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

Chapter: 1 Pathways to Discovery: From Foundations to Frontiers

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Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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:"1 Pathways to Discovery: From Foundations to Frontiers." 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:"1 Pathways to Discovery: From Foundations to Frontiers." 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:"1 Pathways to Discovery: From Foundations to Frontiers." 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:"1 Pathways to Discovery: From Foundations to Frontiers." 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:"1 Pathways to Discovery: From Foundations to Frontiers." 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:"1 Pathways to Discovery: From Foundations to Frontiers." 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:"1 Pathways to Discovery: From Foundations to Frontiers." 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.
×
Page 17
Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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.
×
Page 18
Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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.
×
Page 19
Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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.
×
Page 20
Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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.
×
Page 21
Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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.
×
Page 22
Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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.
×
Page 23
Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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.
×
Page 24
Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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.
×
Page 25
Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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.
×
Page 26
Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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.
×
Page 27
Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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.
×
Page 28
Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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.
×
Page 29
Suggested Citation:"1 Pathways to Discovery: From Foundations to Frontiers." 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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1 Pathways to Discovery: From Foundations to Frontiers We live in a time of extraordinary discovery and progress in astronomy and astrophysics. Since the dawn of the millennium, breakthroughs have come at an astounding rate, with highlights that include the first direct detection of gravitational radiation from astronomical sources; the discovery of thousands of extrasolar planets, including potential Earth-like analogs and the first characterizations of the physical properties and atmospheres for gaseous giant planets; mapping of the nascent disks of other solar systems as they are forming; a unified paradigm for the formation and evolution of galaxies, including deep insights gained from the fossil record of the Milky Way Galaxy; precision measurements of the supermassive black hole in the Milky Way’s center; the first direct image of the shadow of a supermassive black hole; and precision measurements of the dark contents of the universe itself. Six Nobel Prizes for discoveries made using astronomical data have been awarded over the past decade alone (dark energy, gravitational waves, neutrino oscillations, the discovery of exoplanets, cosmology, supermassive black holes). Many ambitious scientific visions have been fulfilled in the past 10 years, but, if anything, momentum has only grown. Every decade, the agencies that provide primary federal funding for astronomy and astrophysics—the National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), and the Department of Energy (DOE) Office of Science—request a decadal survey to assess the status of, and opportunities for the nation’s efforts to forward our understanding of the cosmos. The National Academies of Sciences, Engineering, and Medicine responds by convening a body of experts with diverse interests and expertise to undertake this task, with a resulting report that advises the agencies about how to best deploy resources to advance knowledge in these areas. This survey’s key objective is to map the national and international scientific landscape and to chart a path for investment, identifying programs with transformational scientific potential and new observational capabilities. Also central to the survey’s charge is to assess the health of the profession and the balance of investments in the people and scientific infrastructure crucial to advancing the understanding of the cosmos. This report lays out a strategy for federal investments aimed at paving a pathway from the foundations of the profession to the bold scientific frontiers. This chapter provides an integrated view of the strategy, analysis, and advice contained in Chapters 2-7. It is not a comprehensive summary of the report, but rather describes the recommended program in the broader context and framework in which this decadal survey was conducted, articulating the approach for building a scientifically broad, balanced, sustainable program that seizes the opportunities before us. 1.1 THE SCIENTIFIC OPPORTUNITIES We stand on the threshold of new endeavors that will transform not only our understanding of the universe and the processes and physical paradigms that govern it, but also humanity’s place in it. The tremendous richness of 21st century astrophysics is evident in the 573 science white papers authored by more than 4,500 individuals that lay out a wide array of questions we are now poised to answer. Six PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-1

expert science panels formulated these into key science questions and discovery areas ripe for rapid progress in the coming decade. Three broad themes, described in Chapter 2, encompass these opportunities—Worlds and Suns in Context, New Messengers and New Physics, and Cosmic Ecosystems. The diversity of the science and observational techniques used to advance the associated goals is striking. Because of the balanced and varied programs put forward by prior decadal surveys, small telescopes, inventive experiments, and competed missions operating across the spectrum have harnessed the creativity and technical ingenuity of the community, resulting in an intensely dynamic and rapidly evolving enterprise. As a result, many of the questions at the forefront of the survey’s themes could not have been framed even a decade ago. The richness of these three themes demands that a broad and varied suite of capabilities be sustained over the full electromagnetic spectrum and in the new windows of gravitational waves and high-energy neutrinos. Within each overarching theme, with its multiple science objectives, the survey identifies a priority science area that captures the most transformative and far-reaching goal, where, given new, ambitious facilities, we are poised to take giant strides forward. 1.1.1 Worlds and Suns in Context The science theme of Worlds and Suns in Context captures the quest to understand the interconnected systems of stars and the worlds orbiting them, tracing them from the nascent disks of dust and gas from which they form, through the formation and evolution of the vast array of extrasolar planetary systems so wildly different than the one in which Earth resides. This is an area where advances over the past decade have been stunning, and progress in the next decade will be similarly rapid. By 2020, just 25 years after the discovery of the first exoplanet, the inventory of known exoplanets had exceeded 4,000, with more being identified nearly every week, thanks to ground-based radial velocity measurements and surveys of systems where the exoplanet partially eclipses its star (transit surveys), as well as dedicated space missions. The Kepler Discovery-class mission,1 launched in 2009, revolutionized exoplanet studies by monitoring more than 150,000 stars to detect thousands of transiting planets, enabling astronomers to explore the structure and vast diversity of planetary systems for the first time. Combining Kepler’s data with ground-based radial velocity measurements is providing essential information on exoplanet masses and densities. The Transiting Exoplanet Survey Satellite (TESS) Explorer-class mission,2 launched in 2018, is surveying the entire sky to find nearby exoplanets, thereby providing the best sample for detailed follow-up studies using current and future ground- and space-based facilities. These same missions, along with the European Space Agency (ESA) Gaia astrometric and photometric observatory, launched in 2013, and large ground-based spectroscopic surveys have also enabled great leaps in the understanding of the physics of stars, the stellar populations of stars of the Milky Way, and the Milky Way’s formation history. The astronomical community and the public alike have been galvanized by the extraordinary progress in detecting and studying exoplanets. The 2018 National Academies report Exoplanet Science Strategy3 captures this progress in rich detail. For the coming decade, key goals include applying spectroscopic and photometric observations to characterize exoplanet surfaces and atmospheres, and fully characterizing not only individual planets but also the properties of entire extrasolar planetary systems. The past decade has revealed how diverse and often different these are from our own solar system. But far more is needed to reliably assess the relative numbers of different system architectures. The upcoming Nancy Grace Roman Space Telescope, with launch expected in 2026, will conduct a microlensing survey 1 The Discovery Program is a series of small to medium-sized competed solar system exploration missions funded by NASA Planetary Science Division. 2 The Astrophysics Explorer Program is a series of small to medium-sized competed missions. 3 National Academies of Sciences, Engineering, and Medicine, 2018, Exoplanet Science Strategy, Washington, DC: The National Academies Press, https://doi.org/10.17226/25187. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-2

