3

Existing Programs

INTRODUCTION

Astronomers observe the universe in unique ways from space, from the ground, and even from underground. The variety of astronomical objects and the number of instrumental techniques are remarkable. The committee summarizes in this chapter the most important instrumental projects that are operating or under way. The committee also describes programs in theoretical and laboratory astrophysics and in particle astrophysics that are important to the research base in astronomy. In the discussion below, the committee highlights planned enhancements to certain ongoing programs that would be particularly effective scientifically. The committee emphasizes that a vigorous research program requires grants to individual astronomers and broad access to telescopes; these issues are addressed in Chapter 1 and Chapter 7. For any of the programs discussed in this chapter or the next ( “New Initiatives”) to be effective, there must be adequate support for data analysis, interpretation, and theoretical studies.

GROUND-BASED ASTRONOMY

Optical and Infrared Astronomy

Ground-based observations in optical and infrared regions of the spectrum are central to our understanding of astronomical objects and the physical processes that shape their evolution. In the past decade, ground-based observations



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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS 3 Existing Programs INTRODUCTION Astronomers observe the universe in unique ways from space, from the ground, and even from underground. The variety of astronomical objects and the number of instrumental techniques are remarkable. The committee summarizes in this chapter the most important instrumental projects that are operating or under way. The committee also describes programs in theoretical and laboratory astrophysics and in particle astrophysics that are important to the research base in astronomy. In the discussion below, the committee highlights planned enhancements to certain ongoing programs that would be particularly effective scientifically. The committee emphasizes that a vigorous research program requires grants to individual astronomers and broad access to telescopes; these issues are addressed in Chapter 1 and Chapter 7. For any of the programs discussed in this chapter or the next ( “New Initiatives”) to be effective, there must be adequate support for data analysis, interpretation, and theoretical studies. GROUND-BASED ASTRONOMY Optical and Infrared Astronomy Ground-based observations in optical and infrared regions of the spectrum are central to our understanding of astronomical objects and the physical processes that shape their evolution. In the past decade, ground-based observations

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS have changed the scientist's paradigm of the universe from that of an almost boringly uniform expansion to one of startling inhomogeneity that requires huge amounts of unseen matter. Technological breakthroughs in fabricating large mirrors of superb optical quality and in correcting for the distortions introduced by turbulence in the earth's atmosphere now make it possible to build new facilities with unprecedented sensitivity and spatial resolution. The recommendations for new equipment (Chapter 1) are designed to make a new generation of powerful facilities available to the U.S. astronomical community. The specific recommendations for investment at the federal level reflect the assumption that, in addition, private and state funds will enable ongoing collaborations to complete one or more 8- to 10-m telescopes in the 1990s. The Keck 10-m telescope, the largest optical telescope in the world, developed by the California Institute of Technology and the University of California, is already under construction on Mauna Kea. The Smithsonian Institution and the University of Arizona are converting the Multiple-Mirror Telescope in Arizona to use a single 6.5-m mirror. Other private consortia are designing additional large telescopes. Pennsylvania State University and the University of Texas have designed a segmented 8-m telescope suitable for spectroscopic surveys. The Columbus project, consisting of two 8-m telescopes on a single mount, is being planned by the University of Arizona, Ohio State University, and a foreign partner, Italy. The Magellan project, currently under consideration by the Carnegie Institution, the University of Arizona, and Johns Hopkins University, aims to put a single 8-m telescope at Las Campanas, Chile (Table 3.1). Although, under current guidelines, little or no observing time on these large new telescopes will be available to the general astronomical community in the United States, the private telescopes, including the Keck, Columbus, Magellan, and the Spectroscopic Survey telescopes, are important to a balanced program of astronomical research and greatly augment the national capability. The scientific problems described in Chapter 2 require more observing time on powerful telescopes than two national instruments can provide. It is the combination of the federal investment in large telescopes recommended in Chapter 1 with the private initiatives, described only briefly here, that will assure all-sky access for the U.S. astronomical community. Increased investment in instrumentation at all levels—federal, state, and private —will be required to make full use of the capabilities of these telescopes. Several private-federal partnerships to build new 4-m-class telescopes have been formed; the committee views these partnerships as an innovative way to make the best use of limited federal funds. The WIYN (University of Wisconsin, Indiana University, Yale University, and the National Optical Astronomy Observatories) telescope project will put a 3.5-m telescope funded by the three universities on Kitt Peak. Operating expenses will be shared by the National Optical Astronomy Observatories (NOAO) and the universities; the national

