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Astronomy and Astrophysics in 1995 Expected Status OVERVIEW . Astronomy Han advanced dramatically over the past two to three decades, stimulated by the development of major ground- based facilities and the introduction of space techniques. A promi- nent feature of the astronomy program has been its reliance on detailed, Tong-term planning, exemplified most recently by the As- tronomy Survey Committee report, Astronomy and Astrophysics for the 1980s. Looking forward to 1995, the task group antici- pates progress in realizing the plans detailed in that report. Such progress will involve the principal elements of the Shuttie-based program, as well as HST, GRO, AXAF, and STRTF, which form the first family of comprehensive observatories in space and have been called "the Great Observatories." These instruments will provide a powerful capability for detailed astrophysical studies at optical, gamma-ray, x-ray, and infrared wavelengths. Thus, the next decade will see space astronomy move strongly from an era of exploration to a program of in-depth study. The Great Observatories are the foundation of this program. Their effective use will require the development of new state-of- the-art instruments for these telescopes during their useful lifetime and the replacement of the entire observatory when its useful life 19
20 ends, due either to out-of-date components or scientific obsoles- cence. In the case of out-of-date components, it might be adequate simply to replace the observatory with one of equal capabilities. Replacement could also be seen as an opportunity for improve- ments that would make the observatory more capable to investi- gate the astrophysical questions of greatest interest at the time. Scientific obsolescence can occur either when the unique capabili- ties for which an observatory was built have been fully exploited or when general improvement of other observational capabilities diminishes the facility's productivity. The core program that the task group envisages is one in which we would attempt to maintain the four observatories as forefront research instruments. Different considerations will apply to the different wavelength regions. Along with this evolution of technical means and scientific aims, the task group foresees two other trends a further shift in emphasis from ground-based to space observations and a continued strengthening of the international base of space astronomy, with the realization of major European and Japanese missions, such as the Roentgen Satellite (ROSAT), the Infrared Space Observatory (ISO), and Japan's Explorer-cIass x-ray satellite (ASTRO-C). In the event that the international base of space astronomy continues to strengthen, as present indications in Europe and Japan suggest, the task group also looks forward to further im- provement in the cooperation and planning of the major programs of this era on a worldwide basis. Such a development would be fully in keeping with the international nature of astronomy. During the coming decade, it will be vital to: ~ maintain adequate support for the ongoing, long-lived ob- servatories; ~ provide, not only for major new initiatives, but also for medium- and small-scale missions, for supporting analysis, and for research and development; and . introduce substantially less expensive ways of conducting space activities so that new facilities such as those outlined in Chapter 4 can become a reality. RADIO ASTRONOMY The results of extending observational capabilities into new domains are well illustrated by examining the recent history of
21 radio astronomy. Naively, one might have expected that the rela- tively long wavelengths characteristic of the radio spectrum would have precluded the attainment of high angular resolution. This turned out not to be the case, because of the exquisite control of the time domain achieved by modern electronic techniques. Start- ing in the late 1960s, very long baseline interferometry (V~Bl) extended the effective size of observing apertures to worldwide dimensions. Angular resolution of a milliarcsecond became com- monplace, superior to the angular resolution achieved in any other part of the electromagnetic spectrum, and a number of new discov- eries were made. The relativistic jet phenomenon was clarified and shown to have alignments over scale factors differing by more than a million. Two puzzling phenomena velocities that apparently exceed the speed of light, and interstellar masers still remain un- explained; both could potentially influence our basic assumptions about physical phenomena in the universe. The first V[B! observations used simple two- and three- element configurations, but it rapidly became evident that more complete arrays were needed to give reliable pictures on the rnilli- arcsecond scale. This led, eventually, to the establishment of the V~BI array project of the National Radio Astronomy Observa- tory (NRAO), which is expected to be in use by 1995. This gives the angular resolution of an aperture nearly equal to the Earth's diameter, provided a source is high in the northern sky. The prob- lems at hand reading down toward the site of acceleration of the relativistic jets, for example require still longer baselines, but the Earth is too small. This led to the concept of space V[BI, and it is expected that the launching of QUASAT in mid-1995 will represent an important early step in exploring this new ob- servational realm. The orbiting antenna of QUASAT will have an apogee of the order of 20,000 km, which will be further extended by earthbound telescopes covering another 10,000 km baseline. The wavelengths to be used are 1, 2, and 6 cm. Images will comprise up to 2000 x 2000 picture elements (pixels), with pixel dimensions as small as 90 ,uarcsec, and perhaps 40 parcsec if coordinated inter- national programs are effected. This will probably provide a first step toward understanding the problem of the "synchrotron-self- Compton effect" that theoretically limits the brightness of sources to 10~2K. It will lead to greater understanding of superTuminal