of the Milky Way’s galactic bulge, filling out the census by finding exoplanets in the outer reaches of planetary systems that are inaccessible by other detection techniques. Ground-based 6–10 m optical and infrared telescopes with custom instrumentation will continue to broaden demographic samples and diagnose their properties. For the study of atmospheres of exoplanets in close-in orbits, as were found in abundance by Kepler and TESS, spectroscopy with the James Webb Space Telescope (JWST), to be launched by the end of 2021, will be transformational. Millimeter, radio, and infrared observations of the gas and dust disks of forming protoplanetary systems are providing complementary clues to the factors shaping the extent and architectures of solar systems, and this is an area of great discovery potential. A rich agenda of discovery and scientific opportunity also lies ahead for stellar astrophysics. Over the coming decade, attention will focus on the most important unanswered questions, including understanding the effects of stellar multiplicity on the evolution of the stars in the system, the nature of stellar activity and activity cycles, and reconstructing the formation and assembly of the Milky Way as derived from its ancient stars. Precise distances, until recently only available for ~100,000 stars, are now available for hundreds of millions of stars, along with high-precision photometry, thanks to the ESA Gaia mission. With such a large sample, even rare types of stars and short-lived stellar evolutionary stages are well represented. At the same time, precision time-domain measurements of thousands of stars from Kepler, TESS, and the National Centre for Space Studies (CNES)/ESA Convection, Rotation and planetary Transits mission (CoRoT) have provided detailed asteroseismological measurements of their oscillations, which, like seismic measurements on Earth, unveil the internal structures and motions of material. Ground-based spectroscopy of the stars measured by the space missions will be crucial to obtain orbital velocities, chemical compositions, surface gravities, masses, rotation rates, and other fundamental properties. Spectroscopic survey telescopes in the 4-10 m class capable of observing thousands of stars simultaneously promise major advances. Finally, the Daniel K. Inouye Solar Telescope (DKIST) will revolutionize observations of the Sun’s atmosphere. Priority Area: Pathways to Habitable Worlds Over the past two decades, thousands of extrasolar planets have been discovered, almost all of them extremely different from any world in our own solar system. This decadal survey’s science theme of Worlds and Suns in Context encompasses the interlinked studies of stars, planetary systems, and the solar system. Within this broader science theme, the survey has identified the priority science area of Pathways to Habitable Worlds with the goal of trying to discover worlds that could resemble Earth and answer the fundamental question: “Are we alone?” Such planets will be found in the “habitable zone” of their parent stars—not too close and hot and not too distant and cold. Measurements indicate that around 30 percent of stars possess such a planet. The task for the next decades will be finding the easiest of such planets to characterize, and then studying them in detail, searching for signatures of life. Life on Earth has profoundly altered the planet’s atmosphere (Figure 1.1). Interpreting such “biosignatures” is not simple, but the interplay of atmospheric components such as water, oxygen, methane, and carbon dioxide can be modeled to search for evidence of life on other planets. Astronomers have already demonstrated the ability to use spectroscopy to study the atmospheres of large, hot worlds; with future facilities, the same techniques will measure the composition of small, habitable planets. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-3

FIGURE 1.1 Evolution of the reflectivity spectrum of Earth. Simulated spectra of Earth before life had significantly altered its atmosphere (top, Archean era 2.5 to 5 Gyr ago), before the development of complex life (middle, Proterozoic era from 0.54 to 2.5 Gyr ago), and the modern oxygen-bearing Earth (bottom). SOURCE: LUVOIR Report; G. Arney, S. Domagal-Goldman, T. B. Griswold (NASA GSFC). The pathway to searching for biosignatures on habitable worlds depends strongly on the properties of their parent stars. The most common stars in the Milky Way Galaxy are dim, red “M dwarfs.” Their habitable zone will be very close to the star, making the systems accessible with the transit technique. JWST will observe a few of the very best target systems. To expand that sample will require the spectroscopic sensitivity of ground-based 25-40 m extremely large telescopes (ELTs). However, M dwarf stars may not be the best harbor for life—they have massive super-flares and intense, potentially life-destroying energetic emissions. The planets around more placid Sun-like stars are essentially inaccessible to the transit technique and beyond the reach of ELTs, which must observe PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-4

through Earth’s atmosphere. Only an ultra-stable, space-based telescope equipped to block the star’s light and directly image the planet can reach this level of sensitivity. The larger the telescope, the larger the number of stars whose planets can be searched for signatures of life. Properly interpreting these observations will also require a scientific context—understanding the formation and history of these planetary systems to see how life-enabling chemicals flow onto worlds, laboratory studies and simulations of planetary atmospheres, and deeper knowledge of the stars themselves—driving a large part of the overall Worlds and Suns in Context theme. Key capabilities required on the pathway to habitable worlds include the following:  Ground-based extremely large telescopes equipped with high-resolution spectroscopy, high- performance adaptive optics, and high-contrast imaging;  A large, stable, space-based infrared/optical/ultraviolet (IR/O/UV) telescope with high- contrast imaging capable of observing planets 10 billion times fainter than their star, and UV, visible, and near-IR exoplanet spectroscopic capabilities;  A high spatial and spectral resolution X-ray space observatory to probe stellar activity across the entire range of stellar types, including host stars of potentially life-sustaining exoplanets; and  Laboratory and theoretical studies of planetary formation, evolution, and atmospheres. Life on Earth may be the result of a common process, or it may require such an unusual set of circumstances that we are the only living beings within our part of the galaxy, or even in the universe. Either answer is profound. If planets like Earth are rare, our own world becomes even more precious. If we do discover the signature of life in another planetary system, it will change our place in the universe in a way not seen since the days of Copernicus—placing Earth among a community and continuum of worlds. The coming decades will set humanity down a path to determine whether we are alone. 1.1.2 New Messengers and New Physics Our understanding of the universe has been repeatedly transformed by looking at the sky in new ways, from exploiting the full range of electromagnetic phenomena, to making large-scale, high-cadence astronomical movies, to exploring the universe in non-electromagnetic messengers. This has led to remarkable progress in astronomy over the past century, including the ever-growing impact of astronomy on basic physics. The New Messengers and New Physics theme captures the key scientific questions associated with a broad range of inquiries, from astronomical constraints on the nature of dark matter and dark energy, to the new astrophysics enabled by combined observations with particles, neutrinos, gravitational waves, and light. The unknown physical natures of dark matter and dark energy, both discovered through astronomical measurements, remain outstanding grand challenges in both physics and astronomy, and great observational progress will be made in the coming decade. Addressing these profound mysteries were prime motivations for the Roman Space Telescope, with a field of view 100 times that of the Hubble Space Telescope (HST); the NSF/DOE Vera C. Rubin Observatory, a wide-field 8.4 m telescope devoted to a decade-long mapping of the entire southern sky; as well as ESA’s Euclid mission, with a planned launch in 2022. These telescopes are all poised to address the nature of dark energy through large optical and infrared surveys aimed at measuring the distribution of galaxies on large scales, and by detecting distant supernovae. These measurements will also provide a lasting astronomical legacy, with data that can be mined to answer a variety of foundational astronomical questions. High-sensitivity and wide-angle mapping of the cosmic microwave background (CMB) has the potential to create virtual 3D tomographic maps of the matter distribution between the young universe—when there were free electrons that could readily scatter CMB photons—and Earth. These measurements can also be used to map the cosmic structure by mass (rather than by light, which is the structure traced by Roman, Rubin, and Euclid through PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-5