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS TABLE 3.1 Large Nonfederal Telescope Projects Project Location No. of Telescopes × Size Status Keck U.S. (Caltech, Univ. of California) 1 × 10 m Construction Very Large Telescope Europe (ESO) 4 × 8 m Funded MMT conversion U.S. (Smithsonian Inst., Univ. of Arizona) 1 × 6.5 m Funded Spectroscopic Survey Telescope U.S. (Penn State Univ., Univ. of Texas) 1 × 8 m Proposed Columbus U.S. (Ohio State Univ., Univ. of Arizona), Italy 2 × 8 m Proposed Japan National Telescope Japan 1 × 7 m Proposed Keck II U.S. (Caltech, Univ. of California, NASA) 1 × 10 m Proposed Magellan U.S. (Carnegie Inst., Univ. of Arizona, Johns Hopkins Univ.) 1 × 8 m Proposed Note: Access to all these telescopes is restricted largely by institutional affiliation or nationality. community will have access to 40 percent of the telescope time. The 3.5-m ARC telescope is being built by a consortium of universities (University of Chicago, University of Washington, Princeton University, New Mexico State University, and Washington State University). The NSF provided a fraction of the capital funding for ARC with the stipulation that about 10 to 15 percent of the time on the telescope would be available for users outside the consortium. Harvard University and Cambridge University are planning to build a 4-m telescope in the Southern Hemisphere. LARGE MIRRORS The Steward Observatory Mirror Laboratory at the University of Arizona operates at the forefront of optical technology and constitutes a crucial element in the U.S. astronomy program. The laboratory is developing advanced techniques of optical design, casting, polishing, and testing, which are necessary to make the 8-m-diameter mirrors needed by the next generation of telescopes (Plate 3.1). The laboratory also plays a unique role in training students in optical engineering and in strengthening interactions between industry, government, and the university community.

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS ADAPTIVE OPTICS AND INTERFEROMETRY The implementation of new technologies has permitted astronomers to achieve enhanced spatial resolution from the ground at progressively shorter wavelengths, first in the radio region of the spectrum and, during the coming decade, at infrared and optical wavelengths. Recently, European astronomers have demonstrated impressively that adaptive optics can be successfully applied to astronomy. Adaptive optics permits diffraction-limited imaging over the full aperture of large telescopes. The pioneering U.S. effort in this area was cut back because of budget reductions at the NSF. The committee believes this program should be renewed so that the United States can take a leadership role in adaptive optics. Hence the committee has established adaptive optics as its highest-priority, moderate ground-based program (Chapter 1). As discussed in more detail in Chapter 4, optical and infrared interferometry can achieve high spatial resolution over baselines longer than the aperture of a single mirror. The technology needed for the application of optical interferometry to faint sources was carried out on some bright stars in the 1980s. The Department of the Navy supported the building of two interferometers on Mt. Wilson. The NSF also established a number of modest programs. These demonstration projects deserve continued support. Radio Astronomy CENTIMETER WAVELENGTH ASTRONOMY The Very Large Array (VLA), recommended by the Greenstein Committee (NRC, 1972), operated with great power in the 1980s, annually providing observations for about 600 astronomers and data for over 200 research papers. During the 1980s, new receivers and computing techniques enhanced the power of the VLA to more than 10 times that of the original instrument, all without major changes in telescope hardware. However, the declining NSF budget has caused major problems at the VLA and its parent National Radio Astronomy Observatory (NRAO), as discussed by the Radio Astronomy Panel in the Working Papers (NRC, 1991) and summarized briefly here. A decade of diminishing funds has led to deferred maintenance that directly affects the reliability of the array. For example, the railroad track system over which the antennas are moved has deteriorated (Figure 3.1), making it difficult to configure the VLA for observations at different angular resolutions. Some of the vital instrumentation of the VLA is out of date: needed are low-noise receivers, fiber-optic transmission lines, a broadband digital correlator, and more powerful computers for data analysis. These items could improve the sensitivity of the instrument by up to a factor of 10, improve the frequency coverage and spectral resolution, and increase the maximum image size.

