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Executive Summary The half-decade since the publication of The Decade of Discovery in Astronomy and Astrophysics, the 1991 report of the National Research Council's (NRC's) Astronomy and Astrophysics Survey Committee chaired by John Bahcall, has been one of the most productive periods in the history of astronomy. Remarkable advances in understanding have been achieved, in no small part owing to the successful operation of space facilities such as the Hubble Space Telescope (HST), the Compton Gamma-Ray Observatory (CGRO), and several smaller missions including the Cosmic Background Explorer (COBE). The community consensus embodied in the Bahcall and earlier decadal surveys has proved to be a major factor in the initiation of many of NASA's space astronomy missions. But at a critical phase in NASA's planning cycle and midway between decadal surveys, the list of unexecuted consensus missions was too small to serve as the foundation for NASA's next strategic plan for the space sciences. Accordingly, in December 1995, NASA's Office of Space Science (OSS) requested that the Space Studies Board (SSB) update the scientific priorities for space astronomy and astrophysics in the context of recent discoveries and the likelihood that all but one of the space missions recommended by the Bahcall report will have been started before the year 2000. To undertake this study, the SSB early in 1996 established the Task Group on Space Astronomy and Astro- physics (TGSAA), under the aegis of the NRC's Committee on Astronomy and Astrophysics (CAA). To encom- pass the wide range of topics relevant to a study of astronomy and astrophysics beyond the solar system, TGSAA organized itself into four panels: Planets, Star Formation, and the Interstellar Medium; Stars and Stellar Evolution; Galaxies and Stellar Systems; and Cosmology and Fundamental Physics. Forty-six experts (including 10 of the CAA's 13 members) were selected to serve on these panels. The work of the four panels was coordinated by a steering group consisting of the chairs of the four panels, the two co-chairs of the CAA, an at-large member, and the chair of TGSAA. From among the leading topics for study identified by each of the four panels through debate, discussion, and a series of ballots, the steering group established a draft series of final priorities based on scientific goals. These priorities were later ratified in the same way at a joint meeting of TGSAA's steering group and the CAA. Thus, as input to OSS's development of its 1997 strategic plan, this report poses and prioritizes what TGSAA considers to be the most important scientific questions for researchers in space astronomy and astrophys- ics to address during the remainder of this decade and the beginning of the next. Astrophysicists employ a broad variety of tools to study electromagnetic radiation over the entire spectrum as well as energetic cosmic-ray particles. As a result of this diversity of techniques, TGSAA considered a wide range of space-based astrophysical opportunities. From among the many options, TGSAA identified four particularly

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2 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS important and timely priorities in space astrophysics for the early years of the coming decade. In ranked order these recommended priorities are as follows: 1. Determination of the geometry and content of the universe by measurement of the fine-scale anisot- ropy of the cosmic microwave background radiation; 2. Investigation of galaxies near the time of their formation at very high redshift; 3. Detection and study of planets around nearby stars; and 4. Measurement of the properties of black holes of all sizes. The second and third priorities were given virtually the same weight. Four additional scientific objectives were judged by TGSAA to be of high priority but to be less urgent at this time than the primary four listed above, or less achievable in terms of possible space missions. These recom- mended secondary objectives, unranked, are the following: 1. Study of star formation by, for example, high-resolution far-infrared and submillimeter observa- tions of protostars, protoplanetary disks, and outflows; 2. Study of the origin and evolution of the elements; 3. Resolution of the mystery of the cosmic gamma-ray bursts; and 4. Determination of the amount, distribution, and nature of dark matter in the universe. TGSAA places determination of the fine-scale structure of the cosmic microwave background radiation at the top of its list of priorities because of the enormous impact of COBE's observations. Not only have these observations provided confirmation of the hot big bang cosmological model, but they also have yielded new information on the primordial seeds responsible for the large-scale distribution of matter in the universe. More- over, there is a very strong likelihood that moderate follow-on missions with higher-resolution instruments can, in the fairly near term, solve some of the deepest, and hitherto intractable, problems of cosmology and physics. TGSAA believes that NASA would be making a mistake of major proportions if it