the starlight of galaxies). Comparing the two will reveal vital information about the structure itself, its evolution, and the evolution of differences between the distribution of light and mass. In the past decade, a new, perplexing inconsistency between the expansion rates of the universe (Hubble constant) measured from nearby stellar distance ladders versus the CMB and other cosmological yardsticks has also emerged. The latter could be an observational issue, but it could also conceivably point to a missing element of physics in the current cosmological model. New measurements of the Hubble constant made by combining gravitational wave signals with associated redshift measurements will be an entirely independent way to resolve (or confirm) this tension. The power of near-continuous monitoring of large regions of the sky in the X-ray, gamma-ray, optical, infrared, and radio bands has been dramatically demonstrated over the past two decades. Time- domain astronomy is now a mature field central to many astrophysical inquiries, from diagnosing the wide array of stellar explosions, to exoplanet detection, probing stellar structure, and measuring dramatic and unexplained changes in the appearance of active galactic nuclei—the regions closely surrounding supermassive black holes. New phenomena such as fast radio bursts, besides being events of mysterious origin, can provide a means of probing the tenuous gas in and in between galaxies. Progress in this subject will accelerate dramatically with the commissioning of the Rubin Observatory and later in the decade with the launch of the Roman Space Telescope. In particular, Rubin’s unique time domain mapping of the southern sky is expected to detect roughly 10 million variable events per night, providing optical color information necessary for rapid characterization and unique scientific inquiries. The ground-based Laser Interferometer Gravitational Wave Observatory’s (LIGO) discovery in 2015 of gravitational waves from a pair of merging 30 solar mass black holes is certainly one of the watershed moments in physics and astronomy of the last decades. Future upgrades of LIGO, the European Virgo interferometer, and the Japanese Kagra, together with the launch of the Laser Interferometer Space Antenna (LISA) low-frequency gravitational wave observatory in the early 2030s have tremendous promise to answer fundamental questions in physics and astronomy and to open vast new discovery space. Upgrades of NSF’s IceCube high-energy neutrino detector will enable these nearly massless subatomic particles to be associated with individual astrophysical objects, probing extreme environments where particles are accelerated to near-light speeds. The recent addition of the entirely new messengers— gravitational waves and high-energy neutrinos—to time domain astrophysics provides the motivation for the survey’s priority science theme within New Messengers and New Physics. Priority Area: New Windows on the Dynamic Universe This report’s science theme of New Messengers and New Physics captures the broad array of science made possible by observing the sky in new ways. Within this theme, the decadal survey has identified the priority science area of “New Windows on the Dynamic Universe”—the study of neutron stars, white dwarfs, collisions of black holes, and stellar explosions using the complementary perspectives provided by the wide range of messengers from light in all its forms from radio to gamma rays, gravitational waves, neutrinos, and high-energy particles. In parallel to remarkable advances in observations with multiple messengers from the LIGO/Virgo/Kagra gravitational wave and the IceCube high-energy neutrino observatories, the combination of large detectors, big data, and software advances for handling that data continues to transform the previously static view of the sky to one with nearly daily movies. Future upgrades of ground-based gravitational wave facilities, together with the launch of LISA make this a high priority for discovering new physics, and making astronomical measurements that will change paradigms. Just like our everyday experience benefits from combining the information provided by sight, sound, taste, and smell, so too observations with these complementary messengers open new ways of doing astronomy and new ways of testing the laws of physics. This will reshape the understanding of topics as diverse as the origin of the carbon in bones and the metal in phones, the history of the expansion PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-6

of the universe since the Big Bang, the life and death of stars, and the physics of black hole event horizons. New, coordinated advances in several areas are required to unlock the workings of the dynamic universe. These include the following:  A suite of small and medium-scale ground and space-based observational facilities across the electromagnetic spectrum to discover and characterize the brightness and spectra of transient sources as they appear and fade away.  Ground-based 20-40 m optical-infrared telescopes and an IR/O/UV space telescope significantly larger than HST to see the light coincident with colliding neutron stars detected in gravitational waves—most of these are sufficiently distant to be undetectable with current facilities. These same telescopes will diagnose in exquisite detail the elements produced in stellar explosions.  A sensitive next-generation radio observatory more powerful than the Very Large Array (VLA) to detect the jets of relativistic gas produced by neutron stars and black holes, including those in mergers observed by ground and space-based gravitational wave facilities.  Next-generation CMB telescopes to search for the polarization signatures of gravitational waves produced in the infant universe.  Upgrades to improve the sensitivity of current ground-based gravitational wave detectors, and development of technologies to enable next-generation facilities.  Improvements in the sensitivity and angular resolution of high energy neutrino observatories.  Strong software and theoretical foundations to numerically interpret the gravitational wave signals from merging compact objects to extract new physics in the extremes of density and gravity, and ensure easy user access to the wealth of data on the dynamic universe and to model and interpret astronomical sources whose physical conditions cannot be replicated in laboratories on Earth. 1.1.3 Cosmic Ecosystems The universe is characterized by an enormous range of physical scales and hierarchy in structure, from stars and planetary systems to galaxies and a cosmological web of complex filaments connecting them. A major advance in recent years has been the realization that the physical processes taking place on all scales are intimately interconnected, and that the universe and all its constituent systems are part of a constantly evolving ecosystem. The seeds of galaxies were planted during the first moments of the Big Bang, and modern numerical simulations trace the gravitational growth of cosmic structure from 300,000 years after the Big Bang to the structures and galaxies seen today. The galaxies are ecosystems of their own, with further condensation of matter to form stars and planets balanced by “feedback” from stellar winds, outflows, and supernovae that return mass and energy to the gaseous environment. The supermassive black holes that form and grow within nearly all massive galaxies also play a key role in this feedback process. Unraveling the nature of this connection is one of the key science goals of the decade. The time is ripe for major breakthroughs. JWST will provide definitive observations of the earliest stages of galaxy formation and evolution, and the histories of star formation, chemical enrichment, and feedback processes over cosmic time. The combination of wide-area observations of distant galaxies by the Rubin Observatory, Roman, and Euclid will provide imaging and spectral energy information for millions of galaxies, complementing the in-depth observations from JWST and HST. The upcoming observations with JWST, the Rubin Observatory, and Roman will be profound but will not on their own be able to address the central problem of understanding how galaxies grow. Probing the heart of the galactic feedback process requires detecting and measuring the tenuous gases at the boundaries of galaxies and their intergalactic surroundings, the circumgalactic medium (CGM), where the PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-7

