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1 Framework of This Study ORIGIN AND RELATIONSHIP TO DECADAL SURVEYS For the last three decades, U.S. astronomers have set forth their future plans and priorities in a series of so- called decadal surveys. Beginning in 1964 with the work of the Whitford committee and continuing with that of the Greenstein committee in 1972, the Field committee in 1982, and, most recently, the Bahcall committee in 1991, these reports have charted the future of astronomical research in the United States.~~4 The decadal reports' approach to setting priorities is to define the means by which widely held, but unwritten, scientific priorities of the astronomical community are to be implemented. In other words, they prioritize in terms of projects and initiatives designed to address a broad range of community goals. This report of the Task Group on Space Astronomy and Astrophysics (TGSAA) is not a decadal survey and does not replace the wider-ranging, consensus-building activities associated with the Bahcall report and its prede- cessors. TGSAA's deliberations were motivated by NASA's realization that there was a significant gap in the advisory foundation on which its Office of Space Science (OSS*) has to construct strategic planning priorities in space astronomy and astrophysics to be addressed in the first decade of the next century. This gap developed because of the rapid and successful implementation- of several priority space missions, an unfortunate mismatch of schedules, and changes in the way government agencies are required to plan their future budgets. In recent years NASA has adopted a systematic approach to strategic planning for all of its scientific activities, in part because of the changing environment for research funding and also in response to the requirements of the Government Performance and Results Act (GPRA) of 1993. Besides an agencywide plan, each of NASA's components, including OSS, must maintain its own long-term strategic plan. GPRA stipulates that, by the end of the decade, agency budget submissions to the Office of Management and Budget and to Congress must be formulated in terms of these strategic plans and performance plans based on them. OSS currently revises its strategic plan every 3 years, with the next such update scheduled for the first half of 1997. At the same time, formulation of NASA's budget for FY 2000, the first year in which any major new space astronomy initiative could be proposed, will take place in early 1998. The Bahcall report, issued in 1991, recommended a series of priorities for ground- and space-based astronomy *Frequently used acronyms and those with lengthy definitions, together with definitions of selected terms and concepts, can be found in the glossary that follows Chapter 6. 4

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FRAMEWORK OF THIS STUDY and astrophysics projects. By the end of this decade, NASA expects to have fulfilled the report's remaining two priorities for space-based research in astronomy and astrophysics: initiation of the Space Infrared Telescope Facility (SIRTF) and completion of advanced studies of an astrometric interferometer facility. Thus, at a critical phase in NASA's planning cycle and midway between decadal surveys, the list of remaining consensus missions was too small to serve as the foundation of NASA's next strategic plan. As part of its efforts to develop a new pool of missions from which a few with broad-based support might emerge, NASA canvassed the astronomical community for ideas by issuing a research announcement for propos- als on new mission concepts in astrophysics.5 From the resulting response, NASA chose some 30 mission concepts for advanced study, with the aim of selecting a few of the most promising candidates for development at a later date. To solve its more immediate needs for planning advice, NASA asked the Space Studies Board (SSB) to initiate the TGSAA study to identify new priorities around which OSS could build its 1997 strategic plan. The current absence of a consensus suite of well-developed mission concepts, such as the one in place at the time of the last decadal survey, mandated that TGSAA set priorities based on scientific goals. In other words, this report assesses current knowledge across a broad range of topics in space astronomy and astrophysics and then goes on to pose and prioritize the most important scientific questions that can be formulated at the present time. Thus, TGSAA's recommended strategy is not an implementation plan and is, in fact, independent of the means of implementation.: It soon became clear in TGSAA's deliberations that the lack of emphasis on missions and the associated broadening of perspective meant that TGSAA's report would have an important secondary role as an adjunct or precursor to the next decadal survey. In essence the current report could be viewed as a framework around which the preliminary activities of the next survey committee could be organized. RELATIONSHIP TO THE SPACE STUDIES BOARD'S OTHER SPACE SCIENCE STRATEGIES Given the breadth of topics encompassed by space astronomy and astrophysics and the even wider range of scientific enquiries utilizing facilities and techniques usually associated with astronomical studies, it is not surpns- ing that certain research topics fall within the spheres of influence of several different scientific communities. This compartmentalization frequently exists for bureaucratic reasons such as differences in the sources of funding; the danger is that certain areas of study can appear to "fall between the cracks." Failure to mention a particular topic in a report such as this thus does not necessarily mean that it is unimportant. Areas of relevance to astronomy but not covered in this report include, for example, planetary astronomy and solar physics. The scientific priorities in these areas can be found in the reports of the SSB's committees on Solar and Space Physics (CSSP) and Planetary and Lunar Exploration (COMPLEX). Detailed discussions of topics such as helioseismology, solar neutrinos, and cosmic rays can be found in CSSP's A Science Strategy for Space Physics.6 A more specific discussion of cosmic rays is contained in Opportunities in Cosmic-Ray Physics and Astrophysics.7 Topics relating to studies of planetary bodies and how planetary systems and life originate can be found in COMPLEX's An Integrated Strategy for the Planetary Sciences: 1995-2010.8 Topics related to the Kuiper Belt and its relationship to circumstellar disks and the search for extrasolar planets will appear in COMPLEX's forthcoming report, Exploring the Trans-Neptune Solar System.9 APPROACH TO PRIORITIZATION The basic goals for astronomy and astrophysics are many and varied. Observations of numerous objects in the universe make unique contributions to important scientific questions. Moreover, astronomy has a long history of major discoveries associated with the opening of new wavelength regimes and observing windows. Thus, it can be tThe TGSAA study was not always completely divorced from studying missions. NASA did ask TGSAA to assess the projects selected for study under the aegis of the New Mission Concepts in Astrophysics program, but later withdrew the request.

