B TRACE analysis, normalized to a total integrated cost equivalent to JWST inflated to current year dollars.11 The survey committee believes this is a conservative assumption: the JWST telescope incorporates a 6.5 m segmented primary mirror, and as it operates in the mid-infrared it has many tight thermal requirements. A ~6 m aperture high contrast imaging mission would have the added complexity of extreme starlight suppression, but would operate at and could be tested at room temperature. Many factors, including insufficient technology development prior to Phase A start, sub-optimal funding profiles, and management and execution challenges increased the overall cost of JWST relative to what should be achievable. Here the survey is recommending an aggressive early technology phase so that development can proceed optimally after a mission start. The result of the analysis is that a total cost of $11 billion (FY2020), with a real-year funding profile shown in Figure 7.10, is likely to bound the mission cost. The survey committee believes that the scientific goals of this mission, when achieved, have the potential to profoundly change the way that we as 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, and 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. 11 LUVOIR-B was chosen as a basis for the estimate because it was more scalable with its segmented mirror, internal coronagraph, and single launch vehicle. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-17

7.5.3 Sustaining Activities The two recommendations below are aimed at capitalizing on the upcoming Roman, Rubin, Athena and LISA observatories, and balancing scientific progress among the survey’s priorities, thereby addressing the extraordinary richness of 21st century astrophysics. 7.5.3.1 Time Domain Astrophysics Program Exploring the cosmos in the multi-messenger and time domains is a key scientific priority for the coming decade. The ability to probe time-variable, explosive and transient phenomena has been propelled by large format detectors, by dramatic computational advances, and in the last decade by the advent of entirely new means of discovering transient phenomena through gravitational waves and high energy neutrinos. As a natural result of their large fields of regard and cadenced observations, time domain observations are a central element of the top ground and space-based projects supported by Astro2010; the Vera Rubin Observatory (referred to as the Legacy Survey of Space and Time [LSST] in 2010); and the Nancy Grace Roman Space Telescope (WFIRST in 2010). Time-domain studies offer tremendous new discovery space for both observatories, as well as for many other U.S. space and ground-based facilities (e.g., the Transiting Exoplanet Survey Satellite [TESS], Zwicky Transient Facility [ZTF], Deep Synoptic Array 110 [DSA-110], the Laser Interferometer Gravitational-wave Observatory [LIGO], and IceCube). Many of the exciting opportunities in multi-messenger astrophysics rely on observations of astrophysical transients discovered via gravitational waves, and neutrinos, but requiring electromagnetic observations across the spectrum to identify and probe the fundamental physical nature of the phenomena. The LIGO/Virgo-discovered binary neutron star merger, GW170817 is a prime example (Chapter 2, Box 2.2). Looking to the future, the LISA mission will open enormous discovery space for probing larger mass black holes, as well as white dwarf binaries, where electromagnetic observations will be equally essential. While ground-based measurements by observatories large and small are essential, several key capabilities that must be sustained to enable time-domain and multi-messenger astrophysics can only be realized in space. The most important of these are wide-field gamma-ray and X-ray monitoring, and rapid and flexible imaging and spectroscopic follow-up in the X-ray, ultraviolet (UV), and far-infrared (far-IR). In addition, space platforms can be designed to access much of the sky at any given time, essential for the study of short-lived transients or rapidly variable sources. Space missions can also observe near- continuously compared to ground-based telescopes. As discussed in the report of the EOS-2 panel, many of the necessary observational capabilities can be realized on Explorer-scale platforms (Missions of Opportunity (MoO), Small Explorers (SMEX) and Medium-class Explorers (MIDEX), while others could require larger efforts, but still less than half the scope of a Probe Mission. In addition to the threats of lost capabilities resulting from the aging of Swift and Fermi, there are also potential new international opportunities to meet the scientific needs, such as the Space Variable Objects Monitor (SVOM). Contribution of instruments to international efforts is another possibility for achieving some elements of the program. The specific needs to sustain and enhance the optimum suite of space capabilities will change over the upcoming decade, and it is likely that these capabilities will be most effectively achieved by a complement of missions on different scales, including contributions to international efforts. One effective mechanism for achieving the above goal would be for NASA to appoint a standing planning committee in time-domain astrophysics. This committee could, over the decade, periodically review and advise NASA about what the critical needs are to maintain and expand a vibrant and effective system of time-domain and transient follow-up observatories, evaluated in the international context, and considering what can effectively be done by ground-based facilities. The committee’s considerations would be guided by the key scientific questions identified by the survey. NASA could then respond to PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-18

these needs either through targeted calls that are part of the Astrophysics Research and Analysis Program (APRA) or the Explorer program, or through dedicated announcements of opportunity. Conclusion: A standing planning committee or advisory structure could provide tactical advice to NASA on impending needs and priority capabilities for time domain and multi-messenger follow- up, evaluating these needs in the international landscape. Recommendation: NASA should establish a time-domain program to realize and sustain the necessary suite of space-based electromagnetic capabilities required to study transient and time-variable phenomena, and to follow-up multi-messenger events. This program should support the targeted development and launch of competed Explorer-scale or somewhat larger missions and missions of opportunity. The estimated cost of this program would range from $500 million–$800 million over the decade. This lower range would support competed missions of opportunity, SMEX and MIDEX scale missions. As described in Section 6.2.1.1.3, the survey notes this funding is intended to be added above the current funding level of the Explorer program, so as not to negatively impact the rate of selections through entirely open, non-targeted calls. These expenditures would take place throughout the decade. 7.5.3.2 Astrophysics Probe Mission Program Advancing the survey’s science program depends on the existence of a panchromatic suite of capabilities. Given the long development timescales for large strategic missions, establishing a ‘probe’ class line with mission costs of ~$1.5 billion and launches every decade, will address the need for broad wavelength coverage and scientific balance. Through advances in technology, combined with focused science, missions at this scale can achieve more than an order of magnitude leap in capability, and address scientific areas of high priority. This is supported by the large number of white papers for missions in this category evaluated by EOS-1 and EOS-2. Both panels concluded that probe-scale missions offer exceptional scientific opportunities. Missions at this scale would also address the significant gap in cost, capability, and development time scales between Explorers and strategic missions. A mission cost-cap of $1.5 billion affords an appropriate balance between capability and launch cadence. For many of the projects considered by the program panels, a cost cap of $1 billion was constraining. This conclusion is supported by NASA’s Probe Cost Assessment Team report, which found that only one of the nine NASA-supported probe concept studies is likely to be achievable within a $1 billion cap.12 At the other extreme, pushing the cap to $2 billion would decrease the launch rate to less than one per decade, given other demands on the program. In contrast to entirely open competitions (such as in the Explorer program), probe class mission calls that target areas where there are strategic scientific gaps in decadal programs will advance survey priorities in a more optimal way. A good model for achieving this is the approach taken by NASA’s Planetary Science New Frontiers Mission line. Conclusion: The large gap in cost and capability between Explorer class missions and large strategic missions is a significant impediment to achieving the broad set of Astro2020 decadal scientific priorities. Institution of a Probe class line of missions, with an individual mission cost cap of ~$1.5 billion, selected from priority areas identified by decadal surveys, would broaden NASA’s astrophysics program in a way that better addresses the richness of 21st century astrophysics. 12 NASA, “PCAT Final Report,” 2019, Independent Probe Cost Assessment Team (PCAT), https://science.nasa.gov/science-red/s3fs-public/atoms/files/PCAT_FinalRpt-2019-Nov8.pdf. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-19

