National Academies Press: OpenBook

A New Science Strategy for Space Astronomy and Astrophysics (1997)

Chapter: 6 Conclusions and Recommendations

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Suggested Citation:"6 Conclusions and Recommendations." National Research Council. 1997. A New Science Strategy for Space Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/5873.
Page 58
Suggested Citation:"6 Conclusions and Recommendations." National Research Council. 1997. A New Science Strategy for Space Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/5873.
Page 59
Suggested Citation:"6 Conclusions and Recommendations." National Research Council. 1997. A New Science Strategy for Space Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/5873.
Page 60
Suggested Citation:"6 Conclusions and Recommendations." National Research Council. 1997. A New Science Strategy for Space Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/5873.
Page 61
Suggested Citation:"6 Conclusions and Recommendations." National Research Council. 1997. A New Science Strategy for Space Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/5873.
Page 62

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6 Conclusions and Recommendations The four preceding chapters describe in detail the great scientific progress that has occurred since 1991 on a broad range of astrophysical topics of interest to NASA. Among the many highlights, general-purpose instruments such as the Hubble Space Telescope (MST) and the Keck telescope have transformed astronomers' thinking about the distant universe and have provided remarkable evidence regarding the process of star formation in our own galaxy. The Cosmic Background Explorer (COBE) has provided stunning confirmation of the hot big bang cosmology, and it has found the long-sought primordial seeds of large-scale structure, providing a quantitative basis for theories of how structure evolved. The Compton Gamma-Ray Observatory (CGRO) has detected more than 1,000 gamma-ray burst events and has demonstrated that they are isotropically distributed on the sky, implying an origin outside the disk of our galaxy. The signature of massive planets in orbit around a number of nearby stars has been detected by ground-based spectroscopy. The evidence for the existence of black holes in several x-ray binary systems now is quite solid, and the existence of giant black holes in the centers of several galaxies is now established beyond any reasonable doubt. Astrophysics covers an enormous diversity of physical phenomena, but there is much cross-fertilization, and advances in one subfield are often crucial to advances in another. The existence of numerous important topics in astrophysics, ripe for further progress by judiciously selected NASA missions beyond those assumed in Chapter 1 (MST, CGRO, Advanced X-Ray Astrophysics Facility, Space Infrared Telescope Facility (SIRTF), Stratospheric Observatory for Infrared Astronomy (SOFIA), and Far-Ultraviolet Spectroscopic Explorer), presents extraordi- nary opportunities. The research activities identified in the concluding sections of Chapters 2 through 5 are the foundation on which TGSAA's overall recommended priorities for space astronomy and astrophysics are based. The priorities recommended in Chapters 2 through 5 (Box 6.1 J encompass highly diverse spatial, temporal, and energy scales, ranging from extrasolar planets to supermassive black holes, from the origin of the universe to its ultimate fate, and from submillimeter radiation emitted by forming stars to ultrahigh-energy cosmic rays of unknown origin. Some of the recommended priorities may be achieved with a limited series of observations, while others will require long-term, multifaceted observing programs. In setting its priorities, TGSAA made every effort to be realistic and to consider recent progress and the prospects for the requisite technology. After considerable productive discus- sion and debate, TGSAA was able to make a robust selection of a ranked list of scientific endeavors that are most compelling as the foci of NASA's mission planning over the next 5 to 10 years. 58

CONCLUSIONS AND RECOMMENDATIONS Box 6.1 Recommended Priority Research Activities Identified in Chapters 2 Through 5 Planets, Star Formation, and the Interstellar Medium · Obtain a census of planetary systems around enough stars (~1,000) so that the frequency, separa- tions, and masses of planets comparable to or larger than Uranus can be investigated for a range of types of stars and stellar systems. · Characterize the very earliest stages of star formation by observing the structure and dynamics of protostellar regions. · Determine the large-scale three-dimensional structure of the interstellar medium and the star-forma- tion regions within it. · Detect indirectly terrestrial-mass planets. · Perform ultraviolet spectroscopic studies of the connection between galactic disks and halos with a sensitivity significantly higher than that now provided by the Hubble Space Telescope. · Conduct near-infrared imaging and spectroscopic studies of young embedded star clusters. Stars and Stellar Evolution · Understand the origin and astrophysical manifestations of black holes. Study the behavior of maker at extremes of gravity, rotation, magnetic field, and energy density. Investigate fundamental issues concerning the origin of the elements. Improve understanding of stars as markers of the size and age of the universe. · Understand the effects of rotation and magnetic fields, and the effects of binary companions. Galaxies and Stellar Systems · Detail the processes at work in the high-redshift universe by conducting a census of the universe as it was 1 billion years after the big bang. · Link the high-redshift objects to their descendants by following the evolution to lower redshift, and undertake a detailed study of the underlying physical processes. · Understand the formation and evolution of supermassive black holes in the nuclei of galaxies, and elucidate the processes at work there. Cosmology and Fundamental Physics · Determine the geometry of the universe by measuring the anisotropy of the cosmic microwave back- ground radiation. · Test theories for the origin and evolution of structure in the universe. · Determine the amount, distribution, and nature of dark matter. · Test predictions of general relativity in the strong-gravity regime. · Investigate the nature of cosmic gamma-ray bursts and the origin of ultrahigh-energy cosmic rays. · Extend radio astronomy to ultralow frequencies. The scientific rationale for TGSAA's leading four recommended priorities is as follows: 59 1. Determination of the geometry and content of the universe by measurement of the fine-scale anisot- ropy of the cosmic microwave background radiation. The establishment of the hot big bang cosmology is, as explained in Chapter 5, one of the great scientific achievements of the twentieth century. The 80-year-old goal of