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6 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS argued that the best way to devise a science strategy for space astronomy and astrophysics is to consider the most important scientific questions to be tackled using each observing technique. Adopting this approach would greatly assist mission planners, in that the priority investigations for a particular mission proposal could be readily determined. Organization by observing windows is, however, at variance with the multiwavelength character of today's frontline astronomical research. It is now common for researchers to specialize in the study of a particular class of astronomical object (e.g., active galactic nuclei) and their manifestations across the spectrum, rather than concen- trating on the application of a particular technique (e.g., those of x-ray astronomy) to study a broad range of objects. In summary, TGSAA's basic approach to prioritization was as follows: Concentrating on scientific objectives rather than emphasizing the methods by which the objectives are implemented; Adopting a multiwavelength approach by prioritizing scientific questions of significance to whole classes of astronomical objects and not limiting discussion to individual observing bands; and Being realistic with respect to such practical matters as cost and technical feasibility. ~ ~ con - ~ ASSUMPTIONS For a variety of reasons ranging from redirected federal budgets and hardware failures to technological advances and changing scientific emphasis, the U.S. space program must remain flexible and alert to new oppor- tunities. In addition, changing national policy goals can profoundly affect the resources available for space research, a recent example being the reconfiguration of the Advanced X-Ray Astrophysics Facility (AXAF) into AXAF-I and AXAF-S, followed by the termination of AXAF-S. This report, like previous SSB strategies, promotes systematic investigations in which new understanding resulting from one mission provides the justification and impetus for further studies. Thus, a break in the logical progression of missions, whether due to cancellation or a failure of one sort or another, will usually require that the lost mission, or an improved version of it, be reflown. As a result, a fundamental assumption of this report is that certain ongoing or approved space missions will either be successfully implemented or will continue to operate as planned. In particular, in conducting this study, TGSAA made the following assumptions: The Hubble Space Telescope (HST), the first of NASA's Great Observatories, will continue to operate until at least 2005, and planned upgrades in instruments (including NICMOS, STIS, and ACS) will proceed as currently scheduled and will operate successfully. HST is a 2.4-meter optical, ultraviolet, and near-infrared (following the installation of NICMOS in February 1997) telescope operating in low Earth orbit. The Common Gamma-Rav Observatorv (CGRO). the second of NASA's Great Observatories. will ~ ~ ~ ~ ~ ~ , operate at least until 2004. Although its high-energy gamma-ray telescope, EGRET, has ceased routine opera- tions, three of the satellite's four instruments continue to provide the only gamma-ray observations from space, including the continuing study of the mysterious gamma-ray bursts. The Advanced X-Ray Astrophysics Facility (AXAF), the third of NASA's Great Observatories, will be launched according to schedule in late 1998 and will be operated for at least 5 years. AXAF is equipped with grazing-incidence mirrors and four scientific instruments designed to perform high-spatial-resolution imaging and spectroscopy in the 0.1- to 10-keV x-ray band. The Far-Ultraviolet Spectroscopic Explorer (FUSE), nominally the first of NASA's mid-size Explorer (Midex) spacecraft, will be launched according to schedule in late 1998 and will be operated for approximately 3 years. FUSE is designed to conduct high-resolution spectroscopic observations in the far-ultraviolet spectral band between 90 and 120 nm.