Recommendation: NASA Astrophysics Division should implement a line of probe missions with a mission cost cap of ~$1.5 billion fiscal year 2020 and a targeted launch rate of approximately one per decade. These missions should be competed, with solicitations calling for concepts in priority areas identified by decadal surveys. The survey identified two priority areas as the most compelling for probe missions this decade. These are highlighted, because they address crucial scientific gaps, and wavelength ranges and/or observational capabilities important to the survey’s scientific objectives, but where no mission is currently planned either nationally or internationally. In calling out these areas, the survey is not endorsing any of the specific mission concepts from submitted white papers, or any of the NASA probe studies. Rather, the probe implementation will require NASA to support the development and study of concepts that fit within the cost cap, most likely through a concept study/down select process. The two areas for the first probe competition are listed below. Allowing both areas to compete for the first opportunity will enable a robust number of mission concepts, providing a high likelihood that a compelling mission is selected and developed within the cost cap of the program. A third area is listed where investment in technology development this decade would prepare for a subsequent probe call early in the 2030’s. 7.5.3.3 A Far Infrared Imaging or Spectroscopy Mission A far-IR imaging or spectroscopy probe mission would address scientific objectives central to Astro2020, and would fill an important gap in world-wide capabilities. Since the EOS-2 report was completed, ESA made the decision to remove the joint ESA/Japan Aerospace Exploration Agency (JAXA) Space Infrared Telescope for Cosmology and Astrophysics (SPICA) far-IR mission from consideration for its M5 slot. SPICA was identified as a priority for NASA participation by Astro2010, would have flown a powerful set of spectrometers covering the 12 – 230 μm range, as well as a mid- infrared imager. SPICA was positioned to make significant progress in a number of the science areas highlighted by this survey. Recent improvements in far-IR technology mean that major scientific advances can be made compared to the Herschel Space Observatory. The EOS-2 panel considered the landscape for a future far-IR mission prior to ESA discontinuing its consideration of SPICA. The survey committee believes that considering this change in landscape there are many unique opportunities for a properly scoped far-IR probe to advance high priority science, and a probe scale mission is an extremely timely and compelling opportunity to do so. These scientific areas include tracing the astrochemical signatures of planet formation (within and outside of our own Solar System), measuring the formation and buildup of galaxies, heavy elements, and interstellar dust from the first galaxies to today, and probing the co-evolution of galaxies and their supermassive black holes across cosmic time. These goals are all central to the broader scientific themes of the survey. The ultimate scientific focus of the far-IR probe will depend on the outcome of the competitive selection. 7.5.3.4 An X-ray Probe to Complement ESA’s Athena Observatory Observations of the universe in the X-ray band probe many energetic phenomena and processes not accessible through other wavebands. The ESA L-class Athena X-ray mission has many of the capabilities for a next-generation X-ray observatory that were recommended by Astro2010. With its very large aperture, moderate spatial (5”) resolution imager, and non-dispersive spectrometer (ΔE of 2.5 eV) operating from 0.1 – 8 keV, Athena will be a major step forward. However, Athena lacks the high spectral resolution (R ~ 7500), broad bandpass, and spatial (<1”) resolution needed to address multiple Astro2020 key science questions. These include understanding the invisible drivers of galaxy formation and evolution through observations of effects of feedback from accretion of gas onto supermassive black PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-20

holes, searching for the first seed black holes at high redshifts (z~10), and characterizing the activity of stars and studying their evolution. Because of the unique science that can be addressed with a mission complementary to Athena, a targeted X-ray probe is one of the priorities for a probe mission competition. While a probe would not fulfill all objectives of a large strategic mission such as Lynx, a probe mission could enhance Athena, and also address important capability gaps. Which of the scientific objectives mentioned above can be achieved by a probe scale mission is unclear at this time. For a mission properly selected and scoped, there are multiple potential science opportunities that are both unique and timely. Depending on the scientific focus, it is not necessary for the X-ray probe to be operational simultaneously with Athena, rather the survey envisions strong complementarity of the science focus. 7.5.3.5 An Early Universe Cosmology and Fundamental Physics Probe As detailed in the report of the Panel on Cosmology, studies of the cosmic microwave background continue to provide data that address profound and fundamental questions about the universe on the largest scales and during its earliest moments. As noted by the EOS-2 panel report, “space observations will unquestionably be needed for the best foreground separation and the lowest systematic errors on all angular scales, and especially on angular scales of greater than about ten degrees.” With investment in technologies this decade, combined with ground-measurements, cosmic microwave background (CMB) probe mission could potentially be a compelling candidate for the future probe call in the 2030’s, complementing the survey’s ground-based CMB-S4 recommendation. 7.5.4 Prioritization of NASA Sustaining and Frontier Activities The survey’s top priority for medium and large programs is for NASA to complete the major astrophysics facilities and missions currently in development, including its commitments for participation in major ESA missions, and to maintain the Explorer program at the current healthy rate (see Table 7.1 for a list of these activities).For new missions and programs, the survey does not prioritize projects between sustaining activities and advancing large strategic missions, as both achieve equally important goals for the program. This parallels previous surveys that have not prioritized programs in the large category relative to medium or small scale activities. The rationale for this is the overriding need for the balance of mission and program scales required for the success of the astronomy and astrophysics enterprise. In the sustaining program category the survey prioritizes the time-domain program over the probe line, due to the urgent need to maximize return from major U.S. investments in LIGO and Rubin Observatory on the ground, and Roman in space. For large strategic missions, the highest priority is for NASA to rapidly establish the Great Observatories Mission and Technology Maturation program, with the most important element in that category being to commence maturation of the large IR/O/UV mission. The largest budgetary increase associated with the recommended program arises in the latter half of the decade, assuming that the large IR/O/UV mission is technically ready and sufficiently mature to commence detailed design and implementation (See Figure 7.10). If, for budget or technical reasons, this must be delayed, it is still important for NASA to proceed with the mission and technology maturation programs for a Far IR observatory and a high-resolution X-ray observatory. This will help to minimize the time between the ultimate realization of the IR/O/UV mission and the subsequent large strategic mission. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-21

7.6 ROADMAP FOR GROUND: NEW ACTIVITIES The sections below describe the new ground-based activities that the survey recommends that NSF and DOE undertake in the next decade, divided into three categories: sustaining activities that broaden science and the timescales on which new capabilities emerge; enabling activities that advance future MREFC-scale facilities; and the frontier observatories that are ready for implementation in the coming decade, and that will ensure the U.S. community continues to advance the scientific forefronts. 7.6.1 Frontier Facilities and Observatories Large observatories provide the transformative capabilities that achieve breakthrough discoveries, and advance a broad range of scientific objectives. The survey recommends phased NSF investment in three large programs this decade. The highest priority is participation in the United States. Extremely Large Telescope (ELT) Program, because of its transformative nature, and because of the urgency of this investment to the projects’ success. Next, equal priority is placed on developing and implementing the CMB-S4 observatory together with the DOE, and on beginning design, cost studies and prototyping for the next-generation Very Large Array radio telescope. Finally, if these studies are successful, and if budgets allow, the survey recommends commencing construction of the next generation Very Large Array (ngVLA) toward the end of the decade. While the IceCube Gen-2 neutrino observatory is not funded out of AST, an assessment is provided of its relevance to the science recommended by this survey. 7.6.1.1 The U.S. Extremely Large Telescope Program The U.S. ELT Program as proposed to the survey is made up of three elements: the Giant Magellan Telescope (GMT), the Thirty Meter Telescope (TMT), and NSF’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab) (See Figure 7.7). The primary mirror of the GMT has a total diameter of 24.5 meters and the telescope has a 25 arcmin field-of-view (FOV). The GMT will be located at the Las Campanas Observatory in Chile. The majority of the GMT partners are U.S. institutions, with international partners in Australia, Brazil, and Korea. The TMT primary mirror has a diameter of 30 meters and the telescope has a 20 arcmin FOV. The TMT will either be sited on Maunakea in Hawaii, or at Roque de los Muchachos Observatory on La Palma in the Canary Islands. The majority of the TMT partners are international, with the participation of institutions in the United States, Canada, China, India, and Japan. For comparison, the European Southern Observatory (ESO) is building the ESO ELT on Cerro Armazones in Chile with a 39.3 m diameter and a 10 arcminute FOV, with first light expected in 2028. Both the TMT and the GMT are well into development; both projects have mature designs and have commenced fabrication of key elements, although challenges remain. They are expected to commence operations in the mid 2030’s, contingent on a U.S. funding commitment. The scientific potential of the ELTs is vast. The combination of large collecting area (4-9 times that of a 10m Keck telescope) and diffraction-limited imaging (0.01-0.02” FWHM with adaptive optics in the near-IR) provides observational capabilities unmatched in space or the ground, and opens an enormous discovery space for new observations and discoveries not yet anticipated. A resolution of 0.01” (12 times that of the Hubble telescope at similar wavelengths) projects to a linear scale of 25 km at Jupiter, 1 AU for a protoplanetary disk at distance 100 pc, 0.8 pc at the distance of the Virgo Cluster, and 60 pc for galaxies at redshift z=2.5 (comparable to the scales resolved by ground-based telescopes with natural seeing at Virgo). For unresolved sources the sensitivity of these telescopes scales as their diameter to the fourth power, a gain of 36–81 times over current 10 m telescopes. The large collecting areas of the telescopes also makes them powerful spectroscopic machines, especially for high-resolution spectroscopy where measurements are often limited by detector noise. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-22