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS FIGURE 3.1 A decade of deferred maintenance and refurbishment has led to a variety of problems at the national observatories, including the deterioration of the railroad tracks used at the VLA to reconfigure the array for operation at different spatial resolutions. Courtesy of the National Radio Astronomy Observatory/Associated Universities, Inc. The Very Long Baseline Array (VLBA), the highest-priority ground-based initiative recommended by the Field Committee (NRC, 1982), has been funded by NSF and will begin operating in 1992. The VLBA will provide detailed maps of the cores of active galaxies and quasars with sub-milliarcsecond angular resolution and will determine distances to objects in our own and other galaxies from measurements of H2O masers. The committee is confident that this next step in the application of very long baseline interferometry (VLBI) techniques will return exciting results. The most spectacular closing of an old facility was the unanticipated collapse of the Green Bank 300-ft telescope. The replacement for the 300-ft telescope will be the Green Bank Telescope, a fully steerable instrument of comparable diameter. The telescope will incorporate novel design features, such as an active surface, that may eventually permit operation at wavelengths as short as 3 mm. The telescope will begin operation in 1995, initially at centimeter and longer wavelengths, for the study of pulsars, active galaxies, and 21-cm hydrogen emission in our own and in distant galaxies. The upgrade of the 1,000-ft Arecibo telescope will improve the sensitivity of that instrument by a factor of 3 to 4 for nonradar observations. Because of its large collecting area, the Arecibo telescope will continue to play a critical role in pulsar studies and

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS TABLE 3.2 Millimeter and Submillimeter Telescope Projects Project Location No. of Telescopes × Size Status Single Dishes Nobeyama Japan 45 m Operational IRAM Europe 30 m Operational Maxwell Observatory (JCMT) U.K. 15 m Operational Five Colleges U.S. 14 m Operational NRAO U.S. 12 ma Operational CSO U.S. 10 mb Operational Bell Laboratories U.S. 7 m Operationalc Submillimeter Telescope U.S., Germany 10 m Funded Interferometers Nobeyama Japan 6 × 10 m Operational Owens Valley U.S. 3→4 × 10 mb Operational Berkeley-Illinois-Maryland U.S. 6→9 × 6 mb Operational IRAM Europe 3 × 10 m Operational Smithsonian submillimeter array U.S. 6 × 6 m Funded NRAO Millimeter Array U.S. 40 × 8 ma Proposed a Telescopes without restrictions on access by U.S. astronomers. b Approximately 30 to 50 percent of time available to U.S. community. c To be decommissioned in 1991-1992. in extragalactic surveys of HI in galaxies even after the Green Bank Telescope is built. MILLIMETER AND SUBMILLIMETER WAVELENGTH ASTRONOMY The focus of U.S. activity in millimeter and submillimeter astronomy (Table 3.2) shifted during the 1980s from single dishes to interferometers, beginning with the inauguration of millimeter interferometers at Owens Valley (Caltech) and Hat Creek (Berkeley, University of Illinois, and the University of Maryland). These small arrays of 6- to 10-m telescopes have made important contributions to many fields of galactic and extragalactic astronomy. Important results include, for example, the discovery of molecular gas concentrated at the center of infrared-luminous galaxies and the imaging of protoplanetary disks associated with young stars. Submillimeter astronomy is expected to become a major field of research in the 1990s. The Caltech Submillimeter Observatory began operations in 1990 with significant support from the NSF and with 50 percent of the observing time available to the national community. This telescope, a small program recommended by the Field Committee, has taken advantage of sensitive receivers and clever telescope fabrication techniques to pioneer observations in

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS the submillimeter wavelength band. Important new initiatives are under way with the forthcoming construction of the Smithsonian Institution 's submillimeter array and the University of Arizona-Germany Submillimeter Telescope. The Smithsonian array will make subarcsecond images with high spectral resolution for the first time in this important wavelength band. The NSF is supporting construction of a 1.7-m telescope to investigate the feasibility of conducting submillimeter observations from the South Pole. Planetary Astronomy NASA's Infrared Telescope Facility (IRTF) and the Kuiper Airborne Observatory (KAO) continue to provide important new results in planetary astro-physics, including the direct detection of water vapor in Halley's Comet, the imaging of volcanoes on Io, and the detection of numerous molecules, including water, phosphine, and germane, in the atmosphere of Jupiter. Radar results play a critical role in planetary astronomy. The radar sensitivity of the Arecibo telescope will be increased by a factor of 10 or more in a joint undertaking by NASA and the NSF. The enhanced telescope will make high-spatial-resolution images of many asteroids and comets and probe the sub-surface properties of many of the natural satellites such as Phobos, Deimos, Io, and Titan. The committee commends NASA and the NSF for collaborations on the Arecibo telescope. Solar Astronomy The sun can be studied at a level of detail that is impossible to achieve for any other star, establishing the foundation of our understanding of all stars. The physical regions inside the sun, especially in the convective zone and below, can be probed by observing the oscillation (or “ringing”) of the sun's surface. The technique is similar to studying the interior of the earth by observing seismological waves on the earth's surface. The Global Oscillations Network Group (GONG) project is an international collaboration, supported in this country by the NSF, to set up a chain of telescopes around the world to monitor the sun's oscillations continuously. With GONG data, it will be possible to constrain the interior temperature and density structure of the sun, and to infer its differential rotation as a function of radius, latitude, and depth. The space-borne complement of GONG is a NASA-funded helioseismology instrument on the European Space Agency's (ESA's) Solar-Heliospheric Observatory (SOHO) mission. The committee regards the completion of the GONG network, with adequate support for its continued operation and data analysis, as being of fundamental importance.

