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Recommendations

INTRODUCTION

Our Place in the Universe

Astronomy and astrophysics address questions about the origin and evolution of the planets, the stars, and the universe. In this century we have learned that the climates and weather patterns of planets in the solar system are driven by many of the same physical processes that create the earth's environment; that stars form out of clouds of gas and eventually die either in quiet solitude or spectacular explosions; that most of the common chemical elements are created in explosions of stars; that stars group together in isolated galaxies; that galaxies and clusters of galaxies stretch in sheets and filaments as far as the largest telescopes can see; and that the universe itself was born in a violent explosion some 15 billion years ago. Most amazingly, we have learned that the laws of nature that humans have discovered on the earth apply without modification to the farthest reaches of the observable universe.

Yet each new answer leads to new puzzles. What kinds of planets form around other stars? What triggers the formation of stars in our own galaxy and in other galaxies? What powers the enormous bursts of energy seen in some galaxies? How did galaxies themselves arise in the primitive universe? Where can black holes be found, and what are their properties? What is the ultimate fate of the universe? These are a few representative questions that capture the



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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS 1 Recommendations INTRODUCTION Our Place in the Universe Astronomy and astrophysics address questions about the origin and evolution of the planets, the stars, and the universe. In this century we have learned that the climates and weather patterns of planets in the solar system are driven by many of the same physical processes that create the earth's environment; that stars form out of clouds of gas and eventually die either in quiet solitude or spectacular explosions; that most of the common chemical elements are created in explosions of stars; that stars group together in isolated galaxies; that galaxies and clusters of galaxies stretch in sheets and filaments as far as the largest telescopes can see; and that the universe itself was born in a violent explosion some 15 billion years ago. Most amazingly, we have learned that the laws of nature that humans have discovered on the earth apply without modification to the farthest reaches of the observable universe. Yet each new answer leads to new puzzles. What kinds of planets form around other stars? What triggers the formation of stars in our own galaxy and in other galaxies? What powers the enormous bursts of energy seen in some galaxies? How did galaxies themselves arise in the primitive universe? Where can black holes be found, and what are their properties? What is the ultimate fate of the universe? These are a few representative questions that capture the

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS imaginations of astronomers and the general public and that stimulate young people to study mathematics, science, and engineering. Discoveries of the 1980s Observations with underground, ground-based, airborne, and orbiting telescopes during the 1980s produced important discoveries that advanced our knowledge in many areas of astronomy. The following is a selection of some of the more important advances and consolidations. The theory of the origin of the elements in the “Big Bang” received support from both astronomical observations of stars and sensitive experiments in particle physics. An orbiting satellite launched in 1989 began observing the relict radiation from the earliest years of the universe. Preliminary results indicate the need to revise existing theories of the formation of galaxies and clusters of galaxies. Evidence gathered shows that the radiation from as much as 90 percent of the matter of the universe has so far gone undetected. Quasars were found at extremely large distances and must have been formed when the universe was less than 10 percent of its present age. Einstein's prediction that the gravitation of matter could bend rays of light found application in the discovery that galaxies can act as lenses, refracting the light from more distant quasars. Surveys of large numbers of galaxies revealed that the universe is organized on larger scales than predicted by many cosmological theories. Increasing evidence suggested the possibility of giant black holes in the centers of some galaxies and quasars. An orbiting satellite surveyed the sky at infrared wavelengths and discovered disks of solid material, possibly the remnants of planet formation, orbiting nearby stars. It also found ultraluminous galaxies emitting 100 times as much energy in the infrared as at visible wavelengths. Supernova 1987A burst into prominence in our closest neighbor galaxy, the Large Magellanic Cloud. Subatomic particles called neutrinos from the supernova were detected in underground observatories, confirming theories about the death of stars and the production of the heavy elements crucial to life on the earth. Neutron stars spinning at nearly 1,000 revolutions per second were discovered by their regular pulses of radio radiation. Signals from these objects may constitute the most stable clocks in the universe, more accurate than any made by humans, and can be used to search for gravitational waves and as probes of the dynamics of star clusters. A deep probe of the interior of a star—our own sun—was achieved through a technique analogous to terrestrial seismology, measuring pressure