22 motions in quasars and of the structure of interstellar and circum- stelIar masers. When used in conjunction with the VLBA, the system will have miTTijansky sensitivity, allowing the study of a vast array of interesting objects. INFRARED AND SUBMILI`IMETER ASTRONOMY In 1983 the Infrared Astronomical Satellite (IRAS), the first cryogenically cooled telescope to orbit in space, catalogued approx- imately 300,000 new infrared sources. Among its major discoveries were dust systems orbiting Vega and many other nearby stars; a "cirrus" cloud component on a galactic scale extending well above and below the galactic plane; two new zodiacal dust bands strad- dling the ecliptic plane; hosts of infrared sources, which, because of their very Tow luminosities, appear to be solar-type stars in their earliest stages of formation; high infrared fluxes from interacting galaxies; and large numbers of galaxies that are orders of magni- tude brighter at infrared wavelengths than in the optical domain and are rivaled in their energy production only by quasars. There are galaxies that at first sight might appear much like our own Milky Way-a spiral galaxy among billions of others. At infrared wavelengths, however, these galaxies have exhibited an enormous population of recently formed stars still shrouded by clouds of the gas and dust from which they formed. Massive star formation appears to have taken place there in a sudden outburst of activity. We will need to understand the origins and underlying mechanisms at work in these starburst galaxies. {RAS discovered disks of finely structured material orbiting about several nearby stars. These disks may provide clues to the way in which planetary systems are formed around a star. SIRTF will be able to study a number of these disks to determine the spatial distribution and chemical composition of this circumsteliar matter. In order to understand the various physical processes that re- sult in infrared emission under these diverse conditions, we need considerably higher spatial resolution coupled with high sensitiv- ity. We also need to obtain spectral information about the chemical structure and physical conditions in many of these sources. The task group expects that by 1995 a number of major space- and ground-based infrared facilities will be available for these
23 studies, as well as for future observations that we cannot yet anticipate. These facilities are described briefly below: ~ Cosmic Background Explorer (COBE) will have searched for deviations from the isotropic mucrowave background, examin- ing the conditions when galaxies and stars first formed. It will also search across the entire infrared domain, mapping the large-scale, diQuse features of the galaxy. The Space Infrared Telescope Facility (SIRTF) and In- frared Space Observatory (ISO) will draw upon advanced detector and detector-array technology to provide imaging at sensitivities orders of magnitude higher than TRAS, together with greatly en- hanced spectroscopic capability. ~ A near-infrared imaging and spectrometry experiment will extend the Hubble Space Telescope's (HST's) advantages of in- creased angular resolution and reduced background from space into the short-wavelength infrared region of ~ to 3 ~m. At the 3-,um wavelength there is a natural window that opens on the uni- verse. In that region of the spectrum both the optically scattered zodiacal light and the infrared emission from zodiacal dust are particularly low, and our search deep into the universe will extend out to exceptionally great distances. ~ A new generation of large, ground-based, optimized in- frared telescopes, such as the loom Keck telescope, will provide large collecting area for spectroscopic observations and high angu- lar resolution (~0.2 arcsec) imaging with high sensitivity to point sources. A number of 1~ to l~m-cIass subm~limeter telescopes will be in operation at high mountain sites, providing limited access to the submillimeter regime through selected atmospheric windows. Observations in the far-infrared and submillimeter spec- trum carried out from aircraft and balloons will continue to fill in the spectral coverage at lower angular resolution and sensitivity. . High-resolution spectroscopic studies will be possible throughout the subrn~llimeter range with the 8-m-clam Far-Infrared Space Telescope (FIRST). The major shortcomings in infrared observational capabilities in the mid-1990s will be the limited angular resolution from space observatories other than HST, the lack of a major submillimeter space observatory, and the absence of milliarcsecond interferomet- ric capability.