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS The facilities of the National Solar Observatory (NSO) are critically important to the solar community, both because few university astronomy departments maintain solar facilities and because NSO telescopes are among the best in the world. Yet budgetary pressures, accumulated over many years, have weakened the NSO. For example, the observatory has been unable to purchase state-of-the-art infrared or visible-light detector arrays to support solar adaptive-optics experiments at an appropriate pace. This situation should be corrected and is one reason that this committee's highest priority for ground-based astronomy is renewed investment in the infrastructure. Solar neutrino experiments constitute a complementary way of looking inside the sun. The nuclear fusion reactions that cause the sun to shine occur deep within the sun's core but are revealed directly by observations of particles called neutrinos. The United States operates the chlorine solar neutrino experiment, which detects rare high-energy neutrinos of the electron type. The United States also collaborates with the Soviet Union, Germany, France, Italy, and Israel on experiments using gallium detectors to detect electron neutrinos from the basic proton-proton reaction and with Canada and the United Kingdom to construct a detector of heavy water to observe higher-energy neutrinos of all types. In addition, the United States has a potentially important collaboration with Italy and the Soviet Union to observe the beryllium neutrino line with a liquid scintillator. These observatories have complementary functions, including the study of variations of the neutrino flux with phase in the solar cycle. The Search for Extraterrestrial Intelligence Ours is the first generation that can realistically hope to detect signals from another civilization in the galaxy. The search for extraterrestrial intelligence (SETI), involves, in part, astronomical techniques and is endorsed by the committee as a significant scientific enterprise. Indeed, the discovery in the last decade of planetary disks, and the continuing discovery of highly complex organic molecules in the interstellar medium, lend even greater scientific support to this enterprise. Discovery of intelligent life beyond the earth would have profound effects for all humanity. NASA's decade-long Microwave Observing Program is based on a particular set of assumptions and techniques for exploring the SETI problem. This committee, like the Field Committee before it, believes strongly that the speculative nature of the subject also demands continued development of innovative technology and algorithms. A strong peer-reviewed, university-based program should be an integral part of this effort. SPACE ASTRONOMY During the 1980s, the initial reliance on the Space Shuttle for access to space, followed by the Challenger disaster, slowed the rate of progress in U.S.

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS space science. In contrast to the fruitful 1970s, only two American astronomical satellites were launched in the 1980s, and leadership in areas the United States had pioneered, such as x-ray astronomy, moved to Europe, the Soviet Union, and Japan. The currently planned program in space astronomy, described in the Strategic Plan (NASA, 1988, 1989) for NASA's Office of Space Science and Applications, can reverse this trend. NASA's plan includes telescopes that range from small payloads like the Submillimeter Wave Astronomy Satellite to be launched in 1995, to the four Great Observatories (discussed below). This section describes the ongoing program in space astronomy that underpins the recommendations for new projects made in Chapter 1. The Great Observatories The first Great Observatory, the Hubble Space Telescope (HST), was launched in April 1990. The second, the Gamma Ray Observatory (GRO), is currently scheduled for launch in 1991, after the publication of this report. Construction of the third, the Advanced X-ray Astrophysics Facility (AXAF), the highest-priority major new program of the Field Committee in 1982, is under way. As discussed in Chapter 1, this committee endorses the plan to complete the Great Observatories, which are essential for reaching the frontiers of the universe. The Space Infrared Telescope Facility (SIRTF) is the only Great Observatory awaiting initiation and is this committee's highest-priority large equipment initiative for the decade of the 1990s. HUBBLE SPACE TELESCOPE A manufacturing flaw in the primary mirror of the HST will prevent it from forming images of faint objects with a resolution greater than about 1 arcsecond, until after the installation of a second generation of instruments with correcting optics. But important observations will be possible with HST even with reduced performance. The General Observer program for using the HST observatory, which is conducted by the Space Telescope Science Institute, makes possible the use of HST's frontier astronomical facilities by U.S. and foreign scientists. The Hubble Fellowship Program for recent PhDs helps to train some of the best young researchers in space astronomy. At this stage, it appears likely that close to the full potential of HST can be achieved by installing either new instruments or corrective optical elements, although there is no guarantee that all technical and practical problems will be overcome. A replacement, with corrective optics, for the general-purpose Wide-Field/Planetary Camera (WF/PC) is under construction and is scheduled for installation in 1993; the replacement camera is essential to carry out the fundamental goals of this observatory. In addition to the improved camera, NASA selected in 1988 two other new