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS waves on the solar surface. These measurements established the extent of the solar convective zone and the dependence of rotation speed on depth in the sun. Experiments done with solar neutrinos hinted at new physics not included in standard textbooks. The mass and radius of Pluto were determined from observations of its satellite, Charon. Other studies of Pluto revealed the surprising fact that this small, cold planet has an atmosphere. Deuterium was discovered in the Martian atmosphere, and this isotope was used to measure the loss of water from Mars in the past. The 1990s: The Decade of Discovery The 1990s promise to be a decade of discovery. The first 10-m telescope, the Keck telescope in Hawaii, will come into operation early in the decade. This telescope and the others to follow will be the first very large optical and infrared telescopes constructed in this country since the epoch-making installation of the Hale 5-m telescope on Palomar Mountain over 40 years ago. The technological revolution in detectors at infrared wavelengths will increase the power of telescopes by factors of thousands. New radio telescopes will reveal previously invisible details at millimeter and submillimeter wavelengths. A technique called interferometry will combine optical or infrared light from different telescopes separated by hundreds of meters to make images thousands of times sharper than can be achieved with a single telescope. The four Great Observatories of the National Aeronautics and Space Administration (NASA) will view the cosmos across the infrared, visible, x-ray, ultraviolet, and gamma-ray portions of the electromagnetic spectrum. These instruments, orbiting above the earth's distorting atmosphere, will answer critical questions and may reveal objects not yet imagined. PURPOSE AND SCOPE OF THIS STUDY Charge to the Committee The charge to the committee was as follows: The committee will survey the field of space- and ground-based astronomy and astrophysics, recommending priorities for the most important new initiatives of the decade 1990–2000. The principal goal of the study will be an assessment of proposed activities in astronomy and astrophysics and the preparation of a concise report addressed to the agencies supporting the field, the Congressional committees with jurisdiction over these agencies, and the scientific community. The study will restrict its scope to experimental and theoretical aspects of subfields involving remote observation from the earth and earth orbit, and analysis of astronomical objects; earth and planetary

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS sampling missions have been treated by other National Research Council and Academy reports. Attention will be given to effective implementation of proposed and existing programs, to the organizational infrastructure and the human aspects of the field involving demography and training, as well as to suggesting promising areas for the development of new technologies. A brief review of the initiatives of other nations will be given together with a discussion of the possibilities of joint ventures and other forms of international cooperation. Prospects for combining resources—private, state, federal, and international —to build the strongest program possible for U.S. astronomy will be explored. Recommendations for new initiatives will be presented in priority order within different categories. The committee will consult widely within the astronomical and astrophysical community and make a concerted effort to disseminate its recommendations promptly and effectively. The committee agreed that the primary criterion determining the order of priorities would be the committee's best estimate of the scientific importance of each initiative. In forming its judgment of scientific importance, the committee also took into account cost-effectiveness, technological readiness, educational impact, and the relation of each project to existing or proposed initiatives in the United States and in other countries. In a letter to the committee commenting on the initial charge, NASA 's associate administrator for space science pointed out that NASA 's solar physics research program contains investigations of the sun viewed both as a star and as a power source for the solar system, and that many of NASA's solar physics missions have a strong coupling to in situ measurements, which lie outside the purview of this committee. NASA requested that, for these reasons, solar physics space missions not be prioritized together with purely astronomical missions. The committee concurred with this request, since it reflected the nature of the subject and of the funding sources, but considered that ground-based solar astronomy remained within its charge. Independently, the Solar Astronomy Panel established by the committee [see the Working Papers (NRC, 1991) of this report] elected to develop an integrated plan for solar research incorporating both ground- and space-based initiatives. The committee surveyed the entire field of astronomy and astrophysics as defined by its charge and attempted to engage everyone in U.S. astronomy who had an interest in being heard. More than 300 astronomers, listed in Appendix C, served on the 15 panels whose separately published reports (Working Papers) contain important advisory material that was considered by this committee. An additional 600 or so astronomers contributed directly to this report by their letters, essays, or oral presentations at open meetings; more than 15 percent of all U.S. professional astronomers played an active role in some aspect of this report. Distinguished colleagues from throughout the world contributed valuable essays and letters. The committee also profited from discussions with dedicated people in Congress and on congressional staffs, and with personnel

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS in the funding agencies, in the Office of Management and Budget, and in other executive offices. In carrying out its charge, the committee describes prioritized equipment initiatives that reflect its best judgment about what facilities will most advance the central goal of astronomy: understanding the universe we live in. However, the committee recognizes that there can be no research without researchers, teaching without students, or observational progress without advances in technology. An infrastructure of students, researchers, and equipment and a vigorous program of theoretical research must exist to support new work, or the new initiatives will not succeed. This committee therefore prefaces its discussion of new initiatives with (1) recommendations for strengthening the infrastructure for ground-based astronomy and (2) a discussion of the need for a balanced strategy for space astrophysics. Contents of This Report This report presents a prioritized program for the 1990s that balances the development of new facilities with support for existing facilities and for the research of individual scientists. The present chapter, Chapter 1, summarizes the prioritized recommendations for new instrumental initiatives. Other recommendations appear in the context of specific discussions in the chapters on existing programs, on computing, on the lunar initiative, and on policy opportunities. Chapter 2 describes some of the scientific opportunities of the next decade and Chapter 3, some of the most important ongoing programs. Chapter 4 presents a more detailed scientific and technical justification for the recommended new initiatives. Chapter 5 outlines the influence of the computer revolution on astronomy. Chapter 6 evaluates the potential role of observatories on the moon in the nation's Space Exploration Initiative. Chapter 7 discusses some important policy issues in ground- and space-based astronomy. Finally, Chapter 8 highlights some of the ways astronomy benefits the United States and the world. Appendix A defines some of the most common and important astronomical terms used in this report. Appendix B gives some basic statistics on the current demography and funding of astronomical research. Appendix C lists the scientists who served on the panels established by the committee to help carry out this decennial survey. RECOMMENDATIONS FOR STRENGTHENING GROUND-BASED INFRASTRUCTURE The highest priority of the survey committee for ground-based astronomy is the strengthening of the infrastructure for research, that is, increased support for individual research grants