did not thoroughly and vigorously exploit the great breakthroughs achieved by COBE. The required observations are clearly defined, the necessary technology exists, and the costs appear to be fairly modest and well constrained. TGSAA's recommendation for the study of galaxies near their time of formation in the early universe has a similar motivation. MST's observations in this field and in deep extragalactic research, in general, have been especially successful. Studies performed by HST and the Keck 10-meter, ground-based telescope are providing researchers with their first direct look at the evolution of galaxies only a few billion years after the beginning of the universe. Astronomers now have, as never before, the ability to build instruments allowing detailed observation of galaxy formation one of the major missing links in current understanding of cosmic evolution. Additional work in this field will certainly be one of the central facets of astronomical research over the next few decades. Observations from space will be crucial to this enterprise, and the HST, the Space Infrared Telescope Facility (SIRTF), and possible successor instruments will be at center stage. TGSAA's recommendation for a concerted search for extrasolar planetary systems and black holes should be intelligible to any reader of Science or Nature, or even the daily newspaper. At least 10 planets, all very massive compared to Earth, have now been detected around nearby stars. Study of these objects and many more can probably be conducted with optical or infrared interferometers in space. Such an endeavor will be a major scientific activity bridging the interface between astronomy, astrophysics, and the planetary sciences. TGSAA's recommendation for the detection and study of extrasolar planetary systems is a broad one, calling for a census of the most readily observed planets whatever their type. In TGSAA's judgment it would be premature to focus attention solely or primarily on terrestrial planets at this time. The detection and study of planets like Earth are very difficult tasks that should be viewed as constituting the culmination, not the beginning, of the process of extending our knowledge of planetary bodies beyond the confines of the solar system. Black holes have long been thought to power the central engines of quasars and to be responsible for the x-ray

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EXECUTIVE SUMMARY 3 emission from a handful of somewhat problematical binary stars. In the last few years these conjectures have been confirmed. Improved observations of active galaxies, combined with the discovery of new black hole candidates in binary systems, have brought about a wide consensus that black holes have, in fact, been detected. Confirmed examples have masses ranging from a few times that of the Sun, for those in binary systems, to about a billion times greater, for those in active galaxies. Thus, within a few years, the status of these bizarre objects has gone from hypothetical entities predicted by general relativity whose existence was doubted by many- to important constituents of the universe. A systematic study of black holes across the spectrum is extremely timely and, in the judgment of TGSAA, should be a central theme in space research during the coming decade. Detailed justifications for TGSAA's recommended priorities are given in Chapters 2 through 5, each of which was contributed by one of TGSAA's four panels, and in the concluding Chapter 6. The panels' chapters discuss recent progress and current problems in a wide variety of astrophysical topics, among which the recommended priorities listed above are judged to be the most scientifically important, the timeliest, and the most plausible as the centerpiece of NASA's program in space astronomy and astrophysics during the start of the decade ahead. The additional key activities listed in Chapters 2 through 5 serve a number of roles, including providing scientific justification and support for small missions that could be proposed by individual principal investigators, offering guidance to peer-review panels that will select small missions, and suggesting a focus for technology development efforts that will enable future space astronomy and astrophysics missions. Throughout its deliberations TGSAA assumed that all currently approved NASA astrophysical missions either would be operational by the early years of the coming decade or would be approaching launch. Missions of particular importance include the Advanced X-Ray Astrophysics Facility (AXAF), SIRTF, the Stratospheric Observatory for Infrared Astronomy (SOFIA), and the Far-Ultraviolet Spectroscopic Explorer (FUSE). Although TGSAA was not asked to make explicit recommendations about missions to address the scientific priorities outlined above, planned or proposed missions are implicit in the priorities recommended by TGSAA. Thus the Microwave Anisotropy Probe (MAP), approved by NASA while TGSAA's deliberations were under way, and the Planck mission (formerly COBRAS/SAMBA), a European Space Agency project with possible U.S. participation, are both dedicated to studying the anisotropy of the cosmic microwave background radiation.