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS instruments, the Space Telescope Imaging Spectrometer (STIS) and the Near-Infrared Camera and Multi-Objective Spectrometer (NICMOS). Both of these instruments can also be fitted with appropriate optics to remove the aberrations in the HST images and so achieve the high spatial resolution possible with an orbiting telescope. These new instruments will greatly enhance the power of the HST observatory. STIS will increase the speed with which some critical ultraviolet and visible-light observations can be made by a factor as large as 100 or more and will make possible spatially resolved spectroscopy. NICMOS will use two-dimensional arrays for imaging faint, complex fields with 0.1-arcsecond spatial resolution, and for spectroscopy with frequency resolution up to 10,000 at near-infrared wavelengths. In this wavelength range, the background radiation affecting HST is 100 times smaller than that affecting terrestrial telescopes, which must observe through the earth's time-variable atmosphere. The committee considers the prompt installation of the new WF/PC to be of critical importance to space astronomy. Corrective optics may restore most of the capabilities of the other instruments. All three second-generation instruments (WF/PC, STIS, and NICMOS) must be installed and work well in order for HST to attain its full scientific goals. GAMMA RAY OBSERVATORY The Gamma Ray Observatory will study a broad range of topics, including accretion processes around neutron stars, the origin of gamma-ray bursts, nucleosynthesis in supernovae, interactions of cosmic rays with interstellar matter, and energy production by giant black holes in galactic nuclei. GRO's instruments have sensitivities and angular resolutions more than an order of magnitude better than those available on previous missions. The expected value of GRO's dataset mandates a vigorous, peer-reviewed program of investigations by the broad astronomical community. ADVANCED X-RAY ASTROPHYSICS FACILITY The Advanced X-ray Astrophysics Facility, the number-one-priority major new program recommended by the Field Committee in 1982, will return the United States to preeminence in x-ray astronomy, a field pioneered by NASA's earliest x-ray-detecting sounding rockets and satellites. Construction of AXAF is under way, with a launch planned for the latter part of the 1990s. AXAF will have a major impact on almost all areas of astronomy, including studies of the coronae of nearby stars, mapping of energetic galaxies, and detection of hot gas within distant clusters of galaxies. AXAF has a strong technical and scientific heritage from previous x-ray missions; in many ways, it is a scaled-up version of the successful Einstein Observatory launched more than a decade ago. This

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS heritage, together with management from a single NASA center, designation of a prime contractor, project involvement of experienced NASA and non-NASA scientists, and scheduled end-to-end testing, means that AXAF should avoid many of the problems that plagued the HST program. The committee reaffirms the Field Committee decision that made AXAF the highest-priority large program of the 1980s and stresses the importance to all astronomy of deploying AXAF as soon as possible. The committee expresses its strong support for the early establishment of a science center that would help NASA maximize the scientific return from AXAF. The Explorer Program The Explorer queue for astronomy and astrophysics currently includes five missions (Table 3.3) that, according to current NASA plans, will be completed before new projects in either the Delta class or the Small Explorer (SMEX) class can be flown. The currently planned flight rate for astrophysics is one Delta-class Explorer and one SMEX-class Explorer approximately every two years. As discussed in Chapter 1 and Chapter 7, the committee has recommended acceleration of the Delta-class and SMEX-class Explorer programs. Two Explorer missions will operate at ultraviolet wavelengths shorter than HST's limit of 120 nm. The Extreme Ultraviolet Explorer (EUVE) will carry out an all-sky survey in several bands covering wavelengths between 7 and 76 nm, extending the ultraviolet survey of the Roentgen Satellite (ROSAT) to longer wavelengths. Spectroscopic observations with a resolving power of 250 will be carried out on EUVE through a guest observer program. A deep survey will cover about 1 percent of the sky with a sensitivity at least 10 times greater than that of the all-sky survey and will provide guest investigators the opportunity to make spectroscopic observations of interesting new objects. EUVE will add greatly to our understanding of hot, young white dwarfs, cataclysmic variables, stellar coronae, and the local interstellar medium. The Far Ultraviolet Spectroscopy Explorer (FUSE) will make spectroscopic observations shortward of 120 nm with unprecedented sensitivity (100,000 times greater than the sensitivity of the Copernicus telescope launched 20 years ago) and will bridge the spectral gap between HST and AXAF. FUSE will open a window that contains the fundamental resonance transitions of atomic and molecular species that can be used to probe physical processes in the early universe, measure the abundance of deuterium in a variety of environments, and determine the physical conditions in the interstellar medium in distant and evolving galaxies. As discussed in Chapter 1 and Chapter 4, the highest-priority, moderate space-based initiative is the acquisition of an independent, dedicated spacecraft for FUSE. The X-ray Timing Explorer (XTE) will make spectroscopic and photometric