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS and for the maintenance and refurbishment of existing frontier equipment at the national observatories. By any quantitative measure, the research infrastructure has deteriorated seriously in the last decade: support for maintenance and refurbishment of facilities and for individual research grants in astronomy and astrophysics has declined as a fraction of the total budget of the National Science Foundation (NSF), as a fraction of the NSF's total astronomy budget, on a per-astronomer basis, and on the basis of real-dollar expenditures. NSF funding for astronomy has decreased for nearly a decade despite an explosion in research discoveries, a major expansion in the number and complexity of observational facilities, and a large increase in the number of practicing astronomers (see Appendix B). The consequences of this decline include the loss of key technical personnel, limitations on young scientists' participation in the research program, the delay of critical maintenance, the inability to replace old and obsolete equipment, and a lack of funds to pay for scientists to travel to observatories or to reduce data. The situation has reached critical dimensions and now poses a threat to the continued success of U.S. astronomy. The committee recommends that the NSF increase its support for annual operations, instrument upgrades, and maintenance of national research facilities to an adequate and stable fraction of their capital cost. The NSF should include appropriate financial provision for the operation of any new telescope in the plan for that facility. The committee estimates that appropriate remedial actions will require increasing the operations, maintenance, and refurbishment budgets for the observatories now in existence by a total of $15 million per year. The recommended annual increase will serve to repair the effects of deferred maintenance at the National Optical Astronomy Observatories (NOAO) and the National Radio Astronomy Observatory (NRAO), upgrade receivers and correlators at NRAO to improve the performance of the Very Large Array (VLA) by a factor of 10, provide needed computational resources to deal with large-format arrays of optical and infrared detectors at NOAO, replace antiquated equipment at the National Astronomy and Ionosphere Center (NAIC), and hire new technical staff to service millimeter receivers, infrared arrays, and advanced optics. If maintained for a decade, the increases will restore the infrastructure to a healthy working condition. The committee recommends that individual research grants be increased to an adequate and stable fraction of the NSF's total operations budget for astronomy. In order to gather and analyze the large amounts of data that will become available with new instrumentation, to allow young researchers to take advantage of

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS the new opportunities for discovery, and to restore support for theoretical astrophysics, the individual grants budget should be increased by $10 million per year. The grants program supports the activities of graduate and postdoctoral fellows who become the trained scientists of the future; it supports the design of new instruments, the purchase of computers and the analysis of data, and the development of theories to explain and motivate observations and to predict new results. Many of the fundamental discoveries of the 1980s were made using ground-based facilities and with support provided by individual research grants. At present, only one grant in two is able to support a graduate student, and only one grant in four can support a postdoctoral fellow. The recommended annual increase for the grants program is meant to accomplish three purposes: first, improve the chances for first-time applicants, many of whom are young scientists with no other means of research support, to receive grants ($1.5 million per year); second, increase the average grant size by $20,000 to a total of $80,000, which is approximately the size needed to support and train an individual postdoctoral fellow ($7.5 million per year); and third, as discussed below, help establish an astrophysics theory program within the NSF ($1 million per year). Within the astronomy grants program of the NSF, the committee recommends that theoretical astrophysics be given additional visibility and resources. Theory provides the basic paradigms within which many observations are planned, analyzed, and understood. As discussed in Chapter 3, adequate support for theory is necessary in order to realize the full benefits from existing and recommended observing facilities. ACHIEVING A BALANCED SPACE PROGRAM Overall Strategy For space-based astrophysics, the most urgent need is to continue to develop a program that balances the enormous power of the largest observatories with smaller missions of reduced complexity and shorter development times, while maintaining a healthy research and analysis infrastructure. Large missions are essential for solving certain fundamental scientific problems, but their development can require as long as two decades. Moderate and small missions can respond more rapidly to changing scientific priorities and to new technical or instrumental breakthroughs. Rapid access to space attracts talented young instrumentalists and stimulates innovation. The committee believes that a greater involvement of scientists in engineering and management issues could improve the efficiency and scientific return of space astronomy missions of all