FIGURE 7.7 The Thirty Meter Telescope mirror (left) and the Giant Magellan Telescope mirror (right). Both ground-based projects are at an advanced stage of development and have commenced construction of their primary optics. SOURCE: Left: https://www.tmt.org/page/uselt. Courtesy of the TMT International Observatory. Right: Courtesy of the Giant Magellan Telescope – GMTO Corporation. This powerful combination of capabilities can be brought to bear on nearly all of the important science questions laid out by this decadal survey, across all three of our key science themes.13 They will be able to detect, image, and characterize temperate rocky planets around low-mass stars, measure their atmospheric compositions including searches for oxygen, image protoplanetary disks, and through precision radial velocity measurements measure the masses of the planets, vital information only possible with the ELTs. Fundamental physics will be probed through a variety of pathways, including measurements of stars orbiting the Milky Way’s central black hole SgrA*, to perform tests of relativity and gravity. Measurements of the cosmic expansion rate using different methods (variable stars, gravitational lensing, merging neutron star “standard sirens”) will test for the reality of the current Hubble tension and reveal whether the current CDM cosmological model fully describes the expansion. Measurements of the faint spectra of gamma-ray bursts and supernovae beyond redshifts z=10 will probe both the physics of stellar explosions at early cosmic times and probe this epoch of reionization itself. The impacts of the ELTs for revealing the workings of the Cosmic Ecosystem promise to be especially powerful. These telescopes alone will have the sensitivity to make spectroscopic measurements of the faintest galaxies, stellar explosions, and black holes detected by JWST; the result of these studies will be a record over cosmic time of the buildup of matter, stars, heavy elements, and the assembly of the galaxies themselves from hundreds of thousands of years after the Big Bang to the present. Likewise, these telescopes will have the unique ability to trace the chemical and dynamical buildup of the Milky Way and nearby galaxies out to the Virgo cluster, through deep high-resolution imaging and spectroscopy of their oldest stars. Many of these unique capabilities complement those of our top-ranked space project, the IR/O/UV space telescope, extending the powerful synergies between the ground-based 6-10 m telescopes and HST over the past 30 years (and soon with JWST and the Roman telescope). As demonstrated by the 16 6.5-12m OIR telescopes currently in operation around the world (not counting the Rubin Observatory or others under construction), the versatility of these instruments and the large range of top-priority scientific applications will more than fully occupy even three ELTs for decades. Conclusion: Because of their transformative scientific potential, as well as readiness, the success of at least one U.S. ELT is a critical priority for investment for ground-based astronomy in the coming decade. Although the U.S. astronomy community would benefit enormously by an NSF investment in even one of the TMT or GMT, there is considerable benefit to pursuing a coherent two-telescope U.S. ELT Program that would combine capabilities of both A two-telescope U.S. ELT program would offer 13 See Table 1 in the report of the Panel on Optical and Infrared Observations from the Ground. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-23

full-sky coverage, important for leveraging the current U.S. multi-billion dollar bi-hemispheric system of ground-based OIR and radio astronomical facilities (JVLA and the future ngVLA in the north, ALMA and the Vera Rubin Observatory in the south) and assure observations of rare objects (e.g., nearby habitable exoplanets, rare classes of transient events) regardless of where they lie in the sky. Complementary instrumentation on the two telescopes developed in a coherent manner in partnership with NOIRLab would significantly increase the scientific reach of the overall U.S. ELT program. Investing in two telescopes would also maximize the total number of nights of public-access observing time—potentially as much as 200 nights per year—and far more than remain available for NSF partnership on either of the observatories alone. The enormous scientific potential of the ELTs has also been recognized overseas. Several international organizations are partners in the GMT and TMT project, and in 2008 a European Astronet decadal study identified an ELT as one of its top priorities (along with a Square Kilometer Array radio telescope project). ESO now is constructing a 39 m ELT in Chile, with planned commissioning later in this decade. NSF participation in a U.S. ELT program will position the U.S. community to take full advantage of the promise of these facilities. Although smaller in aperture the TMT and GMT offer a number of unique capabilities, including fields of view 4-6 times larger than the ESO ELT (facilitating multi-object spectroscopy), and high-resolution first-generation spectrometers capable of carrying out groundbreaking observations of exoplanets, ancient stars, and the circumgalactic and intergalactic media, key elements of the Habitable Worlds and Galaxy Growth priority areas. These capabilities are regarded less as competitive advantages than as powerful synergies between complementary facilities which will hasten the advancement of the science frontier objectives highlighted in this survey. As proposed to this survey, the U.S. ELT program would be comprised as a collaboration between the GMT and TMT projects with the NSF NOIRLab. NOIRLab would provide proposer and user support, public data products and archiving, broaden participation in U.S. ELT science, foster research inclusivity, and engage and represent the whole U.S. community in the U.S. ELT governance and scientific planning. NSF partnership would leverage major investments by universities and foundations ($1.5 billion), and international partners ($1.2 billion), and assure that the fruits of these revolutionary facilities are shared by the largest possible community of researchers and students in the United States. The Panel on Optical and Infrared Observations from the Ground (OIR) assessed the programmatic and technical risks and cost of both the GMT and TMT separately, and both underwent an independent TRACE analysis. The TRACE construction cost estimates of $2.4 billion and $3.1 billion for GMT and TMT respectively are within 20 percent of the project cost estimates ($2 billion and $2.65 billion), which is within the uncertainties. While there are technical challenges for both projects, solutions appear to be in-hand. TMT has the added risk that the site has not yet been selected, adding cost and schedule uncertainty. However, the biggest risk for both projects is the large gap between commitments in-hand from the partners, and what is required to complete the projects, even with a significant federal investment by NSF of $0.8 billion per project. This programmatic risk is significant, and the TRACE analysis gave both projects a medium-high programmatic risk rating. The scientific potential of the ELTs is so compelling, and the science so broad, that ideally community access would be at least 25 percent on each of the ELTs (as proposed to the survey). If, however, programmatic or financial challenges preclude the viability of one of the projects, the survey recommends that NSF invest in at least one ELT, with a share of the time proportional to the fractional federal investment in constructions and operations. Recommendation: The National Science Foundation (NSF) should achieve a federal investment in at least one and ideally both of the two extremely large telescope projects— the Giant Magellan Telescope and the Thirty Meter Telescope, with a target level of at least 25 percent of the time on each telescope. If only one project proves to be viable, NSF should aim to achieve a larger fraction of the time, in proportion to its share of the costs and up to a maximum of 50 percent. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-24

7.6.1.2 Criteria and Decision Rules for Investment in the U.S. ELTs It will be necessary for NSF to commence with an external review with a target completion in 2023 in order to evaluate the financial and programmatic viability of both proposed U.S. ELT projects, with the level of federal investment in at least one of the projects determined at the end of the review. Federal investment in either project should be predicated on: 1. Demonstration of financial viability with agreed-upon commitments from partners for all of the necessary capital and operations money, pending only NSF investment. 2. Final site selection in the case of the TMT. 3. A public share of telescope time (run through NSF’s NOIRLab) roughly equivalent to the total federal investment of construction and operations expenses. 4. Full public archiving of all data taken by the ELTs, after a reasonable proprietary period. This applies to both federal and consortium telescope time. 5. Development of a management plan and governance structure for the joint project, agreed by all parties including the relevant observatory corporations and NSF. Approval of the project is also subject to the recommendation in Section 5.1.1 that makes the initiation of any new astronomy MREFC project contingent on NSF developing a plan for managing the operations costs of the new facilities within its projected budget envelope. Recommendation: The National Science Foundation (NSF) should conduct an external review of the U.S. extremely large telescopes, with a target completion date of 2023. If only one of the Giant Magellan Telescope or the Thirty Meter Telescope can meet the conditions enumerated above by the time of NSF’s review, NSF should proceed with investment in that project alone. Depending on the outcome, the decision rules for NSF are the following: In the case that only one project can proceed, NSF’s investment of up to a 50 percent share in the project should be undertaken if doing so will ensure that the project has the financial resources to come to fruition. If NSF investment can only fund partnership in one telescope, but both are viable, NSF’s investment should factor in complementarity to the ESO ELT, the ability to address the science questions of the Astro2020 survey, and the relative advantages of a larger diameter (D), which increases the sensitivity ~ D2 to D4 (depending on the science application), versus a larger field of view, which increases survey speed and the number of targets per observation. 7.6.1.3 CMB-S4 Observations of the CMB have not only been central to establishing the standard model of cosmology, but the telescopes designed to undertake them are becoming increasingly important for understanding phenomena ranging from transients to galactic ecosystems to the formation of cosmic structure. The advances possible with a new generation of receivers include searching for polarization signals from gravitational waves from the Big Bang and, when combined with Euclid, Roman, and Rubin Observatory, revealing a detailed picture of our cosmic web, its composition, and its evolution. At the same time, by tracing the electron pressure in halos of galaxies and galaxy clusters, CMB observations can trace feedback between the intergalactic medium, the circumgalactic medium, and the cores of galaxies. Building on the scientific and technical progress brought about by decades of individual private and public investments by the U.S. community, we are poised in the next decade to make a major step forward in ground-based CMB studies. Over the last two decades, second- and third-generation ground- PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-25