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FRAMEWORK OF THIS STUDY 7 The Space Infrared Telescope Facility (SIRTF), the fourth and final Great Observatory, will be launched according to schedule in 2001 and will operate for at least 2.5 years. SIRTF is equipped with a 0.85-meter cryogenically cooled mirror and three scientific instruments. It is designed to perform very sensitive photometric and imaging observations at wavelengths between 3 and 180 microns and spectroscopic observations in the band between 5 and 40 microns. The Stratospheric Observatory for Infrared Astronomy (SOFIA) will have its first flight in 2001 and will operate for approximately 20 years. SOFIA is a Boeing 747 aircraft modified to carry a 2.5-meter telescope to an altitude in excess of 12,000 meters on a routine basis. SOFIA will be equipped with a variety of instruments designed to operate in the optical, infrared, and submillimeter bands between 0.3 and 1,600 microns. In addition, TGSAA assumed that the major ground-based facilities recommended by past decadal reports either would be completed or would continue to operate as they do currently. These facilities, together with other ground-based observatories such as the 10-meter Keck telescope and the instrumentation at the various national optical and radio astronomy observatories, are essential tools for the long-term study of objects discovered by space-based instrumentation. The complementary role of ground-based observatories is so important that any disruption in the operation of this important infrastructure could have serious, long-term consequences for the effective exploitation of space-based astronomical instruments. TGSAA also believes that laboratory and theoreti- cal studies and instrument development are essential complements to the productive use of space-based research facilities. RELATED CONCERNS International Cooperation Space science is a thoroughly international activity, with a growing number of nations now capable of independently mounting important astronomical missions. As budgets shrink, the possibility of leveraging scarce U.S. resources by encouraging international participation in NASA missions, or by flying U.S. instruments on foreign spacecraft, looks ever more promising. Such collaborative activities have many advantages beyond the better use of limited resources; these advantages include broadening the participation in the space-research en- deavor and promoting communication between diverse elements of the worldwide astronomical community. Although a consideration of the international aspects of space astronomy and astrophysics is technically outside its charge, TGSAA is aware that cooperative projects are increasingly necessary to implement the recom- mendations of the decadal surveys. A good example of this trend is NASA's plan to install the x-ray calorimeter originally designed to fly on the cancelled AXAF-S mission on Japan's forthcoming Astro-E x-ray astronomy satellite. Given current budgetary trends, some consideration of international cooperation is a necessary comno- nent of any space-science strategy. ~.. .. . .. .. ~ - -r -- ~---~~ ~~ ~ ~~~~~~~) ---I- L)esp~te their many attractive features, cooperative activities can, however, have significant drawbacks. Al- though it is supportive of cooperative activities, TGSAA also believes that they must not be entered into lightly. Problems encountered during such activities can cancel all related benefits and have a chilling effect on future collaborations. The following general principles, espoused in past SSB reports,~ should be borne in mind when cooperative activities are being considered: Selection of foreign scientists and experiments for U.S. missions should be based on scientific merit, and the free flow of scientific data and results should be a condition for any cooperative arrangements; NASA should consider all appropriate foreign capabilities available for planning and carrying out its missions and should cultivate those that enhance the scientific return; and NASA should fully involve the scientific community in planning for international cooperation and in assessing proposed cooperative missions.

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8 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS Technology Development Another issue not specifically included in TGSAA's charge, but essential to the successful execution of any science strategy, is technology development. Future advances in space astronomy and astrophysics are very strongly tied to the development of new technology. Calling for significant investment in this area might appear obvious, but report after report has criticized NASA's lack of investment in technology. Increased attention to new technology is now more important than ever before. In the past, much technology development has been funded as an integral part of major missions such as HST, AXAF, and SIRTF. Small missions with short development schedules the type currently in favor at NASA provide little or no opportunity for technological advance. With the end of the Cold War and the increasing availability of technology developed for military applications, NASA has an opportunity to substantially close this gap. Given TGSAA's lack of in-depth expertise in many technological areas, additional studies by expert groups should be undertaken before some of the scien- tific priorities identified in this report are implemented as missions. ORGANIZATION OF THIS REPORT To set overall scientific priorities from among the broad range of topics in space astronomy and astrophysics, TGSAA organized itself into four panels, each dealing with a number of closely related fields of research, along the following lines: . Planets, star formation and the interstellar medium, including the search for extrasolar planets, how stars and planets form, and the evolution of the interstellar medium in galaxies (Chapter 23; Stars and stellar evolution, including the life cycles of stars, the origin of the chemical elements, and the use of stars as celestial laboratories (Chapter 3~; . Galaxies and stellar systems, including the formation and evolution