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS sizes. In order to achieve its scientific goals, NASA must carry out its plans to devote adequate resources to the Research and Analysis program and to Mission Operations and Data Analysis. This support is crucial for analyzing and interpreting data, for understanding the implications of the data through theory, and for developing new technologies for future space missions. The funding requirements for new instrumental initiatives within a balanced space program are listed in Table 1.1. Before deciding on its final recommendations for new equipment initiatives, the committee considered the implications of the recently discovered problems with the Hubble Space Telescope. Significance of Large Space Observatories The operation of the first Great Observatory, the Hubble Space Telescope (HST), began disappointingly. Initial observations revealed that a flaw in its primary mirror would prevent HST from achieving its design resolution and sensitivity without corrective action. A failure in the testing and quality control of known technology apparently caused the flaw in the mirror. Investigatory committees have been appointed to identify precisely what happened, why it happened, and how to protect future programs from other major mistakes. In Chapter 7, the committee describes its view that a strong involvement by scientists is critical to the success of space projects of all sizes. This committee has reexamined the justification for large-scale space astronomy programs, taking into account both the failure to meet specifications in the HST program and NASA's record of successes in carrying out other complex missions at the frontiers of science and technology. As will be clear from discussions later in this chapter and in Chapter 3 and Chapter 4, the committee has concluded that in some cases only large-scale programs can answer some of the most fundamental astronomical questions. The four Great Observatories currently planned by NASA for the 1990s cover a large fraction of the electromagnetic spectrum with the sensitivity and resolution required to make progress on frontier problems in astrophysics. The Great Observatories are the HST, the Gamma Ray Observatory (GRO), the Advanced X-ray Astrophysics Facility (AXAF), and the Space Infrared Telescope Facility (SIRTF). The Great Observatories embody the ideal of “small science” made possible by large facilities; they allow individual investigators or small groups of investigators to carry out frontier research programs. The typical number of researchers is only four per proposal for the approved HST programs. Investigators at about 200 different institutions were awarded observing time for the first year of HST operations. Nevertheless, the committee believes that a balanced space astronomy program would put increased emphasis on more frequent and less costly missions. Modest, cost-efficient missions can respond more rapidly to changing

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS scientific ideas, to technological developments, and to the need for training young researchers. Thus the committee calls for a greater frequency of astrophysics Explorer missions and limits the recommendations for new, large space programs to one mission, SIRTF. SIRTF has superb science potential, is technologically well developed, and has been preceded by two smaller and successful precursor missions. RECOMMENDED NEW EQUIPMENT INITIATIVES Ground and Space Initiatives The Astronomy and Astrophysics Survey Committee recommends the approval and funding of the set of new equipment initiatives listed in Table 1.1. Table 1.1 lists the recommended hardware initiatives that fall within the committee's charge, along with the committee's best estimates of their costs (in 1990 dollars). Detailed descriptions of the principal programs appear in Chapter 4. The programs are arranged in large, moderate, and small categories, depending on the scale of resources required. Large and moderate programs are listed in order of scientific priority. Since one set of agencies (NSF, DOE, and DOD) supports ground-based programs and a different agency (NASA) supports space-based programs, the committee has separated the new initiatives into ground- and space-based projects. Two areas to which the committee assigns high priority in other parts of this report—science education (Chapter 7) and theoretical astrophysics (Chapter 3)—do not appear explicitly in Table 1.1 because most of their support is provided from grants to individual researchers and not through facilities. The costs presented in Table 1.1 have various origins. Certain programs such as SIRTF, the two 8-m ground-based telescopes, and the Millimeter Array (MMA) have already benefited from extensive design studies; their costs are well defined and should not change appreciably except as a result of inflation or delays in implementation. For SIRTF and other space missions, the costs include construction of the entire facility and operation through launch plus 30 days. For the large ground-based telescopes, the costs include a first complement of instruments and rudimentary computer analysis. Costs for the other recommended programs are based on discussions with agency personnel or on material presented to the panels [see the Working Papers (NRC, 1991)]. In most cases, the programs have been studied in enough detail under the auspices of the funding agencies that the costs are well approximated by the values listed. In arriving at the prioritized list, the committee made its own informal assessment of the realism of the proposed costs, taking into account technological readiness and the heritage from previous projects.

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS TABLE 1.1 Recommended Equipment Initiatives and Estimated Costs Ground-based Decade Cost ($M) Space-based Decade Cost ($M) Large Programs Infrared-optimized 8-m telescope 80 Space Infrared Telescope Facility (SIRTF) 1,300 Millimeter Array (MMA) 115   Southern 8-m telescope 55   Subtotal ground-based 250 Subtotal space-based 1,300 Moderate Programs Adaptive optics 35 Dedicated spacecraft for FUSE 70 Optical and infrared interferometers 45 Stratospheric Observatory for Far-Infrared Astronomy (SOFIA) 230 Several shared 4-m telescopes 30 Delta-class Explorer accelerationa 40 Cosmic-ray telescope (Fly's Eye) 15 Astrometric Interferometry Mission (AIM) 250 Large Earth-based Solar Telescope (LEST) 15 International collaborations on space instruments 100 VLA extension 32   Subtotal ground-based 172 Subtotal space-based 1,050 Illustrative Small Programsb Two-micron survey 5 Small Explorer acceleration 100 Infrared instruments 10 Orbiting planetary telescope 50 Cosmic background imager 7 VSOP/RadioAstron 10 Laboratory astrophysics 10 Laboratory astrophysics 20 National astrometric facility 10   300-m antenna in Brazil 10   Stellar oscillations instrument 3   Optical surveys 6   Neutrino supernova watch 10   Subtotal ground-based 71 Subtotal space-based 180 Total ground-based 493 Total space-based 2,530 DECADE TOTAL   3,023 a Examples include gamma-ray spectroscopy, submillimeter spectroscopy, and x-ray imaging. b Three small ground-based programs and one space initiative are highlighted by italics since they were regarded by the committee as being of special scientific importance. In addition to the instrument initiatives recommended in this section, the committee stresses that progress in astronomy requires a vigorous program of theoretical research and laboratory astrophysics, in addition to a balanced program of observation. The vital role of theory and that of laboratory astrophysics are discussed further in Chapter 3.