based CMB experiments, deployed in Antarctica and Chile, have made significant advances, including detecting lensing B-mode signatures in the CMB, and the CMB-galaxy lensing cross power spectrum. The search for the tell-tale signature of cosmic inflation through its imprint on the B-mode polarization pattern of the CMB has pushed to fainter and fainter levels, disentangling foregrounds, and placing tighter constraints on this primordial signal. These observations have informed us how to analyze vast amounts of data and disentangle complex cosmological signals, and how to build theoretical models to extract parameters. The experiments have propelled progress by university groups and government labs to develop ever more sensitive, highly multiplexed bolometer detectors operating over a wide frequency range, and these efforts have informed the community how to design the next-generation facility to push these ground-based observations to their projected limit. Realizing the ultimate scientific potential of ground-based CMB observations will take an effort far beyond what can be achieved simply by independently scaling up existing experiments. It will require a significant increase in the number of CMB detectors in operation, a wide range of independent frequency bands to separate out foreground contaminations, and it will require probing a combination of both large and small angular scales. While such an effort can be carried out using existing millimeter- wave observing sites in Chile and Antarctica, facilities at both must be carefully designed as part of a systemically planned program. Finally, while the United States has been the unrivaled leader in ground- based CMB observations, the needed project is of a scale that would benefit greatly from international participation in both scientific and technical aspects. The Panel on Radio, Millimeter and Submillimeter Observations from the Ground (RMS) evaluated a number of CMB projects, and suggested that the CMB-S4 observatory as the compelling and timely next leap for ground-based observations. CMB-S4 is a joint effort of NSF and DOE that includes international participation. It will conduct a 7-year ultra-deep survey of a few percent of the sky from the South Pole with a combination of large and multiple small aperture telescopes observing from 30-270 GHz. This will be done in parallel with a 7-year deep/wide survey of roughly half the sky with additional telescopes sited in the Atacama desert in Chile. A TRACE analysis estimated the cost for design, development and construction to be $660 million (FY2020), within 15 percent of the project team’s analysis and within uncertainties for this stage of development. CMB-S4 is well along in planning, and could achieve first-light as early as 2026-27. Although significant scale-up of the detector production is required, plans are in hand to accomplish this. Aerospace evaluated the project risk as medium-low. This project engages the international cosmology communities, building upon the foundation of decades of ground- and space-based measurements of the CMB to take a major leap that will push CMB science to the next level. The scientific reach of this observatory goes well beyond cosmology. CMB-S4 will produce unprecedented maps of ~50 percent of the sky between wavelengths of 1 mm and 1 cm with a cadence that samples the entire area every other day, opening up discovery space and providing scientific data that will engage a broad swath of the astronomical community. Particularly compelling to the survey is the fact that these observations open the opportunity for systematic time-domain studies in this part of the electromagnetic spectrum for the first time. Recommendation: The National Science Foundation and the Department of Energy should jointly pursue the design and implementation of the next generation ground-based cosmic microwave background experiment (CMB-S4). Important to our recommendation is that CMB-S4 is a project with a balanced commitment from both NSF and DOE from inception, to design, implementation, operations and science. NSF nurtures and supports university groups with broad scientific and technical experience who have been leading ground- based CMB efforts both in Chile and in Antarctica, and that have been and will continue to train new generations of talent. DOE brings to bear the technical expertise of its national laboratories, scientific expertise including large scale computation, and importantly systematic management approaches that have proven to be effective for large-scale projects. The agencies have been working jointly and effectively to prepare for initiating this compelling project. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-26

An important requirement for our strong endorsement is that the project broadly engage astronomers beyond the traditional CMB community. CMB-S4 will produce data sets of unprecedented sensitivity, cadence and spectral coverage that will advance general astrophysics and open discovery space opportunities for diverse scientific communities. Previous CMB experiments have not had the charge or funding to make data rapidly available and generally usable. It is essential that CMB-S4 produce transient alerts, as well as calibrated maps in all bands and on all angular scales that are openly usable and accessible on as rapid a cadence as practical. This is not necessarily at the same level of precision needed for CMB analysis. This will both maximize and justify the significant national investment in the observatory, even if it does require some nominal level of additional funding to accomplish. 7.6.1.4 The Next Generation Very Large Array For the last four decades, the Karl Jansky Very Large Array (JVLA) Radio Telescope has been the premier observatory world-wide for accessing the sky at centimeter wavelengths. Likewise, the Very Long Baseline Array (VLBA), with its continental baseline, has extended centimeter radio observations to make images with exquisite angular resolution and perform precise astrometry. Both of these facilities have been upgraded since their inception, but they are now at a stage where further significant performance improvements are fundamentally limited by the quality of the antennae, and by their total number and allowable configurations. The next generation Very Large Array (ngVLA) project is a powerful observatory that will replace both the JVLA and VLBA. The ngVLA is an array of up to 244 reflector antennas distributed across North America, operating at frequencies from 1.2 to 116 GHz. As conceived, it would achieve velocity resolution as fine as a fraction of a m/s, sub-milliarcsecond angular resolution, and high-fidelity imaging capabilities on scales from milliarcseconds to arcminutes. The project would have broad, flexible capabilities and provide science-ready data products accessible to a diverse community of users. Such a facility would advance multiple high priority science questions from each of the six Science Panels,14 and open discovery space. These include searching for diagnostic radio emission in compact object mergers from current and future ground- and space-based gravitational wave observatories, mapping the circumgalactic and intergalactic media, cold gas flows inside distant galaxies, and features on the surfaces of nearby stars. The ngVLA would resolve protoplanetary disks on scales more than 20 times finer than ALMA, potentially capturing images of planet formation in action. The ngVLA facility would be absolutely unique worldwide in both sensitivity and frequency coverage. Conclusion: It is of essential importance to astronomy that the JVLA and VLBA be replaced by an observatory that can achieve roughly an order of magnitude improvement in sensitivity compared to these facilities, with the ability to image radio sources on scales of arcminutes to fractions of a milliarcsecond. A TRACE analysis of the ngVLA was performed, and the RMS panel undertook its own evaluation of the technical, cost and programmatic factors and risks. While there are some schedule threats related to antenna prototyping and the high required delivery rate, the technical risk is low, and the overall risk rating is medium-low. The TRACE budget assessment for design and construction is $3.2 billion (FY2020), which is within 5 percent of the RMS panel assessment, and ~20 percent higher than the project estimate. This discrepancy is reasonable given the early stage of development. The project aims to eventually secure 25 percent of the required funding from international partners, with 75 percent being provided by NSF. If design and prototyping were undertaken soon after the release of this report, 14 See Table M.1 in the report of the Panel on Radio, Millimeter, and Submillimeter Observations from the Ground. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-27

construction could start in 2027, with operations beginning with a partial array starting in 2034. In the survey’s assessment, the biggest uncertainty is whether funding at the required level can be secured, given that this would be by far the largest project to be supported by the MREFC line. Recommendation: The National Science Foundation (NSF) should proceed with a program to support science design, development, cost studies, and antenna prototyping for the Next Generation Very Large Array. After completion of the studies, NSF should convene a review to assess the project’s readiness and available budget and proceed with construction if possible. The project as presented to the survey is extremely ambitious in scale, and this could significantly extend the timeframe for commencement of science observations at design sensitivity. An important element of the design studies is whether the overall project cost can be significantly reduced with an acceptable impact on the science. For example, the survey notes that many of the important science objectives could be met with longer integration times, providing opportunity for rescoping of the hardware. Further, it will be important for the project to work during this time to secure international partners who would contribute to the project design, construction and operation at a significant level. This will not only reduce cost to NSF, increasing likelihood of timely completion, but tap into world-wide scientific and technical expertise. The mid-decadal review will be a timely opportunity to examine the status of the intended design and design trades, and to assess them against the science goals of the survey. If sufficient progress is made on design, prototyping, and refinement of cost, and if budgets allow, an additional external review of the project would be necessary to consider whether to commence with implementation toward the end of the decade. If technical progress is not sufficiently rapid, or, if the required funding cannot be secured, the next decadal will need to weigh the implementation of the ngVLA relative to other opportunities. 7.6.2 Sustaining Activities: The Astronomy Mid-Scale Programs Mid-scale programs across the entire range of scales (~$4 million–$120 million) are vital to the enabling foundation of astronomy research, and for capitalizing and amplifying return on our investment in major facilities. They enable new transformative capabilities by incentivizing creative approaches from the community for cutting-edge instruments and experiments. They ensure robust capabilities for basic research through continually refreshed instrumentation suites. They also provide broad access for the community across public-private partnerships, international system of platforms, observing modes, and wavelengths for individual-investigator initiated programs, large survey programs, and archival research. As described in Chapter 6, in the last decade, NSF established a Mid-Scale Innovations Program (MSIP) within AST, and more recently, in 2018, an agency-wide Mid-Scale Research Infrastructure (MSRI) program. The survey received a large number of APC white papers for midscale projects, concentrated at the higher end of the cost range (~$100 million) that were evaluated by the OIR, Panel on Particle Astrophysics and Gravitation, and RMS program panels. All three panels provided multiple superb examples of compelling mid-scale ideas in this cost range. The panels all emphasize the high science value, cost effectiveness, and agility of mid-scale programs at all cost levels to address new science opportunities throughout the decade. Across the range of project scales, mid-scale programs are essential both for achieving the broad range of science prioritized by the survey, for addressing targeted strategic goals, and for ensuring that existing facilities have modern instrumentation to maximize their scientific productivity and community access (see Section 5.1.3). Accordingly, the survey believes that the return from the MSIP and MSRI funding programs will be maximized if resources are deployed in a balanced manner that simultaneously accommodates: PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-28