accretion and recycling of gas and metals from feedback processes take place. This goal motivates the priority area within Cosmic Ecosystems. Priority Area: Unveiling the Drivers of Galaxy Growth Processes on a wide range of time and length scales shape the behavior of most astronomical objects, from the scales of planet formation in disks around young stars to galaxies and clusters of galaxies. The science theme of Cosmic Ecosystems captures the interconnectedness of these astronomical systems across cosmic scales. Within this theme, the decadal survey has identified the priority science area of Unveiling the Drivers of Galaxy Growth. The allure of galaxies—to scientists and the public alike—stems from their diversity and complexity. Their rich internal structures and tremendous variety make understanding the origin of galaxies one of the most continuously compelling stories in astronomy. The past decades have seen a growing understanding of the origin of this complexity: gas flows into galaxies, fueling new generations of stars and the buildup of central black holes, but these same stars and black holes send matter back out, potentially shutting down any chance for new material to stream in. These processes must have profound effects on galaxies, but astronomers have only a tenuous grasp on the full coupling between the larger galaxy environment that holds the gas transiting in and out of a galaxy, and the properties of the galaxy itself. This profoundly multiscale problem requires connecting galaxies from their central black holes, in a region no larger than the solar system, to their outermost reaches millions of light years from the center. Technologically, these demanding requirements drive investments in reaching high resolution—to uncover the parsec-scale astrophysics powering feedback—and towards high sensitivity—to both detect the most tenuous and diffuse emission and to allow spectroscopy against faint background light sources with sufficient density to sample a dozen or more lines of sight in a single galaxy. Furthermore, the range of gas temperatures (from more than a million degrees kelvin down to temperatures approaching absolute zero) and redshifts naturally motivates a multiwavelength approach. New observational capabilities across the electromagnetic spectrum along with computation and theory are needed to resolve the rich workings of galaxies on all scales. These include the following:  Large 20–40 m ground-based O/IR telescopes to observe the transition-rich rest-frame UV, in both emission and absorption, for galaxies in the young universe. This will reveal the faint networks of gas that surround galaxies and the gas’s chemistry, temperature, density, and motions.  A next-generation VLA radio telescope will, for the same early epochs, map emission lines of molecular gas, tracing the cold gas associated with both the extended galactic environment and fueling AGN and star formation within the galaxy itself.  A next-generation IR/O/UV large space telescope to trace much of the same physics as the ELTs but in the nearby, evolved universe, and in dramatic detail, revealing the full multiphase complexity of the local ecosystem.  To complement these capabilities a capable far-IR and/or X-ray mission will further transform these views by peering into the dusty hearts of galaxies to reveal enshrouded accreting black holes, or tracing the hottest phases of gas driven outward by this same accretion, with the spatial and spectral resolution needed to isolate critical physical quantities in massive galaxies.  Investments in theory and in the community of scientific experts exploring these data are essential for synthesizing a new scientific foundation for galaxy evolution from these observational advances. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-8

1.2 BACKGROUND AND CONTEXT The range and variety of compelling scientific opportunities illustrates the dynamic nature of modern astrophysics, with future directions propelled both by steady evolution and by dramatic revolution, powered by new discoveries, emerging capabilities, and an increasingly diverse set of ideas. The survey’s recommended program is driven by the science, but it is also shaped by the global landscape, and the scientific, technical, and human context of the times. Multifaceted considerations led to the balance of science, the emphasis on sustainable investments in projects and people, and the wide- ranging activities on all scales that are prioritized through recommendations in this report. The major scientific progress in astronomy over the past decade has been mirrored by a continued transformation in the national and international landscape in which this research is being conducted. Astronomy continues to become more global and interconnected, and many of the major space missions in recent decades (HST, JWST, Herschel Space Observatory, and Planck) have been carried out as partnerships between NASA, ESA, and/or the Japan Aerospace Exploration Agency (JAXA). With the XRISM and Athena X-ray observatories, Euclid, and LISA on the horizon, the survey’s scientific goals are crucially dependent on such partnerships continuing and even strengthening going forward. On the ground, the Gemini and ALMA ground-based observatories are international collaborations with NSF participation. This trend is likely to continue; a majority of the large ground-based projects presented to this survey have, or plan to have, significant international partners. Data produced by other European-led observatories such as the ESA Gaia mission and the European Southern Observatory have also contributed to major advances by U.S. researchers, either individually or as members of international collaborations. This international context of current and planned facilities has been fully incorporated into the survey’s science and strategy planning. The imminent launch of JWST is a momentous occasion that will shape the course of astronomy and astrophysics in the coming decades. Arguably the most ambitious robotic science mission that NASA has ever undertaken, JWST will influence essentially every area of astronomy, from peering back in time to view nascent galaxies as they begin to form in the early universe, to exploring the atmospheres of exoplanets in exquisite detail. JWST, more than two decades in the making, reminds us of the transformational nature of the ambitious, large strategic missions that NASA is uniquely capable of undertaking. While large strategic missions are transformative, 21st century astrophysics owes much of its richness to NASA’s panchromatic suite of Great Observatories that spanned the spectrum from gamma rays to infrared, and which were accomplished with a wide range of scales, from what today is referred to as “Probe scale” up to the very ambitious HST and JWST missions. Diverse missions of all scales, national and international, designed to view the universe in a multiplicity of complementary ways are now essential to progress in modern astrophysics. The broad science laid out in this report requires a wide variety of space-based techniques and capabilities spanning not just the electromagnetic spectrum, but, with the launch of ESA’s LISA mission, in which the United States is a significant partner, the gravitational wave spectrum as well. While, as noted above, sustaining broad observational capabilities is crucially dependent on international partnerships and missions, essential capabilities, such as very high- contrast imaging and spectroscopy in the IR/O/UV bands, far-IR imaging and spectroscopy, and high- resolution X-ray imaging and spectroscopy, are not planned in ESA’s Voyage 2050 program,4 or by other international agencies. Because of the significant U.S. leadership in the development of the needed technologies and capabilities, and the high priority these have for this survey, it is essential for NASA to lead their development. However, without a major change in the approach to developing strategic missions, combined with expanding the range of mission scales, it will take many decades to realize the necessary range of observational capabilities. 4 See https://www.esa.int/Science_Exploration/Space_Science/Voyage_2050_sets_sail_ESA_chooses_future_science_mis sion_themes. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-9

On the ground, the astronomical community eagerly awaits the commissioning of the Rubin Observatory, which will be devoted to a decade-long mapping of the entire southern sky in multiple colors and with multiple time-domain cadences. By harnessing the power of the digital revolution, and building on past large surveys, large public data sets, data science, and computational astrophysics, Rubin will leave a legacy of data that will be mined far into the future by a diversity of astronomers. The challenge going forward is to ensure that the vast range of variable and transient phenomena that Rubin will uncover can be quickly discovered and studied by facilities spanning the wavelength spectrum. Concerning new ground-based activities, NSF and DOE strongly urged the survey to be ambitious and challenged it to consider bold, transformative initiatives. At the same time, NSF Division of Astronomical Sciences (NSF AST) is faced with an historic underinvestment in smaller scale, foundational activities such as the general investigator grants that ensure high scientific return from projects of all scales. Together with the lack of a sustainable model for operating new facilities, the agency faces structural issues it must address to capitalize on the opportunities. Nationally, attention is growing on the country’s urgent need to build its infrastructure, technological base, and scientific foundations, and this movement aligns well with NSF AST’s needs. Being a field that captures the imagination of the public, pushes technology, and is a gateway to science, technology, engineering, and mathematics (STEM) education, astronomy is in a good position to argue for addressing these foundational issues through increased basic investments. The activities and deliberations of this decadal survey took place in a time of tremendous national and international upheaval. The global COVID-19 pandemic has disrupted every aspect of life, from seemingly mundane issues of how to conduct the survey’s business, to health, childcare, elder care, and education. The impacts have not been equally felt by women and men, and they also depend on socioeconomic status and race. The careers of many young people, including scientists, have been paused, and this will have a lasting impact on the profession. The pandemic also strongly underscored the important role of science, and scientific reasoning in combatting the epidemic, from the rapid development of mnRNA vaccines, to the factual, analytic presentation of the data necessary to design protective measures. The ultimate economic and social impacts of the pandemic remain unclear, adding to the uncertainty of the future landscape. As a final, important backdrop, this survey was strongly influenced by the urgent need to advance diversity, equality, and inclusion in all aspects of society. This need came into sharp focus with the Black Lives Matter movement, sexual harassment and the inequalities highlighted by the #MeToo movement, the inequitable impacts of COVID-19, and the shocking increase in hate crimes against Asian Americans. These harsh realities have invigorated the nation into a renewed conviction to tackle systemic issues of race, gender bias, and privilege at a local and global scale. There is momentum to effect change, and the time is overdue to actively focus on these activities. Changing the defaults under which astronomy is practiced will only happen with energetic engagement and a diversity-, equity-, and inclusion-focused lens. 1.3 FRAMEWORK FOR THE SURVEY’S RECOMMENDATIONS In the context described above, the decadal survey committee weighed many considerations in designing its recommended program (Figure 1.2). Primary among these is that the portfolio must be scientifically balanced, broad, and sustainable. Also, the program must be structured to draw from the widest range of human talent. The first consideration drives the need for a balance of investments among activities that lay the foundations of the science and the profession, and that advance a variety of projects on all scales. The second consideration requires that the profession and the agencies nurture, structure, and expand programs in such a way that they eliminate barriers, create inclusive environments, and actively encourage broad participation. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-10