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS This report's predecessor, Astronomy and Astrophysics for the 1980's (NRC, 1982), widely known as the “Field Report,” was well received in large part because astronomers made for themselves the difficult priority choices. The two highest-ranking major programs (out of a total of four recommended new programs) were funded; the two highest-ranking moderate programs, plus two other programs, out of a total of seven recommended new programs, were initiated; and the highest-ranking small program, plus one other, received funding. The completion of two of these initiatives [AXAF and the Far Ultraviolet Spectroscopy Explorer (FUSE) space observatories] remains a goal of this committee. The total estimated budget for the Field Committee 's recommended new programs was $1.7 billion (1980), or approximately $2.6 billion in 1990 dollars, compared to an estimated $3.0 billion (1990) for the items listed in Table 1.1. The Combined Equipment List The committee presents in order of priority in Table 1.2 its combined list of new equipment initiatives, independent of agency and independent of the location of the facility, ground or space. The major individual items are described briefly below and more fully in Chapter 4. Table 1.1 and Table 1.2 are prioritized lists for new hardware initiatives. The committee emphasizes again that other areas also have high priority. The restoration of the infrastructure, including the refurbishment of existing equipment and increased support for individual research grants, is the highest-priority recommendation for ground-based astronomy, as discussed above. The need to establish a program with the proper balance between large, moderate, and small missions is the highest priority for space astrophysics. The committee stresses that progress in astronomy requires a vigorous program of theoretical research and laboratory astrophysics, as well as a balanced program of observation. The vital roles of theory and of laboratory astrophysics are discussed further in Chapter 3. In addition, astronomy has a long tradition of support by state, private, and even international sources. The committee did not attempt to prioritize initiatives supported by nonfederal sources of funding, but the contribution of such projects is important and is included in Chapter 3, a discussion of ongoing and planned programs. Small Projects and Technological Initiatives The committee decided to maintain separate lists for ground- and space-based small projects and for technology initiatives in recognition of the ability of funding agencies to respond quickly to new scientific opportunities and to advances in instrumentation. Table 1.1 lists some illustrative small projects that have great scientific merit.

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS TABLE 1.2 Recommended Equipment Initiatives (Combined Ground and Space) and Estimated Costs Initiative Decade Cost ($M) Large Programs Space Infrared Telescope Facility (SIRTF) 1,300 Infrared-optimized 8-m telescope 80 Millimeter Array (MMA) 115 Southern 8-m telescope 55 Subtotal for large programs 1,550 Moderate Programs Adaptive optics 35 Dedicated spacecraft for FUSE 70 Stratospheric Observatory for Far-Infrared Astronomy (SOFIA) 230 Delta-class Explorer acceleration 400 Optical and infrared interferometers 45 Several shared 4-m telescopes 30 Astrometric Interferometry Mission (AIM) 250 Cosmic-ray telescope (Fly's Eye) 15 Large Earth-based Solar Telescope (LEST) 15 VLA extension 32 International collaborations on space instruments 100 Subtotal for moderate programs 1,222 Subtotal for illustrative small programs 251 DECADE TOTAL 3,023 Table 1.3 lists, but not in any prioritized order, the committee's assessment of the most important technological initiatives for the 1990s that will form the basis for frontier science in the decade 2000–2010. The costs given in Table 1.3 represent this committee's best estimate of what is required to make major technological progress in each area. Details of the proposed technology development programs appear in the Working Papers (NRC, 1991). Explanation of New Equipment Initiatives This section provides thumbnail sketches of the large and moderate programs recommended by the committee. More complete technical and scientific descriptions appear in Chapter 4.

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS TABLE 1.3 Technology Development for the 1990s and Estimated Costs Technology Initiative Decade Cost ($M) Ground-based Infrared detector arrays 25 Optical detector arrays 10 Solar neutrino experiments 15 Dark matter detectors 10 Digital archive 15 Gamma-ray airshower detectors 5 Radio technology 15 Subtotal for ground-based technology 95 Space-based Optical and infrared interferometry in space 50 Technology for next-generation observatories: Large space telescope technology 50 Submillimeter receiver and telescope technology 75 High-energy mirror and detector technology 50 Subtotal for space-based technology 225 DECADE GRAND TOTAL 320 LARGE PROGRAMS Ground-based Astronomy Three programs have the greatest importance for ground-based astronomy. They are discussed in the committee's order of scientific priority, which corresponds also to the most appropriate temporal sequence for their implementation. Infrared-Optimized 8-m Telescope. The highest-priority recommendation for a large ground-based facility is for the construction of an 8-m-diameter telescope on Mauna Kea, Hawaii; this telescope and its instrumentation should be optimized for low-background, diffraction-limited operation at infrared wavelengths from 2 to 10 µm. The planned telescope will be a unique facility, using revolutionary infrared array detectors to make high-spatial- and high-spectralresolution observations of objects as various as volcanoes on the satellite Io, the inner parts of protoplanetary disks around nearby young stars, and distant galaxies. The telescope builders will benefit from the past decade of intense development in the technology for casting and polishing large mirrors. Chapter 4 discusses the complementarity of the major infrared and submillimeter facilities recommended in this report. Millimeter Array. The proposed Millimeter Array will be an imaging telescope operating at millimeter radio wavelengths with high spatial and spectral