of galaxies, exotic processes in galac- tic nuclei, the intergalactic medium, and the large-scale structure of the universe (Chapter 41; and Cosmology and fundamental physics, including the cosmic microwave background radiation, large-scale structure of the universe, gravitational astronomy, and cosmic mysteries such as gamma-ray bursts and ultrahigh- energy cosmic rays (Chapter 5~. Not surprisingly, certain topics do not fall neatly within these categories and could, with equal justification, be included in several different chapters. Pre-main sequence stars, for example, can be considered as the end points of star formation or the starting point of stellar evolution. Similarly, gamma-ray bursts could be discussed in the context of a stellar or a cosmological phenomenon. In the interest of brevity and to avoid redundancy between different chapters, several somewhat arbitrary assignments have been made. Gamma-ray bursts, for example, are still such a mystery that all discussion of them can be found in the section titled "New Astrophysical Windows and Cosmic Mysteries" in Chapter 5. On the other hand, topics such as large-scale structure and dark matter have clearly defined aspects that fit naturally within different parts of the text. Thus the linear and nonlinear aspects of large-scale structure are discussed in Chapter 5 and 4, respectively. Similarly, the role played by dark matter in the structure and evolution of the universe is found in Chapter 5, while its influence on galaxies and clusters of galaxies is discussed in Chapter 4. In general, each of the chapters devoted to the four principal topics (Chapters 2 through 5) follows a similar format. Each begins by defining a number of topical themes (e.g., dark matter, origin of the elements, and so on) around which the remainder of the chapter is organized. The text devoted to each theme begins with introductory material and is followed by a listing of key questions for which answers are needed to advance current understand- ing. The discussion of each theme then continues by describing recent progress in relevant studies and ends with a review of thematic priorities outlining suggested future directions for investigation. Each chapter concludes with a summary of the most important discipline-oriented scientific priorities as distilled from each thematic section. The key questions and various priorities outlined in each chapter serve a number of parallel roles:

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FRAMEWORK OF THIS STUDY 9 1. First and foremost they serve as a mechanism by which the diversity of topics found in each chapter could be distilled to a sufficient level that it was possible for each panel to devise a rank ordering between a manageable number of roughly comparable activities. The detailed discussion of the final priorities in each of the Chapters 2 through 5 then constitutes the foundation on which the priorities described in Chapter 6 were constructed. This final step, the selection of a series of overall priorities for the major aspects of NASA's future programs in space astronomy and astrophysics, was performed by TGSAA's steering group following debate, discussion, and a series of ballots. The steering group's final rank ordering was subsequently ratified in the same way at a joint meeting between TGSAA's steering group and the Committee on Astronomy and Astrophysics. 2. They demonstrate the depth and diversity of current activities in space astronomy and astrophysics and their potential for future advances on a broad front. As such they are an advertisement for the richness of opportunities open to enterprising researchers. 3. They provide a measure of justification and support to principal investigators proposing Midex and other small missions, and, conversely, guidance to peer-review panels selecting such missions. With NASA's deemphasis of large missions, the Explorer program and other small-mission lines have assumed a new signifi- cance as a means by which priority topics in space astronomy and astrophysics are addressed. Past decadal surveys have not, however, provided much strategic guidance on the conduct of such missions. REFERENCES 1. National Research Council, Panel on Astronomical Facilities for the Committee on Science and Public Policy, Ground-based As- tronomy: A Ten-Year Program, National Academy of Sciences, Washington, D.C., 1964. 2. National Research Council, Astronomy Survey Committee, Astronomy and Astrophysics for the 1970's, National Academy of Sci- ences, Washington, D.C., 1972. 3. National Research Council, Astronomy Survey Committee, Astronomy and Astrophysics for the 1980's, Volume 1: Report of the Astronomy Survey Committee, National Academy Press, Washington, D.C., 1982. 4. National Research Council, Astronomy and Astrophysics Survey Committee, The Decade of Discovery in Astronomy and Astrophys- ics, National Academy Press, Washington, D.C., 1991. 5. National Aeronautics and Space Administration, Office of Space Science, Astrophysics Division, NASA Research Announcement Soliciting Proposals for New Mission Concepts in Astrophysics, NRA 94-OSS-15, NASA, Washington, D.C., September 12, 1994. 6. Space Studies Board and Board on Atmospheric Sciences and Climate, National Research Council, A Science Strategy for Space Physics, National Academy Press, Washington, D.C., 1995. 7. Board on Physics and Astronomy, National Research Council, Opportunities in Cosmic-Ray Physics and Astrophysics, National Academy Press, Washington, D.C., 1995. 8. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994. 9. Space Studies Board, National Research Council, Exploring the Trans-Neptune Solar System, National Academy Press, Washington, D.C., 1997 (in preparation). 10. See, for example, Space Science Board, National Research Council, Space Science in the Twenty-First Century: Imperatives for the Decades 1995 to 2015-Overview, National Academy Press, Washington, D.C., 1988, pages 79-81. 11. See, for example, Space Studies Board and Aeronautics and Space Engineering Board, National Research Council, Improving NASA's Technology for Space Science, National Academy Press, Washington, D.C., 1993.