 Open competition of new ideas - activating the community’s creativity with minimal restrictions on scientific focus in order to fuel new, inventive, cutting-edge approaches that respond to emerging scientific opportunities.  Targeted solicitations designed to advance decadal priorities – responding to identified scientific objectives that can be achieved using mid-scale facilities.  Opportunities targeted at sustaining and advancing instrumentation on existing telescopes – maintaining U.S. competitiveness in ground-based astronomy, and optimizing scientific returns from current facilities. Recommendation: The National Science Foundation (NSF) Division of Astronomical Sciences (AST) should create three tracks within the AST Mid-Scale Innovations Program and within (its share of) the NSF-wide Mid-Scale Research Infrastructure Program. The first track should be for regularly competed, open calls, the second track should solicit proposals in strategically identified priority areas, and the third should invite ideas for upgrading and developing new instrumentation on existing facilities. All tracks should solicit proposals broadly enough to ensure healthy competition. This survey provides the following advice for each of the three tracks: 1. Open calls would continue to emphasize innovative ideas in any area of astrophysics over a wide range of project scales and scientific objectives, consistent with the approach taken in the current AST MSIP program. 2. The strategic priorities track is an essential addition to the existing mid-scale program structure to ensure that it is responsive to decadal and community strategic priorities. The survey expects that these strategic programs will be at the larger end of the mid-scale cost range (i.e., at the ~$100 million level). Therefore, partnerships with other organizations or agencies, including internationally, may be desirable or appropriate. Program directors would be empowered to weigh programmatic considerations in balance with the recommendations of external reviews. The survey has identified one top priority for this element, a time-domain astrophysics program, and two co-equal areas – highly multiplexed spectroscopy and radio instrumentation: a. A time-domain astrophysics program. This program would support a wide range of activities time domain astrophysics. A priority is to maximize the return from Rubin Observatory and other time domain facilities by, for example, supporting efforts to produce efficient triggers, perform time-domain data analysis, and optimize the identification, classification, and notification of transient events (often referred to as event brokers). It is essential that instrumentation aimed at effective time domain and multi-messenger follow-up and spectroscopy be supported. This element is the highest priority for immediate implementation of the mid-scale strategic areas. Based on white paper inputs, the expected costs for these efforts range from $4 million–$40 million. b. Radio instrumentation. The survey received compelling white papers that lay out exciting new projects in radio astronomy, including a wide-field radio camera, and projects to map the evolution of neutral atomic hydrogen in the very early universe. These are major, MSRI-2 scale efforts that could be competed for implementation starting this decade. c. Highly multiplexed spectroscopy – large surveys, such as that to be carried out by the Rubin Observatory, require extensive spectroscopic follow-up. Many of the science panels, as well as the OIR program panel, emphasized the need for new capabilities and especially those that are publicly available, to advance the survey’s science priorities. Noteworthy science areas included galactic archeology and the spectroscopy of stars on a massive scale for understanding stellar abundances and evolution. In the near term, investments that provide public access to some combination of SDSS-V, DESI, and the PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-29

Subaru Prime Focus Spectrograph (PFS), or similar surveys, would help to advance science this decade with relatively modest funding, and later in the decade a major (MSRI-2 scale) investment could be made in a larger, dedicated facility. 3. The sustaining instrumentation element is intended to address the pressing need to maintain and upgrade capabilities on U.S.-led telescopes and to develop state of the art instrumentation on existing facilities to keep them at the scientific forefront. With the survey’s top large recommendation being investment in the U.S. Extremely Large Telescope (ELT) program (Sec 7.6.1.1), the need for complementary instruments on a range of smaller OIR telescopes will become more pressing in the coming decade. Upgrades to 6–10 m class instrumentation will ensure the ability to conduct supporting and preparatory science. Smaller telescopes will be essential for conducting surveys, and will also serve as testbeds for demonstrating new technologies (Box 6.1). Sustaining instrumentation calls would be open to all facilities, public and private, and would support investments for private telescopes that emphasize community access in exchange for instrument investments (see Sec. 5.1.3 for an extensive discussion of this issue). In addition these calls would support upgrades to public facilities such as the Green Bank Telescope, Gemini, and CTIO. External peer review remains the gold standard for recommendations and rankings in all three tracks. The selection criteria for all mid-scale projects would emphasize broad community access. This access could be gained through negotiated “dollars for community time” agreements (as in the former TSIP program), inter-facility “instrument time swap” agreements, public access to proprietary/consortium survey data, or in other ways. The guiding principle is that midscale investments serve to enhance the capacity of the portfolio of research capabilities to which the community has access. Given the strong endorsement of many projects by the program panels, the analysis performed by the EF panel, the expected endorsement of ground-based solar physics projects by the solar and space physics decadal survey, and the survey’s recommendation to add strategic calls to NSF’s mid-scale programs, current mid-scale funding levels are inadequate. There is strong motivation to support projects across all scales, from $4 million to the large (>~$100 million) efforts, and across all wavebands. The survey estimates that this will require funding at a level of ~$50 million a year dedicated to AST, provided through a combination of MSIP and MSRI. Conclusion: Current budget levels for AST mid-scale projects are not sufficient to advance the full range of astronomy and astrophysics priorities. Recommendation: The National Science Foundation should increase the funding available in its mid-scale programs that support astronomy and astrophysics with a target of reaching $50 million per year for the combination of the Mid-Scale Innovations Program and the Mid-Scale Research Infrastructure Program. The appropriate distribution of funding among the three tracks is best determined by proposal pressure. All elements of the program are essential to the survey’s objectives. 7.6.3 NSF Physics Projects Central to Astro2020 As discussed in Chapter 1, the task of the survey is to “develop a comprehensive research strategy to advance the frontiers of astronomy and astrophysics.” Increasingly facilities and projects that are planned and supported through the Division of Physics at NSF are essential to advancing these frontiers. The LIGO gravitational wave observatory, part of NSF’s Gravitational Physics Program, is the prime example. The discovery of gravitational waves from merging black holes in 2015 propelled LIGO to its current essential position as a premier observatory for understanding the demographics and astrophysical PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-30

implications of black holes, and for identifying the sources of heavy elements in the universe. These are among the most rapidly advancing areas in modern astrophysics, and future discoveries are likely to bring additional surprises. Therefore, the future of gravitational wave detection is central to progress in astronomy and astrophysics, and to this report. Another facility with strong overlap with astrophysics is IceCube, which is part of NSF Physics Astroparticle Physics program. New messengers and new physics is a central theme of Astro2020, and firm associations of high-energy neutrino events with astrophysical objects promises to provide unique information on particle acceleration in some of the most extreme environments near black holes. NSF recognizes that central motivations for future investments in gravitational wave detection and high energy neutrino detection lie in astronomy and astrophysics. In their briefing to the steering committee in July, 2019, NSF emphasized that its Physics Division would like to have an evaluation of the importance of these, and other programs in NSF Physics (in the divisions of Plasma Physics, Nuclear Physics, and Elementary Particle Physics), but that ranking them relative to projects led out of AST would not be helpful, given the different advisory and funding mechanisms. Further, the survey was only given budget guidance for NSF AST, and for the agency-wide MREFC program. Of the NSF Physics Division programs, gravitational wave and high energy neutrino detection stand out for having essential scientific motivation in astrophysics. 7.6.3.1 Technology Development for Future Ground-based Gravitational Wave Observatories Gravitational wave astrophysics is one of the most exciting frontiers in science. One of the survey’s key science priorities is the opening of new windows on the dynamic universe, with gravitational wave detection at the forefront. To achieve this goal, the continued growth in sensitivity of current- generation facilities such as LIGO through phased upgrades is essential. In 2018, the “A+” upgrade was approved to reduce the quantum and thermal noise in the detectors. Installation of this upgrade is beginning, and it is expected to begin operating in 2024, with an ultimate astrophysical reach of about a billion light years for neutron star coalescences, although it may take several years to achieve that sensitivity. New technologies are being developed for more advanced detectors in the current LIGO facilities (named “Voyager”) that would bring additional sensitivity improvements, including the use of silicon rather than fused silica for the test masses and suspension fibers, operation at 100 Kelvin, and a laser source of a different wavelength. In the longer term, the international community is planning for next-generation interferometers, such as the U.S. Cosmic Explorer, and the European Einstein Telescope that will make dramatic leaps in science capability. The rate of binary neutron star detections will be sufficient to make precise measurements of the Hubble constant through the detection of electromagnetic counterparts. For merging black holes the signals will be loud enough for precision tests of general relativity, and for nearby neutron star coalescences tight constraints can be placed on the equation of state of dense material. With these facilities black hole mergers can be detected out to high redshifts. In addition, intermediate mass black hole mergers can be used to probe their cosmic evolution and provide insights into the first seeds. Both concepts for third generation gravitational wave detectors, Einstein Telescope and Cosmic Explorer (CE), have longer baselines to diminish the impact of seismic and thermal noise, requiring large costs in vacuum systems. The CE concept is a single L shaped detector constructed on Earth’s surface with arms 40km long, using technology being developed for the A+ upgrade of the 4 km, for a 10-fold increase in sensitivity. Similar to strategic mission technologies, maturation research is needed for more economical vacuum systems, use of heavier masses, improved vertical seismic isolation and reduction of gravitational influence of nearby terrain (“Newtonian noise”). The use of Voyager technology (cryogenics, different materials and wavelengths) could be used in the future to further improve CE sensitivity. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-31