60 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS determining the geometry of the universe is now within reach. Bold and powerful ideas that can extend current knowledge to encompass events occurring within a tiny fraction of a second after the beginning can now be put to observational test. Accomplishing this goal will not only greatly advance astronomers' understanding of the universe on the largest possible scale, but, paradoxically, at the other extreme of size will also shed light on the unification of the ~ ~ . ~ .. ~ ~ r ~ ~ · 1 , ~ · ~ · . ,' · ~ fundamental particles and forces of physics-an unprecedented marriage of science at the macroscopic and . . . microscopic scale. Determining the geometry of the universe requires precise measurements of three classic parameters: the Hubble constant (Ho), the deceleration parameter (q0), and Einstein's cosmological constant Ail. As a result of the recent discovery by COBE of minute variations in intensity, or ripples, in the cosmic microwave background radiation (CMBR), caused by small initial inhomogeneities in the distribution of matter in the early universe, astronomers now have a remarkably powerful new tool to determine these parameters. This tool is high-angular- resolution mapping, on scales of a few arc minutes and larger, of the anisotropy of the CMBR. The CMBR is fundamental to this effort because it offers a snapshot of the universe at a simpler time, long before stars, galaxies, and other structures existed. Moreover, there are very precise predictions for the anisotropy expected-specifi- cally, the spectrum of the ripples as a function of size as functions of the three cosmological parameters. Such measurements will provide maps of the inhomogeneities in the distribution of matter that seeded all the structure in the universe seen today, and they offer the promise of a determination of the fundamental cosmological parameters to a precision of 1 to 5%. High-resolution mapping of the CMBR will critically test the theory of inflation, specifically the predictions it makes that the universe is spatially flat (euclidian), that the spectrum of anisotropies has a specific form, and that a large quantity of dark matter exists. In addition, the mapping will test the cold dark matter theory, the most detailed picture currently available to explain how large-scale structure evolved. Realizing the full potential of the measurements of the CMBR will require a multiplicity of approaches, both from space and from the ground. The anticipated scientific payoff clearly justifies the effort. Indeed, rarely have theory and technology been better matched to address a scientific question of such transcendent importance. That is the essential reason that TGSAA has placed the prime recommendation of its Panel on Cosmology and Funda- mental Physics at the top of its list of overall recommended priorities. 2. Investigation of galaxies near the time of their formation at very high redshift. The powerful capabili- ties of the HST and the Keck 10-meter telescope have greatly expanded astronomers' knowledge of the distant universe, providing for the first time a direct look at the evolution of galaxies only a few billion years after the big bang. Understanding the origin of galaxies is, as explained in Chapter 4, central to astronomers' picture of how the universe around us came to be the way it is. Studies of the CMBR described above provide a direct glimpse of the condensations that eventually became galaxies and the structures seen by astronomers in the universe today. The birth of stars and galaxies is, however, very complex and still only poorly understood. Fortunately, astronomers can take advantage of the vast distances to galaxies. Because it can take billions of years for light to travel to us from distant objects, observers have the opportunity to look back in time and see galaxies as they were when their light was emitted billions of years ago, the era when galaxies themselves were born. While the Hubble and Keck telescopes have given observers a glimpse of the most luminous objects at a time a few billions years after the big bang, astronomers currently know very little about ordinary galaxies like the Milky Way at that epoch. Research- ers cannot currently obtain spectroscopic information for most of the galaxies shown in the so-called Hubble Deep Field (see Figure 4.1). It is urgent that observers persist in this exploration and Gush out to even greater distances ~ ~ ~ ~ ~ ~ -~ and earlier cosmic epochs. The next generation of space-based instrumentation, beginning with SIRTF, will have the technical capabilities to perform what is, in essence, the highest priority of TGSAA's Panel on Galaxies and Stellar Systems, that is, to trace out the whole process of galaxy formation, from the gaseous precursors of galaxies, through the earliest episodes of star formation, to the formation of supermassive black holes in the nuclei of galaxies. 3. Detection and study of planets around nearby stars. The recent discovery of planets around at least 10 nearby stars has, as explained in Chapter 2, opened a new chapter in astronomy and, not surprisingly, also generated great public interest. Observations to date suggest a great diversity among planetary systems, and in TGSAA's judgment, it would be premature to focus attention at this time on terrestrial planets. Nearly all the