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS TABLE 3.3 Currently Funded Explorers Program Date Selected Planned Launch Datea Delta-Class Explorers Extreme Ultraviolet Explorer (EUVE) 1982 1992 X-ray Timing Explorer (XTE) 1984 1996 Advanced Composition Explorer (ACE) 1989 1997 Far Ultraviolet Spectroscopy Explorer (FUSE) 1989 1999 Small Explorers (SMEX) Submillimeter Wave Astronomy Satellite (SWAS) 1989 1995 a Launch dates after 1995 are uncertain. observations in the 1- to 100-keV range with microsecond temporal resolution. XTE will advance our understanding of the physics of accretion flows around neutron stars and black holes, and of relativistic plasmas in the nuclei of active galaxies. The Advanced Composition Explorer (ACE) will study the isotopic and elemental abundances of cosmic rays over a broad range of energies. The Small Explorer program was initiated in the late 1980s to provide rapid access to space for payloads weighing less than about 200 kg. The stringent requirements of astronomy instruments often require pointing systems that weigh almost this amount by themselves; nevertheless, a number of imaginative proposals have been made. The Submillimeter Wave Astronomy Satellite has been selected as the first space mission to explore this wavelength band. As enhanced launch vehicles become available, a broader range of astronomical projects will become possible. NASA is to be commended for its support of the analysis of data from the Infrared Astronomical Satellite (IRAS). The success of IRAS and of its General Investigator program serve as a model for the active support of Explorer missions. The committee urges NASA to provide strong support for the analysis of data from the Cosmic Background Explorer (COBE), including a vigorous guest investigator program. The Suborbital Program NASA's suborbital program trains students, tests instruments, and explores new scientific ideas by flying telescopes in rockets, balloons, and aircraft. This activity is NASA's only space science hardware program that operates on the time scale of a graduate student's career, allowing students and postdoctoral associates to be involved in all aspects of developing new instruments. Important

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS scientific results, including the discovery of celestial x-ray sources, the measurement of the dipole anisotropy of the microwave background, discovery of variable gamma-ray emission from the galactic center, and critical infrared and gamma-ray measurements of Supernova 1987A, have come from the suborbital program. The Kuiper Airborne Observatory (KAO), a 0.9-m telescope operating for 15 years in a C-141 aircraft, opened up far-infrared and submillimeter wavelengths to scientific investigation, produced over 700 scientific papers, and trained 40 PhD students. The importance of the suborbital program for the training of instrumentalists is exemplified by the fact that 80 percent of the U.S. science team on the successful IRAS program had worked previously on the KAO. As discussed in Chapter 1 and Chapter 4, one of the major recommendations of this report is to fund the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA) as a successor to the KAO. International Collaborations Astronomy has traditionally been an international enterprise, and as space missions have become more complex, collaborations with foreign colleagues have made possible important programs that otherwise would have been unaffordable. NASA has used resources from the Explorer program to support U.S. scientists to fly instruments on foreign spacecraft in the absence of flight opportunities on American spacecraft. The ROSAT x-ray telescope was launched in 1990 as a collaboration between Germany, the United Kingdom, and the United States. ROSAT 's high-sensitivity survey will produce an all-sky catalog of more than 100,000 galactic and extragalactic x-ray sources. Following the survey, U.S. and European researchers will carry out pointed observations of many known and newly discovered objects. U.S. instruments are also planned for the Japanese-U.S. ASTRO-D mission, the Soviet-French Spectrum X-Gamma, and the ESA's X-ray Multi-Mirror Mission. Other important missions include radio interferometry from space with the Japanese VSOP and the Soviet RadioAstron missions and a proposed NASA-ESA collaboration on an orbiting submillimeter telescope. Participation by U.S. scientists in ESA's Infrared Space Observatory (ISO) will provide valuable astrophysical data and help define SIRTF's scientific program. In Chapter 1, the committee recommends a new budgetary line to cover the costs of international collaborations carrying U.S. instruments, an activity that currently is supported out of the Explorer program. Shuttle Payloads After the Challenger accident, NASA revived the mixed-fleet philosophy that utilizes unmanned boosters, like the Deltas. The committee strongly endorses this strategy of making unmanned boosters available as the main