Conclusion: Gravitational wave detection is an essential capability for advancing the frontiers of astronomy and astrophysics. Next-generation ground-based gravitational wave observatories can achieve breakthrough capabilities and accomplish science central to the priority objectives of this survey. Continuous technology development will be needed this decade for next generation detectors like Cosmic Explorer. These developments will also be of benefit to the astrophysical reach of current facilities. While not funded out of AST, these efforts are central to achieving the science vision laid out in the survey’s roadmap, and the survey strongly endorses their importance to astronomy and astrophysics. 7.6.3.2 The IceCube Generation-2 Neutrino Observatory Observations of high-energy neutrinos enable astrophysical advances in the study of some of the most energetic phenomena in the universe. In particular, the most extreme accelerators in the universe produce huge luminosities of charged particles and accompanying gamma rays and neutrinos, with per- particle energies ranging up to the TeV-PeV range, and sometimes higher. The IceCube observations of the diffuse neutrino flux suggest a dominant population of sources that are gamma-ray obscured, showing that neutrino observations are essential for understanding and studying such energetic phenomena. A large-scale MREFC investment by NSF in IceCube-Gen2 would greatly enhance this observatory’s capabilities. “IceCube-Gen2 will increase the annual rate of observed cosmic neutrinos by a factor of ten compared to IceCube, and will be able to detect sources five times fainter than its predecessor. Furthermore, through the addition of a radio array, IceCube-Gen2 will extend the energy range by several orders of magnitude compared to IceCube.”15 The primary scientific objectives for this upgrade are to resolve the bright, hard-spectrum TeV-PeV diffuse neutrino background into discrete sources, make the first detections at higher neutrino energies, and identify neutrino emission with specific astrophysical sources in order to gain insight into sites of extreme particle acceleration. The PAG panel, supported by a TRACE study of the observatory upgrade, finds that the project is well-understood, uses mature technology, and with a cost of $345 million in FY2020 is feasible to implement this decade. This survey was not charged to make project recommendations to NSF PHY; however, we endorse the observatory as important to key astrophysics scientific objectives of this survey. Conclusion: The IceCube-Generation 2 neutrino observatory would provide significantly enhanced capabilities for detecting high-energy neutrinos, including the ability to resolve the bright, hard-spectrum TeV-PeV neutrino background into discrete sources. Its capabilities are important for achieving key scientific objectives of this survey. 7.6.4 Prioritization of NSF Sustaining and Frontier Activities For NSF, the survey’s top priority in the medium and large category is to complete the observatories in development, and ensure they are fully supported for operations and science (Table 7.1). The mid-scale programs, MSIP and MSRI are recommended not just to be maintained, but for a significant augmentation. For new projects, the survey does not rank the sustaining mid-scale program augmentation against major new facilities. Both are essential for an optimal, balanced program. Among the large, frontier AST projects, the survey gives the U.S. ELT program the highest priority due to the large scientific reach, and the pressing funding needs and maturity of the two constituent telescope 15 The IceCube-Gen2 Collaboration: M.G. Aartsen, R. Abbasi, M. Ackerman, J. Adams, J.A. Aguilar, M. Ahlers, M. Ahrens, et al., 2019, IceCube-Gen2: The window to the extreme universe, white paper submitted to the Astro2020 Decadal Survey, arXiv:2008.04323. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-32

projects. The survey does not prioritize between CMB-S4 and the ngVLA design and development; both will provide transformative science in different arenas, and the efforts should both proceed as soon as funding becomes available. In the sustaining programs category the augmentation of mid-scale programs and their restructuring to include strategic calls is the single priority. The funding distribution between open, strategic, and sustaining instrumentation calls needs to be balanced, and adjusted over the decade to respond to proposal pressure and strategic needs. At the request of the agency, the survey does not rank projects led out of NSF Physics. However, the survey strongly endorses the central role played by ground-based gravitational wave observatories to many of the survey’s high-priority science questions, and urges NSF to invest in a healthy program to develop technologies for future LIGO upgrades and next- generation facilities. In the frontier observatory category the survey concludes that NSF Physics Division’s IceCube-Gen2 neutrino observatory will have impact on several of the priority science questions and has a central role in the New Messengers, New Physics theme, but again it is not directly ranked. 7.7 NASA’S PROGRAM OF RECORD In this section we provide an assessment of, and advice where needed on the implementation plans for Roman, Athena, and LISA, because these missions are still at a stage in development where this advice could impact the Astro2020 scientific agenda. 7.7.1 Roman Space Telescope The Roman Space Telescope (formerly WFIRST) was the highest space-based priority of the Astro2010 decadal survey. It was envisioned in that report to be a $1.6 billion, 1.5 m near-IR telescope enabling diffraction-limited imaging and low-resolution spectroscopy over a wide field of view. The primary science drivers included: (1) constraining dark energy through measurements of weak gravitational lensing, supernovae distances, and baryon acoustic oscillations, and (2) statistically assessing the frequency of Earth-mass planets on orbits of ~1 AU and greater by carrying out a microlensing survey towards the bulge of the Milky Way Galaxy. Astro2010 additionally called out an open “guest observer” program that would take advantage of the large field of view. A key aspect of the mission’s recommendation was its relatively low technical and cost risks. The Astro2010 plan called for a 5-year mission primarily focused on survey science, with an additional 5-years to “improve statistical results” and to “further broaden the science program.” The current design for the Roman Space Telescope has many similarities to that originally envisioned by Astro2010, but also has key differences. The National Reconnaissance Office gave NASA a 2.4 m space telescope that became the centerpiece of WFIRST-AFTA (for Astrophysics Focused Telescope Assets). Coincident with the change to a larger telescope, an exoplanet-imaging Coronograph Instrument (CGI) was also added. The primary goal was for technology demonstration, although the initial design had significant scientific capability. Since then, to maintain overall project schedule and budget constraints (NASA has adopted a $3.5 billion cost ceiling for Roman), CGI has had its capabilities significantly descoped. In addition to its wide field of view, a key feature of Roman is its rapid time to slew and settle, with no Earth occultations given its orbit at L2.16 The telescope is currently scheduled for a 2026 launch. 16 L2, or Lagrange point 2, is a location in space directly behind Earth as viewed from the Sun where gravitational forces are balanced such that a telescope placed at this point will stay in line with Earth as Earth moves around the Sun. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-33

A number of reviews were undertaken to evaluate the consistency of the new mission design with Astro2010 recommendations. The 2014 NRC WFIRST/AFTA17 study concluded that the increase in aperture resulted in a powerful mission meeting all of the science goals of Astro2010, and that the addition of CGI was a positive step if it focused on technology development and did not drive key mission requirements. In order to remain consistent with the balanced program recommended by Astro2010, the WFIRST/AFTA report also noted containing costs would be critical, and it recommended an independent review of mission scope prior to formal adoption. The resulting WFIRST Independent External Technical/Management/Cost Review (WIETR) was held in 2017, and in 2020 the mission underwent a successful Key Decision Point-C review, formally confirming the implementation phase. Evaluating Roman in the current landscape, the mission remains both powerful and necessary for achieving the scientific goals set by Astro2010. Roman’s cosmological constraints complement those of Euclid and Rubin, with its main contribution expected to come from the 1.8<z<2.5 redshift range. At lower redshifts its constraints on the expansion history are not expected to improve upon Euclid’s, owing to that telescope’s much wider sky coverage. As the systematic errors of Euclid, Rubin, and Roman are different, the three experiments will provide important verification of each other’s results; this is particularly important if Euclid finds significant deviations from standard models for Dark Energy. Roman is also the only platform in the coming decades that can produce a statistical census of planetary occurrence as a function of orbital separation and mass, from terrestrials to gas giants, beyond 1 AU. Such a survey would “pick up” where Kepler completeness falls off, just inside of ~1 AU, although (like Kepler) the actual sensitivity to true Earth analogs in the habitable zone is relatively low. Finding: The Roman Space Telescope remains both powerful and necessary for achieving the scientific goals set by New Worlds, New Horizons (Astro2010). It will carry out cosmological measurements complementing those of Euclid and Rubin Observatory, and Roman’s microlensing survey will probe planetary occurrence over orbital separations not constrained by Kepler or TESS. Roman also provides substantial scientific capabilities that will contribute to achieving the science vision presented in Astro2020. The Astro2020 Science Panel reports describe in detail where Roman’s capabilities will provide significant advances relative to the key questions and discovery areas. Out of thirty questions and discovery areas, Roman will directly impact fourteen. Although the most obvious advances will be in cosmology and exoplanets, Roman’s immense discovery potential beyond those areas almost ensures that its highest impact results will come from other, and possibly unforeseen, directions. Examples are studies of high redshift galaxies, active galactic nuclei, dark matter in Local Group dwarf galaxies, stellar populations in galaxies in the local volume, the stellar mass function in star clusters, and optical/near-IR counterparts to gravitational wave events. More generally Roman will be a premier facility for obtaining deep, high resolution imaging and slitless spectroscopy at optical/near-IR wavelengths, with a field of view that is a factor of 200 larger than HST. The scientific landscape has changed significantly since Roman was first recommended by Astro2010. However, with the change to a larger telescope, Roman has also become more capable. Compared to the situation in 2010, Roman is now just one of multiple “Stage IV” projects (as defined by the Dark Energy Task Force), including the Vera Rubin Observatory’s Legacy Survey of Space and Time, Euclid, and the Dark Energy Spectroscopic Instrument. As a result of the discovery of gravitational wave sources in 2015, and the burgeoning of time-domain astronomy this decade, Astro2020 identified “New Windows on the Dynamic Universe” as one of its priority science areas for the coming decades. Roman, with its wide field of view, and flexible pointing could provide unique time domain surveys, possibly coordinated with other efforts. 17 National Research Council, 2014, Evaluation of the Implementation of WFIRST/AFTA in the Context of New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-34