CONCLUSIONS AND RECOMMENDATIONS 61 planets detected are far more massive than Earth, having masses comparable to or greater than that of Jupiter. As a first step toward understanding extrasolar planets, it is essential to take a census of planetary systems around enough nearby stars (a 1,000) to determine their characteristics as a function of the properties of their parent stars. While ground-based measurements of radial velocities are suitable for detecting massive planets close to their parent star, high-precision astrometry at the level of 10 microarc see or better is required to detect planets that are much smaller or more distant than those found so far. Space-based interferometry appears crucial for achieving this goal. This activity, the highest priority of the Panel on Planets, Star Formation, and the Interstellar Medium, is the first step in achieving the long-range goal of detecting and observing terrestrial planets around nearby stars. 4. Measurement of the properties of black holes of all sizes. Black holes, hypothetical objects of great scientific interest predicted by general relativity, are fascinating to scientists and the general public because of their bizarre properties. While the reality of black holes has long been questioned, they have now almost certainly been observed in a wide range of astronomical settings. As described in Chapter 3, observations over the past 5 years from x-ray satellites and ground-based radio and optical telescopes have established at least six very strong black hole candidates among the Milky Way's population of x-ray binary stars. Moreover, as explained in Chapter 4, observations with the HST and the Advanced Satellite for Cosmology and Astrophysics have also provided almost certain evidence for the existence of very much more massive black holes in the nuclei of remote active galaxies. These studies lend support to the hypothesis that accretion onto black holes is the fundamental explanation for the exceptional luminosity of quasars. Observations with the Very Long Baseline Array have obtained even more compelling evidence that the nuclei of the much more common, mildly "active" galaxies contain black holes with masses some 10 million times that of the Sun. As a result of these developments, there is general agreement that black holes have been detected over a wide range of masses. TGSAA concluded, based on the rankings given by its Panel on Stars and Stellar Evolution and Panel on Galaxies and Stellar Systems, that a systematic study of black holes, particularly at high energies, should be given high priority by NASA during the coming decade. Key issues to be addressed are the formation of black holes over the entire mass range observed; the mechanisms by which accretion onto black holes produces both a prodigious output of energy and jet-like outflows; the number of stellar-mass black holes in galaxies and galactic halos; and possible tests of the laws of physics in the extreme environment close to black holes. Each of the chapters contributed by TGSAA's panels describes a number of priority topics in addition to the four listed above. Some are more suitable than others for implementation by NASA missions in the coming decade. Continued NASA support for technolo~v development is anoronriate Hanford. ~ nilmh~.r of the othPr~ will be suitable for flight opportunities. it ~ ~ ~ ~ . ~ ~1~ ~ _ _ ~ ~ 1 ~ 1 ~ _ _ __~J __, _~_--~ ^ ~t-~_ v~_ ~ AA~$AAV~A I Ll1 ~J~ll~1 ~VY 111 many or mese aou1t1ona1 sc1ent1~1c priorities are best addressed by multifaceted studies, including additional theoretical investigations. A number of the topics discussed are possible scientific objectives for missions over the coming decade in NASA's ongoing Explorer-class satellites. Others might be best pursued by balloon-borne payloads, and still others are best suited to ground-based studies. TGSAA debated the merits of including an expanded list of recommended priorities and decided that the following four topics are particularly interesting and timely for NASA's consideration, addressing questions of fundamental significance. These additional recom- mended priorities, unranked, are as follows: 1. Study of star formation by, for example, high-resolution far-infrared and submillimeter observa- lions of protostars, protoplanetary disks, and outflows. Chapter 2 explains that although star formation is a continual process in our galaxy, observing the creation of stars from interstellar material is a formidable challenge because these regions are inevitably deeply shrouded in thick clouds of dust. Astronomers' understanding of the process whereby dense clouds form individual young stars and their circumstellar disks is far from complete. Most stars are observed to form in clusters, but astronomers do not understand how this affects the properties of individual protostars. Nor do they understand the distribution of stellar masses, the most crucial factor determin- ing the life history of stars. SIRTF and SOFIA will provide new insights into these questions, but future missions with higher spatial and spectral resolution will be needed. 2. Study of the origin and evolution of the elements. Chapter 3 describes how all the chemical elements on which life depends were produced in early generations of stars. These elements are subsequently expelled by