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS launch vehicles for new astronomy missions. The committee recognizes that some important missions, such as the ASTRO telescope, the HST second-generation instruments, and the High-Energy Transient Experiment (HETE), are dependent on the Shuttle for either launching or servicing. While this report was in its final stages of preparation, ASTRO had a successful nine-day mission producing x-ray, extreme-UV, and UV spectra; UV polarization measurements; and UV images of a wide range of astronomical objects. A preliminary inspection of those data suggests that many discoveries will follow from the full analysis of the dataset. Technology Development An imaginative, innovative program of technology development is a prerequisite for the missions of the next century. Current programs such as the NICMOS instrument for HST and the entire SIRTF mission benefited from NASA's investment in advanced detector technologies. AXAF benefited from the application of bolometers (developed in infrared astronomy) and charge-coupled devices (CCDs; developed in optical astronomy) to x-ray astronomy. Future submillimeter and far-infrared telescopes as large as 10 m may use technology being developed by NASA for lightweight optics with surface accuracies around 1 µm. Technology development set in motion long before critical mission milestones is cost-effective. The committee concurs with the strong support given by the Committee on Space Policy (the “Stever Report ”; NAS-NAE, 1988) for vigorous technology development across NASA's entire space astronomy program. Chapter 1 recommends new technology initiatives in support of a generation of telescopes beyond the Great Observatories. As discussed in Chapter 6, “Astronomy from the Moon,” the committee strongly favors phased technology-development efforts that progress from laboratory test beds, to modest instruments and precursor missions with significant scientific goals, and finally, to large sophisticated observatories. THEORETICAL AND LABORATORY ASTROPHYSICS Theory provides the paradigms within which observations are planned and interpreted, and it must also respond to unexpected observational discoveries. A strong theoretical community also makes motivating, and often surprising, predictions about what might be seen. This predictive capability is often crucial in designing new instruments: for example, the characteristics of the Cosmic Background Imager recommended in Chapter 1 are determined by theoretical calculations of fluctuations in the microwave background, and the proposed gamma-ray-spectroscopy Explorer mission is designed to detect theoretically predicted lines from supernovae in other galaxies. Solar neutrino experiments

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS are meaningful only in relation to theoretical predictions; a few radioactive decays, more or less, in a thousand tons of detector material are significant only in comparison with predicted rates. Theory and observation together provide a coherent view of the universe. The success of modern astrophysics illustrates the close interdependence of theory, observation, and experiment. As described in the report from the Theory and Laboratory Astrophysics Panel in the Working Papers (NRC, 1991), the vitality of astronomy and astrophysics requires strong support for theoretical investigations as well as for experimental and observational programs. NASA responded positively to the Field Committee's recommendation that it establish a strong, broad program in theoretical astrophysics. Many of the arguments made by the Field Committee in 1982 still apply. New observations from telescopes operating in the 1990s are likely to “consume” a great deal of existing theory and point the way to more sophisticated modeling and interpretation. If the full scientific benefits are to be realized for existing and planned space missions, then theoretical interpretations are required. The committee believes that NASA's support for theoretical investigations should grow in approximate proportion to NASA's support for the analysis and interpretation of observational data. Grants to individual investigators have traditionally been one of the strengths of U.S. science, particularly at NSF. Theory is by its nature often both multidisciplinary and interdisciplinary, and therefore it does not readily fit into the object-oriented classification of the Astronomy Division at NSF. The existing organization of the NSF's grants program is damaging to theoretical investigations, which most often are not tied to a specific project or subject area. The committee recommends that the NSF establish a separate, adequately funded theory program in the Astronomy Division. Theory is frequently interdisciplinary and therefore is not easily categorized within the existing object-oriented classification. The interpretation of results from ground- and space-based telescopes requires knowledge of nuclear, atomic, and molecular physics, as well as of the properties of interstellar dust and solid surfaces. The present level of support is inadequate to provide observers and theoreticians with the data they need to interpret, for example, the intensity in a particular spectral line in terms of the physical characteristics of the emitting plasma. It is important that NASA and NSF support the relatively inexpensive laboratory and theoretical work that is important to their astrophysics initiatives and is crucial to the interpretation of the results from their major observatories. The Department of Energy (DOE) has traditionally supported theoretical research in areas of importance and immediate applicability to astronomy