FIGURE 1.2 The recommended program includes elements that pave the way to transformative science by building a strong research and technology foundation, promoting programs on a range of scales that balance and sustain observational capabilities, enabling future large projects, and advancing new frontier observatories. The survey’s organization of projects and activities into categories is a departure from past practice. It emphasizes the function of the activity within the program rather than the cost, although there is a rough equivalence. Prior surveys have divided programs strictly by budgetary requirements (small, medium, and large) and have in general not prioritized projects in one cost category compared to those in another as a means of emphasizing the need for balance. The approach taken by Astro2020 is to adopt functional categories. Projects that build the foundations consist in large part of competed grants to individual investigators and programs that support modest scale activities, and sustaining projects consist PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-11

of competed mid-scale experiments and missions. Two categories capture the large, ambitious initiatives: programs that enable future visions and those that realize frontier facilities. This survey, like its predecessors, places strong emphasis on balance and the need for projects on a variety of scales and does not prioritize one category over another. In most categories, the survey identifies the highest-priority activity, and for others there is a natural time ordering based on scientific urgency and/or project readiness. Another consideration is the budget uncertainty associated both with agency guidance, and with the landscape of federal funding discussed above. NASA and NSF strongly urged the survey to develop a program that is aspirational and inspirational, but that also conforms to budgetary norms. Given the uncertain landscape, the survey committee concluded that it is not possible to imagine and plan for the many possible contingencies. Rather, the recommended program forwards the frontiers of science through ambitious projects, and at the same time strongly advocates for balance. With this guidance in hand, the agencies have the flexibility to seize opportunities that arise on all scales, and the strong motivation to do so given the analysis of this report. For major projects that dominate budgetary requirements, the survey establishes decision rules and off ramps that guide agencies in the event of technical issues, or changes in the budgetary landscape. Interim advice from the mid-decadal survey, and from committees such as the Space Studies Board, the Board on Physics and Astronomy, and the Committee on Astronomy and Astrophysics, are an effective means for the agencies to request input on issues resulting from changing circumstances. These corrections would of course be strongly guided by, and be based on, the full analysis contained in this report. Additional prescriptions are unlikely to be helpful to the agencies given the many constraints, fiscal, political, and organizational, that they are faced with. The greatest challenge faced by the survey committee in developing new recommendations for the nation’s space astrophysics program is how to realize large strategic missions, yet at the same time achieve the wavelength balance, and the overlapping operational lifetimes that characterized NASA’s Great Observatories, a model that so successfully propelled many, varied fields of astrophysics. While international partnerships are essential, they are not sufficient to accomplish the broad and aspirational science program laid out in Astro2020. Doing so will require a range of missions significantly larger than Explorers, yet with a mix of cost and implementation time scales spanning from a less than a decade to the multiple decades required to realize a mission of the ambition and complexity of JWST. As evidenced by the four Large Mission Concept Studies prepared for this survey, the community’s most ambitious and visionary ideas now require timelines that are pan-decadal, and even multi-generational (Chapter 7, Table 7.4). We are poised to tackle some questions that are so grand that the facilities and instruments needed to address them require vision and commitment beyond our individual horizons. But to do this sustainably, and to realize the broad capabilities demanded by the richness of the science requires a re-imagining of the ways in which large missions are developed and implemented. The ambitious strategic missions demand much more significant early investments in co- maturing mission concepts and technologies prior to adoption, with appropriate decadal input on scope, and with checks and course corrections along the way. In addition, adding a competed probe mission line that spans the large gap between Explorers and ambitious strategic missions, with science foci identified by decadal surveys will be a further move toward a capable, panchromatic mission fleet. The greatest challenge for NSF going forward is its need to develop the appropriate programmatic balance of projects spanning the needed range of budgetary levels required to optimize the return on federal investments. Seizing the compelling scientific opportunities, and retaining U.S. competitiveness in astronomy requires capabilities that are uniquely provided by large, ambitious facilities. However, it also requires supporting operations of NSF’s wide range of productive facilities, including upgrading instrumentation, ensuring a balance of project scales, and most importantly supporting the community of individual investigators to realize the scientific goals set out for the decade. The complex challenge associated with achieving this balance has been an impediment to the field for multiple decades, and it must be addressed if we are to reap the scientific rewards going forward. For NASA, NSF, and DOE, overruns and delays in major projects have historically been a significant threat to improving and maintaining program balance. The survey addresses this in two ways. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-12

First, the recommendations in this report emphasize more significant project and technology maturation prior to a commitment to, and commencement of, implementation. This will enable requisite budgets to be more firmly established when the project is adopted by the agency. In addition, for major projects, the decision rules are intended to guide the agencies in how to manage changing circumstances, technical or budgetary. 1.4 CRITERIA FOR ADVANCING NEW ACTIVITIES The program of new activities in this report was conceived in the context of numerous exciting large strategic projects and missions, including international partnerships, that have yet to begin scientific operation (see Table 7.1 for a comprehensive list). This survey assumes that these compelling programs will be all 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 essential capabilities upon which the Survey’s scientific goals rely. Further, programs resulting from decadal recommendations, such as NASA’s expanded Explorer program and NSF’s Mid-Scale Innovations Program, play essential roles in sustaining scientific breadth and ensuring timely response to new opportunities. These continued and future capabilities are essential underpinnings upon which new recommendations are predicated. For NSF, as noted above, the pressure imposed by operations costs of large NSF facilities on grants and other programs has been a systemic issue plaguing astronomy. By the middle of the decade, this will escalate to unsustainable levels unless changes are made to the way that large facilities are supported. The survey’s recommendation is that new, large Major Research Equipment and Facilities Construction (MREFC) recommendations described below be predicated on NSF developing a sustainable plan for supporting the operations and maintenance costs of its astronomical facilities, while preserving an appropriate balance with funding essential scientific foundations and the remainder of the NSF AST portfolio. 1.5 RECOMMENDED PROGRAM OF NEW ACTIVITIES The survey’s recommendations for new programs and program augmentations are organized into steps that form the pathway from the foundations of the profession out to the scientific frontiers (Figure 1.2). The full text of the survey committee’s analyses and recommendations is found in Chapters 2-7, while this chapter provides a broad overview. These recommendations advance transformative science in the coming decade and set the stage for enabling the bold visions in the future (Figure 1.3). 1.5.1 Guiding Principles 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 recognized and responded to the need for synergy with, and complementarity to, activities worldwide. 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 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. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-13