62 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS supernova blast waves or stellar winds back into the interstellar medium and are eventually incorporated into subsequent generations of stars. Astronomers understand the general mechanism of mass loss from stars, but they cannot explain why the material is ejected so anisotropically. The abundance of heavy elements is higher toward the centers of galaxies, as expected. Researchers have, however, no idea how the central regions of active galaxies (e.g., quasars), observed at high redshift, can produce a solar distribution of heavy elements so quickly. The interior structure of the highly evolved stars that eventually eject their metals into space is quite complex. A key question that is poorly understood at present is how mass loss from, and mixing inside, stars affect stellar evolution and nucleosynthesis. 3. Resolution of the mystery of the cosmic gamma-ray bursts. Chapter 3 emphasizes that some of the most challenging problems in astrophysics remain in the exploration of the behavior of systems under extreme physical conditions associated with compact stars. Because of the large gravitational potential and extreme temperatures encountered, the natural form of radiation is in the x- or gamma-ray bands. Study of these high-energy emissions is done most efficiently from space. Perhaps the prime indication of the primitive nature of current understanding of compact objects is the inability of researchers to explain the mysterious gamma-ray bursts. Astronomers have, as described in Chapter 5, known of the existence of gamma-ray bursts for more than 20 years, but only recently have obtained fairly conclusive evidence that the bursts probably originate from outside the disk of our galaxy. The transient nature of the bursts makes it exceptionally difficult to identify a counterpart source in any other waveband. As a result, astronomers do not know the most fundamental property of the gamma- ray burst sources: their distance. Hence major questions are unresolved. Are they at cosmological distances or in an extended halo of our own galaxy? Is the radiation collimated by some sort of jet? Why is such a large fraction of the energy emitted at such a high energy? The recent discovery by the Satellite per Astronomia in Raggi X (SAX) of the soft x-ray afterglow of at least one gamma-ray burst, and the subsequent identification with a fading optical object, may point the way to resolving at least the distance-scale mystery of the gamma-ray bursts. However, detailed understanding of the bursts and their slowly fading counterparts at longer wavelengths (e.g., optical and soft x-ray) poses major challenges. Given the heavy obscuration expected in some galaxies, rapid infrared follow-up may be required to identify the objects and their underlying physics. 4. Determination of the amount, distribution, and nature of dark matter in the universe. The only direct evidence for dark matter derives, as discussed in Chapters 4 and 5, from astrophysical inference of gravitationally bound objects, such as galaxies and clusters of galaxies, but the evidence is ubiquitous and consistent numbers are measured by several very different techniques. Astronomers still have no idea whether this dark matter takes the form of a collection of compact dead stars or primordial black holes, a hitherto unseen elementary particle, or something even more exotic. Theories of elementary particle physics suggest several reasonable candidates for this dark matter, some of which might be detectable in terrestrial laboratory experiments. Continued study of dark matter on a variety of fronts is of critical importance to astrophysics. Beyond this expanded list of scientific priorities, there are many additional unresolved scientific questions raised within the four chapters contributed by TGSAA's panels. Questions that fall outside the scope of the eight priorities listed above are not necessarily of minor interest. Some are best addressed by ground-based programs or by level-of-effort activities such as the peer-selected Explorer missions. For others, the time scale for the required technology development is longer than practical for a mission scheduled for launch within the next decade. The most suitable approach for NASA to adopt to prevent these important scientific questions from being overlooked is to continue a vigorous program of research and analysis as well as theoretical studies throughout the coming decade. In addition, adequately funded technology-development programs will be necessary to answer many of the questions posed in Chapters 2 through 5. Examples include investment in the precision photometric techniques required for the search for extrasolar planets (Chapter 2) and in the development of large-format infrared detectors for the study of the high-redshift universe (Chapter 44. Since the members of TGSAA were selected mainly for their scientific expertise, many issues relating to the technological readiness of some of the priorities in this report were not considered in depth. Additional study by more appropriately constituted groups is thus needed before implementation of some of the present recommendations.

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