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS resolution. The array will consist of 40 transportable antennas, each 8 m in diameter. Scientific projects advanced by the MMA will range from extragalactic programs to solar and planetary programs and include studying flares on the sun, imaging the outer parts of protoplanetary disks, and probing motions in the cores of distant, infrared-luminous galaxies. The MMA builds on techniques pioneered by the VLA, which operates at centimeter wavelengths, and by two university observatories that operate small arrays at millimeter wavelengths. Southern 8-m Telescope. The proposed Southern Hemisphere 8-m telescope will be a twin of the Northern Hemisphere infrared-optimized 8-m telescope, except that the southern telescope and its instrumentation will be optimized for optical and near-ultraviolet wavelengths. The southern 8-m telescope will provide U.S. astronomers with a vital window through which to view objects that are best observed from the Southern Hemisphere, such as the center of our galaxy and our closest neighboring galaxies, the Magellanic Clouds. These three large ground-based initiatives will provide state-of-the-art research facilities for hundreds of researchers working at dozens of different institutions. For example, the 4-m telescopes, the largest instruments at NOAO, typically have 150 users each per year (20 percent of whom are students), with an average of about 1.5 astronomers per approved proposal. The VLA radio telescope has about 600 users per year, with an average of about 3 astronomers per research program. Space-based Astronomy SIRTF. The highest priority for a major new program in space-based astronomy is the Space Infrared Telescope Facility, a 0.9-m cooled telescope in a spacecraft to be launched by a Titan IV-Centaur into high earth orbit. SIRTF will operate as a national facility, with more than 85 percent of the observing time during its five-year lifetime available to individuals and small groups of investigators from the general astronomical community. Across the wavelength region from 3 to 200 µm, SIRTF will be up to a thousand times more sensitive than other space- or ground-based telescopes, and over a million times faster than other instruments for surveying, mapping, or obtaining spectra across large, complex regions (see Chapter 4). The technical heritage of SIRTF includes two infrared telescopes launched and successfully operatgd as part of NASA's Explorer program. MODERATE PROGRAMS Ground-based Projects Adaptive Optics and Interferometry. The two highest-priority moderate programs for ground-based astronomy involve the application of new techniques

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS for optical and infrared observing. The highest priority is to apply technologies collectively called adaptive optics to overcome the blurring effects of the earth's atmosphere on time scales of a few hundredths of a second and thereby enhance the sensitivity and spatial resolution of new or modernized telescopes. The second highest priority is to use techniques of interferometry, previously used successfully by radio astronomers, to join separated infrared or optical telescopes. A linked group of telescopes would have a resolving power equivalent to that of a single instrument as large as the distance separating the individual telescopes. Demonstration projects have proven successful for both of these techniques, suggesting that adaptive optics should be applied to a broad range of telescopes and that several interferometers, more powerful than existing ones, should be supported. 4-m Telescopes. While discovery often follows the opening of new frontiers in sensitivity, wavelength, or angular resolution, detailed understanding usually requires detective work by large numbers of scientists with access to appropriate investigative tools. The basic “detective tool” for standard investigations in the 1990s will be a 4-m-class optical or infrared telescope. Federal funds should be used in combination with state and private funds when possible to construct several new 4-m telescopes. Advances in technology will make it possible to build and operate these facilities more economically than the first generation of 4-m telescopes at NOAO, endorsed by the “Whitford Report” (NRC, 1964). University involvement in operation and management of these facilities will provide opportunities for students to perform exploratory or long-term programs and to help develop instrumentation. Fly's Eye. Cosmic rays consist primarily of protons and the nuclei of heavy atoms. The existing Fly's Eye telescope has detected the fluorescent trails of over 200 cosmic rays more energetic than 1019 eV. At present, no one knows how particles can be accelerated to such high energies, what their composition is, or whether such particles originate inside or outside of the galaxy. A new Fly's Eye telescope, with a factor-of-10 improvement in sensitivity and better spatial resolution, would help determine the anisotropy, the energy spectrum, and the composition of cosmic rays in the energy range 1019 to 1020 eV. Large Earth-based Solar Telescope. U.S. solar astronomers have entered into an international collaboration with scientists from eight other countries to build the world's best ground-based solar telescope, to be located in the Canary Islands. The Large Earth-based Solar Telescope (LEST) is a 2.4-m telescope that will obtain diffraction-limited images and spectra of the sun using the techniques of adaptive optics. The United States will contribute the most technically challenging part of the telescope, the adaptive optics system. VLA Extension. The Very Large Array of the NRAO has proven extraordinarily productive in studying objects as diverse as comets, planets, the sun,