FIGURE 1.3 Timeline for the medium and large programs and projects recommended by this Astro2020 decadal survey. The starting point of each, indicated by the logos, shows the projected start of science operations for missions and observatories, or the start date of the program. The boxes on the right show the survey’s three broad science themes, and the placement of the logos to the left of the boxes indicate which activities address the indicated theme. As evidenced in the figure, advancing each of the survey’s broad science themes requires a range of facilities and programs. The recommendations in this report are also guided by the precepts and principles of diversity, equity, benefit to the nation and the world, and sustainability. Diversity is a driver of innovation, and the astronomy and astrophysics enterprise can be at its most innovative only when it maximizes and fully utilizes the diversity of its human talent, ensures equitable access to opportunities, removes barriers to participation, and when it values diverse forms of expertise in its leadership. Equity demands that what is pursued with the nation’s resources are done in a manner consistent with the principles of fairness and equal opportunity that are core to society’s ideals. Anyone with the ability and the drive to contribute through astronomical discovery should have a fair chance to do so, and be free of fear, harassment, or discrimination. The benefits of astronomy and astrophysics extend beyond its fundamental discoveries. They provide lifelong learning opportunities and science literacy to the public and contribute to the development of the nation’s broader, technically trained STEM workforce. In terms of sustainability and accountability, the substantial investments in people and the use of natural resources in astronomy require responsible stewardship, transparency, and accountability for outcomes. This is a core responsibility of the organizations, agencies, and stakeholders that benefit from the human labor and products of the field. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-14

1.5.2 Foundational Activities The pathway begins with strong support for foundational activities that build the people and the profession, (Chapter 3, The Foundations of the Profession), bolster the core activities necessary for a vibrant research enterprise (Chapter 4, The Research Foundations), and lay the technological foundations for the future (Chapter 6, The Technological Foundations and Small and Medium Programs that Balance the Science). The key new programs with these aims are described below.  Develop and diversify the scientific workforce. The diversity of the astronomy and astrophysics profession remains an area where much improvement is needed. While there have been some notable improvements, especially with regards to the representation of women at the early career ranks of the profession, the overall demographics of the field remain very far from parity with the larger population. Addressing this will require action on many fronts: recommendations in this decadal survey report span the career stages from undergraduate to faculty and beyond, with targeted programs to improve diversity at each level; bridge critical transitions in the pipeline; and work to improve diversity of project teams, participants, and beneficiaries. The ugly realization of continued discrimination in the form of racism, bias, and harassment hampers progress towards building a fully diverse and inclusive workforce, and a recommendation of the report in this area suggests adoption of scientific integrity policies that address discrimination and harassment as forms of research or scientific misconduct. At the core of a diversity-, equity-, and inclusivity-focused approach is the need for data to evaluate equitable outcomes of proposal competitions; such data was sorely lacking in the preparation of this report, and a recommendation to collect, evaluate, and publicly report such data would enable future assessments.  Promote scientific literacy and engage the public. By capturing the public’s attention with discoveries, including the participation of citizen scientists in the research process, promoting science literacy, and realizing advanced technologies that can then find real-world applications, astronomy has a clear benefit to the nation. Astronomy education is effective as a broad gateway to STEM careers. Considering the rapidly increasing need for advanced computational skills in both the public and private sector for students to be competitive, embedding computational training in the undergraduate curriculum is even more important to integrate in the coming decade.  Promote sustainability and accountability. The future of the field requires that greater attention be paid to issues of sustainability and accountability, whether it is in the context of the natural resources required for astronomy research activities at observing sites, or the current crisis of a large number of low Earth–orbiting satellites that will impact wide-field imaging at optical wavelengths and radio frequency observations. Adapting to the realities of climate change requires a decrease of the field’s impact on carbon emissions. Recognizing the need for active, up-front, and sustained engagement with local and Indigenous communities, the survey committee recommends the implementation of a Community Astronomy model of engagement, similar to community-based approaches in other scientific disciplines. The goal for such an approach is to advance scientific research while also respecting, empowering, and benefitting local communities.  Expand the NSF grants program (highest-priority foundational recommendation). Robust individual investigator grant funding is crucial to achieving the science goals of this decadal survey and to ensure more equitable access to resources. The NSF Astronomy and Astrophysics Grants (AAG) program is a cornerstone of the enabling foundation for research in astronomy and astrophysics in the United States, supporting research projects across nearly all subfields of the astrophysical sciences. This program is not currently at a healthy level, and the recommendation for an augmentation over 5 years is designed to restore success rates PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-15

to a healthy competitive environment. This is the highest-priority item amongst the many recommendations for building the foundation of the nation’s research enterprise.  Bolster theory underpinnings. Theoretical investigations, crucial as both a mechanism for driving new discoveries and a framework for interpreting essentially all signals received from space, are, like grants at NSF, lacking crucial funding at a level that can sustain the necessary projects. A recommendation to increase the amount of funding for NASA’s Astrophysics Theory Program (ATP), and restore its cadence to an annual call, reflects increases to recover from past limited funding.  Maximize science from large programs on ground-based facilities. Another survey recommendation in the foundations category urges NSF to establish a mechanism of research funding and production of high level data products for large principal-investigator programs on MREFC-scale astronomical facilities. This would accelerate scientific output and maximize the timeliness and community impact of large key projects.  Support data archives and curation. Astronomy is evolving rapidly into a profession in which archiving of individual observations can produce scientific impacts that rival the original studies, and large-scale surveys are designed for science-ready archival manipulation from the beginning. As demonstrated by space missions and some ground observatories (e.g., ALMA, the European Southern Observatory [ESO]), readily-accessible archival data can substantially increase the scientific impact of facilities for a relatively modest incremental cost. The situation is less uniform for the large number of ground-based optical/infrared (OIR) facilities managed by universities and other institutions. A survey recommendation to NSF and stakeholders for enabling science-ready data across all general-purpose ground- based observatories is an attempt to ensure that all pipelined observations are archived for eventual public use.  Advance crucial laboratory measurements. Laboratory astrophysics is a critical but often hidden and underappreciated cornerstone of the enabling research foundation. It has been chronically underfunded; concerns were raised in both the 2000 and 2010 decadal surveys, but the problem persists. Research in this area needs to be regarded as a high priority, and the existing approaches are not sufficiently advancing the field. A multi-step recommendation in this area urges the agencies to identify the needs for supporting laboratory data to interpret the results of new astronomical observatories, identify resources, and consider new approaches or programs for building the requisite databases. The recommendation also points out the need to include not only experts in laboratory astrophysics, but also users of the data to identify the highest priority applications.  Expand support for early-stage and basic technology development. Analyses of the needs for basic technology funding to support future innovation, as well as to advance identified goals for, for example, high-contrast imaging, adaptive optics, highly multiplexed detectors, and technologies that will drive the next generation of instruments, observatories and missions, identifies increased investments in basic technology as a priority. Another important factor is that basic technology grants are too small to support infrastructure or significant involvement by industrial partners. To be able to fuel innovative future projects on all scales, it is important for the basic technology development portion of the Astrophysics Research and Analysis Program (APRA) to be significantly increased, and for cuts to NSF’s Advanced Technologies and Instrumentation (ATI) program over the last decade be rapidly reversed, and additional funding be added to bring the program to the levels recommended by Astro2010. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-16