In light of the altered landscape and new opportunities, it is reasonable to ask whether the allocations of survey time recommended by the Science Definition Team in mid-2015 are still optimal. The planned imaging, spectroscopy, and microlensing surveys continue to be essential parts of Roman’s mission, and a wealth of science will result from their data products, far beyond the measurement of Dark Energy parameters and exoplanet statistics. However, as currently planned, the balance of these surveys with the equally promising GO program may not be ideal, when evaluated in light of the updated survey scientific objectives. It is beyond the scope of this survey to recommend an appropriate rebalancing of time, however given that there are still 3 or 4 years until launch, a dedicated re-evaluation of the scientific program in light of this survey’s scientific priorities is warranted. Conclusion: The scientific landscape and the Roman Space Telescope’s capabilities have changed significantly since it was first envisioned by New Worlds, New Horizons, and the currently planned balance of surveys and guest investigator-led observations may not be optimally suited to take advantage of new scientific opportunities. Recommendation: NASA Astrophysics Division should hold a non-advocate review of the Roman Space Telescope’s science program to set the appropriate mix of survey time devoted to the weak lensing, baryon acoustic oscillations, supernovae, and microlensing programs relative to guest investigator-led observing programs during the primary 5 year mission. 7.7.2 The Athena X-ray Observatory NASA has joined as a partner in the second of ESA’s Flagship Cosmic Visions missions (L2), Athena. This high energy observatory, currently scheduled for launch in 2031, is oriented towards science themes of the “Hot and Energetic Universe.” Its science instruments enable wide-field X-ray imaging and sensitive spatially-resolved spectroscopy of X-ray-emitting objects. NASA plans to invest ~$200 million to $300 million, split roughly equally between hardware contributions for half the amount, and establishment of a U.S. Guest Observer (GO) program and U.S. Data Center. The planned hardware contributions include components of the two science instruments, use of the X-ray and Cryogenic Facility (XRCF) at Marshall Space Flight Center (MSFC), and a Soft-Ride system to dampen launch vibrations. These contributions leverage unique U.S. capabilities and facilities. Athena’s science instruments map well onto a wide range of the priority science questions identified by Astro2020—19 out of the 30 science questions/discovery areas will be directly addressed by the mission. The newly-established NASA Project Office, and appointment of U.S. members to Athena’s science working groups, keep the U.S. community engaged in the project. Plans for a U.S. Guest Observer program and U.S. Data Center will ensure that U.S.-based scientists will be well-supported in analysis of Athena observations. When Athena begins operations in the early 2030’s it will be the premier X-ray observatory in space, and the United States will be well-positioned to play a significant role in the science it produces. Conclusion: The scope of the U.S. investment in the European Space Agency’s Athena mission is appropriate both for the hardware contribution as well as for the U.S. Guest Observer and science center. This investment will enable substantial scientific involvement by the U.S. community in this exciting mission. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-35

7.7.3 The Laser Interferometer Space Antenna Mission The Laser Interferometer Space Antenna (LISA) was the third highest ranked large strategic mission in Astro2010, after WFIRST (now Roman) and a major augmentation to the Explorer program. At the time that LISA was evaluated by Astro2010, enthusiasm for the science that this low-frequency gravitational wave mission could achieve was very high, however Astro2010 judged that advancement to the highest priority large strategic mission should be contingent on the success of the LISA Pathfinder technology demonstration mission, and also further development of the mission concept, costs and risks. The mission’s scientific potential was judged to be at the very highest level, but more technological development and risk reduction were deemed necessary prior to recommending LISA for a mission start in the 2010–2020 decade. Since Astro2010 there has been major progress on both scientific and technical fronts. The LISA Pathfinder mission demonstrated crucial, high-risk components of the mission’s precision metrology capability, exceeding all of its performance requirements. Pathfinder placed two test masses in a near- perfect gravitational free fall, and it controlled and measured their motion with unprecedented accuracy. To do this it used inertial sensors, a laser metrology system, a drag-free control system and an ultra- precise micro-propulsion system. This demonstrated LISA’s highest risk components in a space environment and with a practical implementation. LIGO’s detection of gravitational wave sources in 2015 was a transformational event that reinvigorated excitement about LISA’s scientific potential. In 2017 ESA accepted a proposal to develop a version of LISA with launch expected 2034 or after. Based on LISA’s promise and the enthusiasm of the U.S. scientific community, NASA established the NASA LISA Studies Office (NLSO) to estimate the cost of, and coordinate U.S. contributions to the mission. NASA currently plans to contribute an equivalent of $400 million in mission hardware by supplying the telescope, laser, and charge management systems as well as phasemeters and micro- thrusters. As LISA develops, additional areas where NASA can make a critical or especially effective contribution may become apparent. The survey committee evaluated the scientific case for LISA in light of recent LIGO/Virgo measurements, and with the priority science questions in mind. The scientific case for the mission remains rich and compelling. For example, LISA will observe hundreds of stellar mass binary black hole systems, some of which would cross into the band of ground-based observatories like LIGO/Virgo weeks to months later. LISA also complements nano-Hz gravitational wave measurements using pulsar timing arrays; the latter are sensitive to billion solar mass black hole mergers while LISA is sensitive to ~104-6 solar mass black hole mergers (Figure 2.14). Several target sources for LISA will produce electromagnetic signals that can be followed up in several different bands with space and ground observatories. Some events may produce particles detectable on Earth. LISA will thus greatly advance our multi-messenger view of the universe. While LISA builds on experience gained from ground-based gravitational wave detectors, it will have distinct data analysis and processing challenges. Unlike LIGO/Virgo, at any instant LISA will measure the superposition of multiple sources, with many signals lasting months or years. The instrument noise (statistical and systematic) will also be qualitatively different than that of ground-based detectors. The LISA analysis will therefore differ significantly from that used for LIGO-like instruments, and new computational techniques must be developed in order to interpret the data. The format of the LISA analysis program within ESA has not been set. Various models are under consideration. There is an opportunity for significant U.S. engagement and coordination with ESA to continue and extend the collaboration begun with the hardware through to the data analysis. With anticipated contributions from NASA, LISA’s sensitivity (the L3 proposal to ESA)18 will be close to that of the Astro2010 reference mission19 and Astro2010 mid-decadal assessment. In regards to scientific 18 Amaro-Seoane et al., 2017, arXiv 1702.00786. 19 Stebbins et al, LISA Astro2010 RFI #2 Space Response PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-36

partnership and data analysis, there are many future opportunities for the U.S. community for NASA to embrace. Conclusion: ESA’s LISA mission remains a very high priority for the U.S. community, and NASA contributions of hardware and data analysis tools are essential to ensuring the full scientific capability of the mission is achieved, as envisioned by New Worlds, New Horizons and the subsequent mid-decadal assessment. It is also essential to maintain a vibrant U.S. community to prepare for data analysis and science. Recommendation: NASA should work with the European Space Agency to ensure the Laser Interferometer Space Antenna (LISA) achieves the full scientific capability envisioned by New Worlds, New Horizons. NASA should continue calls for LISA Preparatory Science with a known cadence during the decade. After a jointly developed plan for LISA data analysis and management are clear, and a few years prior to launch, NASA should establish funding for LISA science at a level that ensures U.S. scientists can fully participate in LISA analysis, interpretation, and theory. 7.8 BUDGETARY ANALYSIS This section evaluates the budgetary requirements for the recommended program separately for NASA, NSF, and the DOE. The evaluation adopts the cost and schedule profiles determined by the TRACE analyses, with minor adjustments made in some cases to reflect the judgement of the program panel regarding technology and programmatic readiness. It is important to note that this analysis assumes that the agencies are able to provide the optimal funding profiles for the given project or mission. It further assumes that the agencies make the recommended early investments in project/mission and technology maturation. Deviations from the optimal budget profiles that reduce funding in years of peak spending will extend development periods and increase the total mission cost relative to this analysis. This survey emphasizes the need for investment in maturation programs (e.g. the Great Observatories Mission and Technology Maturation Program for NASA, and ngVLA design and prototyping efforts for NSF) so that costs and schedules can be more accurately determined prior to mission/project adoption. This will assist NASA and NSF in their planning to meet peak costs successfully, an important factor in containing total project costs. 7.8.1 NSF Analysis 7.8.1.1 MREFC Program The budget profile analysis shown in Figure 7.8 presents the program outlined in the roadmap of new ground-based major projects in terms of the NSF share of expected program/project cost, and it compares the total cost with the budget projection provided by NSF. The chart runs through FY2041 to capture the expected completion of the ngVLA. The TRACE cost and schedule estimates are used for construction, and project or Program Panel estimates for operations. In some cases, the phasing and durations were adjusted to manage several factors: approximate budget, technology development/readiness, and other programmatic factors. For example, the U.S. ELT program, consisting of TMT and GMT, was spread out over an additional 2 years, lowering the peak spending proposed by the projects, but consistent with the OIR program panel’s judgement related to the rapidity with which NSF funding could be provided. For the ngVLA, the RMS panel recommended an additional 2 years of design and development relative to plans provided by the project prior to any major ramp up of construction efforts, and we incorporate that into this analysis. The NSF MREFC share assumed for the ELTs is 25 PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-37