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS other stars, interstellar clouds, and distant radio galaxies and quasars. The arcsecond imaging of the VLA will complement the thousandth-of-an-arcsecond capability of the Very Long Baseline Array (VLBA) when the latter is completed in 1992. The VLA extension will add four new telescopes and ancillary hardware to bridge the lack of baselines between telescopes needed for angular resolutions between a tenth and a thousandth of an arcsecond in the performance of the VLA and the VLBA. Space-based Projects Dedicated Spacecraft for FUSE. The committee recommends that NASA augment the Explorer program sufficiently to convert the Far Ultraviolet Spectroscopy Explorer into a Delta-launched experiment using its own dedicated spacecraft. In 1989, NASA selected FUSE for development in the Explorer program. This committee, like the Field Committee a decade ago, believes FUSE will produce extraordinary scientific results. The committee is concerned that continued linking of FUSE to the Shuttle program through a reusable Explorer spacecraft will unnecessarily delay the mission and increase its cost. A dedicated spacecraft will enhance the scientific potential of the FUSE mission, providing an optimized orbit, simpler operations, and a longer mission lifetime. SOFIA. The proposed Stratospheric Observatory for Far-Infrared Astronomy (SOFIA) is a 2.5-m telescope mounted in a Boeing 747 aircraft and optimized to study infrared and submillimeter wavelengths from above most of the earth's water vapor. SOFIA provides the highest-resolution spectroscopy of any planned facility for wavelengths between 30 and 300 microns, and its large aperture will yield high-spatial-resolution observations that will complement SIRTF's capabilities. In addition to its scientific value, SOFIA will be an excellent facility for the training of the next generation of experimentalists and for the rapid development of advanced instrumentation. The predecessor to SOFIA is the Kuiper Airborne Observatory (KAO), which has trained some 40 PhD astronomers in an environment that subjects both students and instrumentation to many of the rigors of space missions. SOFIA represents the natural evolutionary replacement for the KAO and is a joint project with Germany. Explorers. The committee believes that flying six Delta-class Explorer missions for astrophysics in the next 10 years will play a key role in revitalizing space astronomy. Each of the panels dealing with space observations [Working Papers (NRC, 1991)] identified examples of forefront science that could be carried out with such experiments. The missions should be chosen through the peer review process, with specific emphasis on modest scope, rapid execution, and careful cost control. The active involvement of the selected science team in all phases of the program is critical to the success of these missions. Astrometric Interferometry Mission. Astrometry, which is concerned with

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS the measurement of the positions of celestial sources, ranks among the oldest and most fundamental branches of astronomy and now lies on the verge of a technological revolution. The application of interferometric techniques in space with telescope separations of a few hundred meters may enable a 1,000-fold improvement in our ability to measure positions. An Astrometric Interferometry Mission (AIM) with 3- to 30-millionths-of-an-arcsecond accuracy could detect Jupiter-sized planets around hundreds of stars up to 500 light-years away. International Collaborations. The ability to place U.S. instruments on foreign spacecraft is a cost-effective way to provide access to space. NASA has often taken support for such experiments out of the strained Explorer budget. A budget line for international collaborations will allow NASA to undertake more of these advantageous joint ventures. SMALL PROGRAMS Small programs can be carried out relatively quickly in response to new scientific or technological developments, focusing research into the currently most profitable areas and making possible greater participation by young astronomers. Some of the most exciting scientific results of the past decade have come from modest, cost-effective programs. The committee recommends that an increased emphasis be given in the astronomy research budget to small and moderate programs. However, it would be counterproductive to set detailed priorities for small programs for an entire decade, because this would interfere with the desired flexibility and rapid response. The federal agencies can peer review small programs on a timely basis. The committee therefore lists illustrative examples of small programs that are of high priority at the present time. Over the next several years some of the programs listed here will likely be either funded or replaced by other small initiatives. Highlighted by italics in Table 1.1 are three small ground-based programs and one small program for space that the committee regards as being particularly important at this time. Other small programs of high quality are discussed in the Working Papers; the committee believes that many other outstanding small programs will be identified throughout the 1990s. Ground-based Projects Two-Micron Survey. The only survey of the sky in the near infrared (~ 2 µm) occurred over 20 years ago. Today, a pair of 1-m telescopes, one for each hemisphere, equipped with modern near-infrared array detectors, can completely survey the sky at three wavelengths in less than two years, reaching a level 50,000 times fainter than the earlier survey.