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS and astrophysics, including plasma physics, atomic physics, nuclear physics, radiative transfer, properties of matter, cosmology, and the physics of the early universe. The opportunity is great for fruitful cross-fertilization between astrophysics and the laboratory-oriented physics that is supported by the DOE. PARTICLE ASTROPHYSICS The study of solar neutrinos contributes to our understanding of stellar interiors and of fundamental physics. The discrepancy between the predicted rate of solar neutrino production and the values measured in the Homestake mine since the 1960s, and recently confirmed by the Japanese Kamiokande II experiment, has raised important questions for both astrophysics and particle physics. The first preliminary answers to these questions will become available in the 1990s from the Soviet-American and Western European-Israeli-American experiments using gallium detectors. The important new experimental results are being obtained using neutrino observatories in other countries, including Canada (Sudbury), Italy (Gran Sasso), Japan (Kamiokande), and the Soviet Union (SAGE), although many of the experimental techniques and theoretical ideas were developed in the United States. However, the international collaborations are strong, and some involve talented American scientists working at the frontiers of research. The committee recommends in Chapter 1 the development of the technology for a new generation of U.S. solar neutrino experiments (see Table 1.3). These detectors would also be sensitive to supernova neutrinos. The committee urges the DOE and NSF to continue to support American participation in international solar neutrino experiments. The serendipitous detection of neutrinos from Supernova 1987A confirmed the basic ideas of stellar collapse, including the order of magnitude of the total neutrino energy emitted, the time scale for the neutrinos to escape, and the characteristic energy of an individual neutrino. Only about 20 neutrinos were detected, but the observation validated theoretical insights that originated more than half a century ago. More diagnostic measurements can be made of future supernovae with detectors designed specifically for this purpose. Two fundamental programs in particle astrophysics involve the detection of particles with very high energies: gamma rays with energies in the range 1011 to 1014 eV and cosmic rays up to 1020 eV. The Whipple Observatory of the Smithsonian Institution has detected gamma rays above 3 × 1011 eV from the Crab Nebula, and there are possible detections from other observatories of even higher energies from x-ray binary stars. The “Granite” telescope, currently under construction at Mt. Hopkins, will have improved sensitivity to radiation with energies up to 1012 eV. Upgraded airshower facilities at Los

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THE DECADE OF DISCOVERY IN ASTRONOMY AND ASTROPHYSICS Alamos, New Mexico, and Dugway, Utah, offer the hope of confirming the detection of 1014-eV gamma rays and extending the observations to new objects. The development of new gamma-ray airshower detectors is recommended in Chapter 1 (see Table 1.3). The existing Fly's Eye telescope has convincingly measured cosmic rays with energies greater than 1019 eV, whose origin is still a puzzle. The new Fly's Eye telescope recommended by the committee in Chapter 1 represents an outgrowth of this exciting work. These ongoing programs, which lay the groundwork for more ambitious projects that may be required later in the decade, should be pursued vigorously. The 1980s saw many interactions between astronomers and physicists on important theoretical questions, such as the origin of the predominance of matter over antimatter and the evolution of large-scale structure in the universe. Only in the first few microseconds after the “Big Bang” were the conditions of density and temperature extreme enough to produce some of the reactions predicted by modern theories of elementary particle physics. Some particle physics models can be tested by comparing their predictions for cosmology and for elemental abundances with astronomical observations. One of the most striking examples of these interactions involves the existence and nature of “dark matter.” As discussed in Chapter 2, a feature of much of the recent work connecting cosmology and particle physics is the requirement that the universe contains much more matter than is seen. In fact, for many years astronomers have quite independently been obtaining observations of galaxies and clusters of galaxies that suggest that as much as 90 percent of the matter inferred to be present from its gravitational effects has not been seen. Many of the explanations involve exotic particles such as massive neutrinos, axions, or weakly interacting massive particles. Imaginative experiments have been proposed to detect contributions to the missing mass from various particles. The committee has recommended (see Table 1.3) that the technology for various dark matter detectors be developed in the 1990s.