percent of the total construction costs for each telescope, for CMB-S4 40 percent of the costs are assumed to be borne by NSF (60 percent by the DOE), and for the ngVLA, the NSF share is assumed to be 75 percent of total costs, with 25 percent to be identified in the future international partners. These fractional funding levels were adopted from the project white papers and presentations. The MREFC budget profile also includes current commitments and a growing wedge for the agency-wide MSRI mid-scale programs as provided to us by NSF. Mid-scale projects are discussed elsewhere, but the committee assumed approximately 15 percent of the NSF-wide MSRI budget line shown here might be successful AST projects. The remainder of the mid-scale funding would need to come out of the AST budget to achieve the total $50 million a year target. FIGURE 7.8 Recommended program for the National Science Foundation (NSF) MREFC. The chart assumes that the agency-wide midscale MSRI funding wedge given to the survey by NSF is realized. It also includes agency-wide prior commitments, and the new NSF construction funding required to realize the large AST projects recommended by the survey. The solid line shows the MREFC budget guidance provided to the survey by NSF through 2030, and extrapolated beyond this using 2.7% inflation. Note that for CMB-S4, U.S. ELTs, and the ngVLA, which have additional contributions from other agencies and partners, only the NSF-share of the total funding is shown. The operations costs for new facilities are included in the budget chart for NSF AST; see Figure 7.9. 7.8.1.2 NSF AST Budget Budget projections for NSF AST shown in Figure 7.9 take into account the existing components of the budget in the areas of education, research, and infrastructure, as well as additions recommended from this decadal survey. The specific recommended items in chapters 3, 4, 6, and 7 for the division span all three of these categories; only recommendations for which specific enough guidance is given to be able to assign a dollar amount are included in these projections. The infrastructure component, in particular, encompasses operations and maintenance for facilities including the National Solar Observatory, the National Radio Astronomy Observatory, the National OIR Astronomy Research Laboratory, and others, as well as an AST Portfolio Review Implementation, Midscale Research Infrastructure, and Research Resources. The starting point for projecting the NSF AST budget and PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-38

examining impacts is the FY2019 budget actuals.20 Operations costs associated with new facilities expected to come online in this decade and the next also figure into the calculations. DKIST has already begun initial operations, and the budget ramps up to full operations in 2022. The Rubin Observatory is gearing up for science operations estimated to $30 million in 2024. Operations budgets associated with new MREFC projects are phased in at the appropriate time as new demands on the budget; operations for the JVLA ramp down as the ngVLA ramps up, in accordance with project and panel guidance. Operations for ngVLA also phase in a partial array for limited early science while the remaining antennas are being integrated, based on advice from the RMS panel. FIGURE 7.9 NSF AST budget requirements for implementing the programs recommended by this decadal survey. The required funding is based on existing budget items as well as the recommended additions. Solid colors indicate existing budget components, adjusted for inflation at 2.7 percent. The orange bar indicates research, along with education, research resources, and other minor budget components. Hatched shading indicates the additions to these three areas resulting from recommendations in this report. Small and Medium Research Infrastructure Additions is in large part the ATI augmentation. Figure 7.9 shows the required budget profile for NSF AST that results from these additions. The increases in non-operations funding have only a modest impact on the total budget profile for the division. Adding operations costs for new MREFC-funded facilities on top of the existing budget burden for operations and maintenance clearly becomes unsustainable going forward, without recourse to a new paradigm for accounting for operations impact. There will need to be significant increases in research funding to accommodate the demand based on impending science from new facilities. The increase to individual investigator grants called out in chapter 4 and included here is a minimum amount. Even with a new paradigm for operations funding that is not within NSF AST, the growth in the field spurred by new facilities will need to be matched by similar increases in support for research to ensure a robust science environment. 20 https://www.nsf.gov/about/budget/fy2021/pdf/27_fy2021.pdf PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-39

7.8.2 NASA Analysis The budget profile shown in Figure 7.10 presents the roadmap outlined above in terms of the expected program or project cost, and then compares the total with the optimistic budget projection provided by NASA. The chart runs through FY2050 to capture the expected completion of the IR/O/UV large strategic mission. The phasing of each program element in the roadmap is adjusted in order to manage several factors: available budget, technology development/readiness, pre-cursor programmatic work and studies and science priorities established by the survey committee. Though the IR/O/UV mission drives the total program to exceed the yearly available budget between FY2035 and FY2043, the integrated cost of all elements in the program through FY2043 is approximately $23.5 billion, about $0.15 billion less than the integrated available budget over the same period. As demonstrated in the past, it is expected that NASA will work within the federal budgeting process to assure that peak budgetary requirements are met while sustaining its portfolio as a whole. In Figure 7.10, the Great Observatory Mission Maturation and Technology Development (GOMMTD) program is shown broken into its constituent parts. This program consolidates mission maturation activities, technology development and management activities. All large strategic mission activities start within the maturation program. When a large strategic mission achieves sufficient maturity, and has a scope consistent with decadal recommendations, mission-specific funding begins. In parallel, mission maturation and technology development for additional large strategic mission commence. In Figure 7.10, a notional future (deep blue) $5 billion class GO is shown undergoing the same development strategy as implemented for the IR/O/UV mission. FIGURE 7.10 Astro2020 recommended program for NASA. This budget profile shows approximate funding requirements associated with construction and operation of all space-based medium and large recommendations. For the IR/O/UV mission, operations are assumed to extend beyond 2050. The ultimate project/program profiles and budget requirements will depend on the actual implementation and on NASA’s budgeting process. The chart shows a program whose costs integrated through FY2043 are approximately equal to the budget available over the same period. The solid line indicates the optimistic budget projection that NASA provided to the survey. The jump in NASA’s available astrophysics budget around 2025 reflects completion of Roman and reduction in other current commitments. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-40

7.8.3 DOE Analysis The CMB-S4 observatory is the only new facility where the project assumed a formal commitment of funding from the DOE. Adopting the total cost estimate from the TRACE analysis, the 60 percent DOE share for construction results in a total commitment of $408 million. The operations costs are also assumed at 60 percent for DOE and 40 percent for NSF. 7.9 ANALYSIS OF CONSISTENCY WITH BUDGETARY GUIDANCE The survey committee performed an analysis to assess whether the proposed program of new activities presented in the roadmap is consistent with envisioned budget profiles provided by the agencies. All three agencies urged the survey committee to present an ambitious vision that would motivate increased federal investment in their astronomy and astrophysics portfolios, and we use the optimistic scenarios given to us for planning. The TRACE project cost and schedule estimates for construction have been adopted where available, with some elements shifted in time to reflect technical readiness and other factors. Details of these assumptions are provided in the Program Panel appendixes for the individual projects and activities. Integrated over time, the proposed portfolios fit within the agency budget guidance, even though in years of peak spending for major projects, this guidance is exceeded. The survey considers this to be appropriate for agencies with budget lines that are driven by the requirements of major projects. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7-41

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We live in a time of extraordinary discovery and progress in astronomy and astrophysics. The next decade will transform our understanding of the universe and humanity's place in it. Every decade the U.S. agencies that provide primary federal funding for astronomy and astrophysics request a survey to assess the status of, and opportunities for the Nation's efforts to forward our understanding of the cosmos. Pathways to Discovery in Astronomy and Astrophysics for the 2020s identifies the most compelling science goals and presents an ambitious program of ground- and space-based activities for future investment in the next decade and beyond. The decadal survey identifies three important science themes for the next decade aimed at investigating Earth-like extrasolar planets, the most energetic processes in the universe, and the evolution of galaxies. The Astro2020 report also recommends critical near-term actions to support the foundations of the profession as well as the technologies and tools needed to carry out the science.

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