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS Infrared Instrumentation. The ongoing revolution in infrared technology can improve the data-collection capability of existing telescopes by factors of tens of thousands by replacing single-element detectors with large, multielement arrays. Building instruments that incorporate arrays will enhance both imaging and spectroscopic capabilities. Cosmic Background Imager. On angular scales smaller than a few degrees, the cosmic background radiation reflects conditions in the early universe at an age of only 100,000 years. Recent technological advances suggest that a Cosmic Background Imager could reach the levels of sensitivity required to search for variations in the brightness of the background in different directions. Primordial density fluctuations that may be the precursors of galaxies can be revealed by these variations in brightness. Laboratory Astrophysics. The interpretation of observations from ground-based telescopes often depends on the results of laboratory experiments concerning basic atomic, molecular, or nuclear data. In some cases, theoretical calculations are necessary because the appropriate quantities cannot be measured. Since many important experiments in laboratory astrophysics are primarily of interest to astronomers, some laboratory research will require direct funding from astrophysics resources. Other Programs. Other important small projects listed in Table 1.1 are a ground-based facility for making long-term astrometric measurements, U.S. participation in an international project to build a 300-m radio telescope in Brazil, instrumentation to study the “seismology” of stars, optical all-sky surveys of galaxies with modern electronic detectors, and systematic monitoring to detect neutrino bursts from supernovae. Details of these and other projects are described in the Working Papers. Space-based Projects Small Explorers. The committee highlights an acceleration of the Small Explorer (SMEX) program to be carried out within tight budgetary constraints with the goal of making possible five astronomy SMEX missions in the 1990s. These small new missions should be selected by the peer review process and launched on Scout-class rockets. SMEX missions will help train the future leaders of space astronomy and will provide a rapid method for executing certain well-defined, high-priority projects. Some of the panel reports in the Working Papers contain excellent ideas for SMEX payloads. Other Projects. Other space projects that will return important data at relatively low cost include U.S. participation in a German orbiting planetary telescope, and orbiting very long baseline interferometry (VLBI) experiments being conducted with the Soviet Union and Japan. The success of future

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS NASA missions like AXAF, SIRTF, and SOFIA depends in part on improved knowledge of the physical and chemical properties of atoms, molecules, and dust grains that can be obtained only with a vigorous program of laboratory measurement. Laboratory astrophysics provides an essential key for the success of the NASA missions planned for the 1990s. TECHNOLOGY DEVELOPMENT The technology developed today is used in the science of tomorrow. The ground- and space-based technology programs include new methods for improving the performance of detectors, developing new types of telescopes, and making astronomical data widely available. Ground-based Technology The largest and most efficient detectors allow optimum use of the nation's investment in telescopes. Infrared detectors are improving rapidly with the aid of technology developed for national security applications. With continuing access to the fruits of this development, only a modest investment is required to optimize and purchase infrared detectors for astronomical applications and to improve relatively mature optical devices. Promising new techniques are being developed to observe the solar neutrinos emitted during the key energy-producing reactions in the solar interior, including proton-proton collisions and the decay of 7Be. These techniques can reveal fundamental aspects of how the sun shines and, at the same time, provide important information about particle physics. If technology development proves successful, these experiments could produce data in the second half of the 1990s. Novel or improved detectors must be developed to detect ultrahigh-energy gamma rays from astronomical sources and to observe exotic particles that could constitute the “dark matter” of the universe. The ground- and space-based observatories of the 1990s will produce immense amounts of data. As discussed in Chapter 5, the archiving of these data is a high scientific priority. Developments in three areas have particularly great potential to enhance the efficiency of radio telescopes: low-noise receivers for millimeter and submillimeter wavelengths, broad-bandwidth recording systems and data links for VLBI, and focal plane arrays. Space-based Technology Infrared or optical interferometers in space many kilometers in size, either in orbit or on the moon, offer ultrahigh angular resolution. As discussed in Chapter 6, a phased technology development program in this area with intermediate technical and scientific milestones, including ground-based and

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS modest orbiting experiments, is required to prepare the way for such major programs in the next century. Astronomers will uncover new phenomena with the Great Observatories throughout much of the 1990s and into the next decade. If the past is any guide, however, the problems we solve will lead to deeper questions and new directions. We must begin now the conceptual planning and technological development for the next generation of astronomy missions to follow the Great Observatories. One example of a next-generation space observatory is a large space telescope, a 6-m telescope that would combine the light-gathering power of a large ground-based telescope with the excellent image quality, ultraviolet capability, and low-infrared background that are achievable in space. Other possible missions with great scientific potential include a large x-ray telescope equipped with detectors capable of simultaneous imaging and spectroscopy; a submillimeter observatory consisting of a deployable 10-m telescope or an orbiting array of smaller telescopes operating as an interferometer; a single large radio telescope many kilometers across; and five orbiting radio telescopes of 100-m diameter forming an array that surrounds the earth. The committee recommends that NASA pursue the technology initiatives listed in Table 1.3 as a prerequisite for initial definition studies of the next-generation space astronomy missions to be initiated early in the second half of the 1990s. The panel reports in the Working Papers (NRC, 1991) contain detailed discussions of technologies that require further study. The development of advanced projects should proceed in a step-by-step manner with frequent tests of technologies and the involvement of key personnel by means of scientific missions of increasing scope. These studies would provide the basis for the selection, by the turn of the century, of a new mission to follow the Great Observatories. The scientific imperatives and the infrastructure available at the time of selection will influence which missions are chosen. Technical issues will include the construction and control of lightweight systems, the capabilities of launch vehicles, advances in robotic construction techniques, and the availability of facilities on the moon. The technology development programs listed in Table 1.3 will provide part of the factual basis required for decisions about future astronomical missions.