1.5.3 Programs that Sustain and Balance the Science Chapter 7 (The New Medium and Large Investments that Sustain Science and Forge Frontiers) lays out an ambitious roadmap for high-priority, space- and ground-based, large and medium-scale initiatives that are compelling and ready to begin implementation in the decade 2023–2033. This roadmap has at its core recommendations 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. Time Domain Astrophysics Program (Highest Priority Sustaining Activity for Space) Exploring the cosmos in the multi-messenger and time domains is a key scientific priority for the coming decade, with new capabilities for discovery on the horizon with the Rubin Observatory, Roman, LIGO/Virgo and the Kamioka Gravitational Wave Detector (KAGRA), and IceCube. To advance this science, it is essential to maintain and expand space-based time-domain and follow up facilities in space. Many of the necessary observational capabilities can be realized on Explorer-scale platforms, or possibly somewhat larger. As the international landscape and health of NASA assets change, it will be important for NASA to seek regular advice over the coming decade on needed capabilities and to ensure their development. The open Explorer program calls have reached a healthy funding level, and as noted in Section 6.2.1.1.3, maintaining the current cadence of open calls is a condition for new initiatives. This time-domain program is therefore recommended as an augmentation to those levels, and would be executed through competed calls in broad, identified areas. Astrophysics Probe Mission Program (Space) The large gap in cost and capability between medium-class Explorer missions and the large strategic missions presented to the survey is a significant impediment to achieving the broad set of decadal scientific priorities. Institution of Probe-class line of missions with a cost cap of ~$1.5 billion per mission, a cadence of ~one per decade, and competed within selected priority areas identified by this and future decadal surveys, is a crucial addition to NASA’s astrophysics portfolio. The two priorities for the first Probe-class mission competition are a far-IR probe or an X-ray probe to complement the Athena mission. Both areas represent important observational needs where advances in technology and focused objectives can yield transformative science on a moderate-sized platform. Augmentation and Expansion of the NSF Astronomy Mid-Scale Program (Highest Priority Sustaining Activity for Ground) Mid-scale programs—across the entire range of ~$4 million to 120 million—enable new transformative capabilities by incentivizing creative approaches from the community for cutting-edge instruments and experiments. They also ensure robust capabilities for basic research through continually refreshed instrumentation suites and can respond rapidly to strategic priorities. For these reasons it is essential to expand funding levels for the astronomy funding available through mid-scale programs, MSIP and Mid-scale Research Infrastructure (MSRI). It is also essential to add components to the astronomy mid-scale program to target strategic areas through dedicated calls, and to sustain and advance instrumentation on existing telescopes. For the next 10 years, time-domain astrophysics, highly multiplexed spectroscopy, and radio instrumentation (including radio transient cameras and neutral hydrogen mappers) are the priorities for strategic calls. Dedicated calls are also needed to ensure the regular upgrading of instrumentation on existing facilities, with an emphasis on 4–10 m class PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-17

optical/infrared telescopes. These two new elements would be added, in addition to entirely open competitions of new ideas, in a balanced way that responds to proposal pressure. 1.5.4 Programs that Enable Future Visions Great Observatories Mission and Technology Maturation Program (Highest Priority for Enabling Programs for Space) NASA’s flagship missions are driven by transformative scientific visions, and they advance a broad range of scientific objectives. Overlapping or near-simultaneous wavelength coverage is particularly impactful, as evidenced by the success of NASA’s Great Observatories. Given the large costs and development timescales associated with the large missions presented to this survey, achieving this will only be possible if a new approach is taken to mission maturation, and in particular phasing it with decadal survey advice. The Great Observatories Mission and Technology Maturation Program is aimed at increasing the cadence of large missions by designating appropriate scope at an early stage and making significant investments in maturing missions to the appropriate level prior to ultimate recommendation and implementation. This motivates the recommendation that a large IR/O/UV mission first enter the maturation program, and only when that has been successful as defined by a review, would it proceed to formulation. It is also important that additional missions enter the maturation program in the next 10 years to ensure the needed cadence for panchromatic capabilities, and the priorities for this are a far-IR flagship with some of the capabilities of the proposed Origins, and a high-resolution X-ray mission with some of the capabilities of the proposed Lynx. An important aspect is that both the X-ray and far-IR missions are to be matured with cost targets of $3 billion to $5 billion. Determining the range of capabilities for these missions will be part of this maturation program, and will be informed by the first Probe mission selection. Technology Development for Future Gravitational Wave Observatories (Ground) Gravitational wave astrophysics is one of the most exciting frontiers in science. One of the survey’s key priorities is the opening of new windows on the dynamic universe, with gravitational wave detection at the forefront. The continued growth in sensitivity of current-generation facilities, such as LIGO, through phased upgrades and planning the next-generation observatory, such as Cosmic Explorer, is essential. This will require investment in technology development now. The survey committee strongly endorses gravitational wave observations as central to many crucial science objectives. Because the technology development for future upgrades and observatories is funded by NSF Physics, it is beyond the survey’s charge to formally recommend this investment. 1.5.5 Large Programs that Forge the Frontiers A Future Large Infrared/Optical/Ultraviolet Telescope Optimized for Observing Habitable Exoplanets and General Astrophysics (Highest Priority for Space Frontier Missions) Inspired by the vision of searching for signatures of life on planets outside of our solar system, and by the transformative capability such a telescope would have for a wide range of astrophysics, the priority recommendation in the frontier category for space is a large (~6 m diameter) IR/O/UV telescope with high-contrast (10-10) imaging and spectroscopy. This is an ambitious mission, of a scale comparable to the HST and JWST space telescopes. It is also one that will be revolutionary, and that worldwide only NASA is positioned to lead. A period of mission and technology maturation is required, however with PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-18

sufficient investment this could be completed before the end of the decade, and the mission could commence formulation prior to 2030. (Section 7.5.2) Decision Rules: Prior to commencing mission formulation, a successful Great Observatories Mission and Technology Maturation program must be completed, and a review held to assess plans in light of mission budgetary needs and fiscal realities. The U.S. Extremely Large Telescope Program (Highest Priority in the Ground-Based Frontier Category) Because of the transformative potential that large (20–40 m) telescopes with diffraction-limited adaptive optics have for astronomy, and because of the readiness of the projects, the survey committee’s top recommendation for frontier ground-based observatories is investment in the U.S. ELT program. The U.S. ELT program 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). The primary mirror of the GMT has a total diameter of 24.5 m and the telescope has a 25 arcmin field-of- view (FOV). The GMT will be located at the Las Campanas Observatory in Chile. The TMT primary mirror has a diameter of 30 m, 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. These observatories will create enormous opportunities for scientific progress over the coming decades and well beyond, and they will address nearly every important science question across all three priority science themes. Both projects are essential for keeping the U.S. community’s global scientific leadership, providing important synergistic capabilities that complement those planned for the European ELT. However, both projects have significant remaining risks primarily associated with the need to raise additional private or international contributions. The success of at least one of these projects is absolutely essential if the United States is to maintain a position as a leader in ground-based astronomy. The objective is to achieve a time share that is equivalent to 25 percent in each telescope. If only one project is viable, then a larger fraction on that telescope is required to meet the survey’s scientific goals, with the aim of achieving an NSF share up to 50 percent time in that project. (Section 7.6.1.1) Decision Rules: Successful completion of an external review that will determine the financial viability of both projects, final site selection (in the case of TMT), development of an appropriate management plan and governance structure, and appropriate plans for public access and data archiving. The Cosmic Microwave Background Stage 4 Observatory (CMB-S4) Given technical and scientific progress over the last decades, ground-based studies of the CMB are poised to take a major step forward in the coming decade. The Cosmic Microwave Background Stage 4 (CMB-S4) observatory will leverage this progress and will have broad impact on both cosmology and astrophysics. Realizing the ultimate scientific potential of ground based CMB observations will take an effort far beyond what can be achieved by independently scaling up existing experiments. CMB-S4 observatory, a joint effort of NSF and DOE, is the compelling and timely next leap for ground-based observations. 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. The Survey is also excited by the breadth of science, including time- domain and transient studies, and the potential engagement of a community well beyond traditional CMB cosmologists. To maximize the science, transient alerts and well calibrated maps from all surveys will need to be made available to the entire community in a timely fashion, even if it requires some extra resources to do so. (Section 7.6.1.3) PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-19

The Next Generation Very Large Array (ngVLA) For the past four decades, the Karl Jansky Very Large Array (JVLA) and the Very Long Baseline Array (VLBA) have been the premiere observatories worldwide for accessing the sky at centimeter wavelengths. 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 at centimeter to millimeter wavelengths on scales of arcminutes to fractions of a milliarcsecond. The ngVLA is such a facility; however, it is immature in its development, and considerable effort must be put into studies to understand and reduce the cost relative to current estimates, secure international partnerships, and prototype the antennae. With such an effort commencing soon, the ngVLA would be ready to commence construction by about 2030. It will be important to begin implementation as soon as it is technically and fiscally possible. (Section 7.6.1.4) Decision Rules: Implementation is contingent on a successful design, development and prototyping program, cost studies, and commitments from any foreign partners. A review will determine the project’s readiness and consistency with budgetary constraints prior to commencement of construction. The IceCube-Generation 2 (IceCube-Gen2) Neutrino Observatory Observations of high-energy neutrinos enable astrophysical advances in the study of some of the most energetic phenomena in the universe. The IceCube-Gen2 would greatly enhance the capabilities relative to IceCube, would be able to resolve the bright, hard-spectrum TeV-PeV diffuse neutrino background into discrete sources, and would make the first detections at higher neutrino energies. Multi- messenger astrophysics is a major theme of this report, and the survey endorses the IceCube-Gen2 observatory as important to many key survey scientific objectives. Because it is funded by NSF Physics, it is beyond the survey’s charge to recommend this investment. (Section 7.6.2.3) 1.6 ADDITIONAL ADVICE In addition to the vision for new, recommended future endeavors, this decadal survey report offers advice on aspects of the agencies’ programs aimed at optimizing returns for their existing programs. Data Archives. An important component of creating effective archives is coordinating with cross-agency and international archiving services to develop best practices and interoperability. As a step toward this, it is important for NASA and NSF to explore mechanisms to improve coordination among U.S. archive centers and to create a centralized nexus for interacting with the international archive communities. The goals of this effort are best defined by the broad scientific needs of the astronomical community. Solar Physics. Solar physics is directly relevant to astronomy, as it is the study of our nearest star, and interacts with stellar astrophysics; is input to studying the Earth-Sun connection and expanding to stellar- planetary interactions; and is vital to understanding Earth’s climate and space weather. The survey committee concluded that an appropriate role for astronomy and astrophysics decadal surveys is to comment on the value of ground-based solar physics projects for astronomy and astrophysics priorities, with the solar and space physics decadal survey being the more appropriate body to prioritize and rank ground-based solar physics projects within the context of the full range of multi-agency activities in solar physics. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-20

NSF Portfolio Reviews. Regular reviews of more mature observatories are essential to determine how to optimize their scientific return and cost effectiveness, and to determine when a facility is at the end of its operational life. While some aspects of ground-based facility reviews are considered as part of the review of operating agreements for observatories, these are not an appropriate substitute for a review that considers the entire portfolio on a self-consistent, holistic basis. It is essential that NSF AST establish a regular cadence of reviews of its operational portfolio, at a frequency sufficient to respond to changes in scientific and strategic priorities in the field. An appropriate target is two reviews per decade. SOFIA. The survey committee has significant concerns about SOFIA, given its high cost and modest scientific productivity. The NASA portion of SOFIA’s operating budget is out of balance with its scientific output, which is a fraction of that of comparable cost missions (e.g., HST, Chandra) and often less than those of Explorer missions. The survey committee finds no evidence that SOFIA could transition to a significantly more productive future and notes the minimal mention of SOFIA science by the science panels. The committee found no path by which SOFIA can significantly increase its scientific output to a degree that is commensurate with its cost and endorses NASA’s current plan to discontinue operations in 2023. NASA’s Balloon Program. NASA’s balloon program plays an important role in offers access to a near- space environment with a wide variety of options for duration and sky coverage, for developing technologies, and training future generations of technologies and mission leaders. It is, however, clear that the balloon program is not yet achieving the potential promised by the advent of ultra-long duration balloon (ULDB) flight capabilities. It is important that the balloon program be critically reviewed to evaluate how to optimally support innovative payload development and to increase the cadence and reliability of LDB and ULDB flights. NASA’s Program of Record. NASA’s upcoming Roman Space Telescope, and ESA’s Athena X-ray Observatory and LISA mission, in which NASA is a significant partner, are essential to the survey’s science program. Advice on how to optimize the science return includes: holding a non-advocate review of Roman Space Telescope’s science program to set the appropriate mix of survey time to guest investigator-led observing programs; and at the appropriate time, establishing funding for LISA science at a level that ensures U.S. scientists can fully participate in LISA analysis, interpretation, and theory. 1.7 CONCLUSION The integrated program forwarded in this report advances a vision for discovery and progress for the coming decades. The content of the remaining chapters, together with the panel reports, represent an enormous effort that took years of preparation on the part of a large fraction of the astronomical community, and more than 2 years for the survey and its committees to complete. The full context of the recommendations and advice summarized in this chapter can only be appreciated by reading the report in its entirety. Realizing the opportunities presented in these pages will only be possible with the continued dedication and energy of the community, the agencies, and the excitement of the nation to explore the cosmos and answer some of humanity’s most profound questions. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1-21

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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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