24 ULTRAVIOLET AND OPTICAL WAVELENGTHS By 1995 one or two ground-based telescopes of the 15-m class are expected to have been in operation for several years. In ad- dition, a few 8-m-cIass ground-based telescopes should have been developed by the United States and Japan, and located at good observing sites in the northern and southern hemispheres. Despite major efforts to improve astronomical seeing, angular resolution from the ground will be limited to about 1 arcsec, with occasional glimpses of 0.3- to 0.5-arcsec image quality. The Hubble Space Telescope (MST) was planned to overcome these limitations by providing better than 0.1-arcsec resolution in the ultraviolet and optical wavelength regions. By 1995 the HST will have been in operation for nearly a decade. If funded adequately, it will by then be equipped with second-generation instruments approaching detector quantum ef- ficiencies of 80 percent over the spectral range 1200 ~ to 2 ,um. By 1995 the Extreme Ultraviolet Explorer (EUVE) and ROSAT will have surveyed the sky between 80 and 900 ~ and the Far Ul- traviolet Spectroscopic Explorer (FUSE/LYMAN) telescopes will perform detailed studies of objects between 900 and 1200 A. By the mid-199Os important astrophysical problems to be addressed with ground-based and space facilities will be the deter- mination of: An accurate cosmological distance scale. Evolutionary and chemical states of normal and active galaxies to modest red shifts. . Dynamics of disk and bulge regions in nearby galaxies and the distribution of associated dark matter. . Physical and chemical properties of the interstellar and intergalactic media including deuterium in the local neighborhood. . Physical and chemical states of degenerate stars and hot plasmas in stellar chromospheres and coronas. Activity and mass loss in every kind of star and stellar . system. . Initial search for protoplanetary disks and planets. These studies will be aided by: . Deep surveys with detection capabilities of point sources to large red shifts in small angular areas.
25 . Follow-up to sources discovered in new surveys (COBE, EUVE, and ROSAT). Although point sources will be detectable to very large red shift limits with the HST, studies of galaxies will be limited by the rapid dimming of surface brightness for these highly red-shifted objects. Thus, for studies of galaxies, the HST will not be capable of exploiting the ultraviolet/optical region over substantial cosmic Took-back times. Moreover, by 1995 there will be many funda- mental astrophysical problems that will be limited by the angular resolution capabilities even of the HST. Undoubtedly the next step will require higher angular resolu- tion combined with large collecting areas so that our studies can answer the many questions that will have arisen about the spatial structure of the objects that HST will likely discover and examine. X-RAY ASTRONOMY X-ray astronomy will achieve a substantial increase in obser- vational capability with the anticipated launch of the Advanced X-ray Astrophysics Facility (AXAF) in about 1994. AXAF will be the first major observatory for x-ray astronomy to be operated on a Tong-term basis. It will permit detailed, high-angular-resolution (about 0.5 arcsec) studies at x-ray energies from about 0.1 to 10 keV. It will observe the broad range of astronomical objects shown by the Einstein Observatory to be x-ray emitters a range that runs the gamut from coronas around nearby coo! stars to distant quasars. AXAF will not only permit detection of distant Quasi- Steller Objects (QSOs) and galaxy clusters but also will enab] moderate-resolution spectral analyses of those sources. These ob- servations will begin to address problems such as the formation and evolution of galaxies and galaxy clusters. In studies of brighter objects, AXAF will allow high-resolution x-ray spectra to be obtained for the first time on objects as diverse as supernova remnants, accreting neutron stars, black holes, and normal stars. Once again, the exploratory spectroscopic studies carried out with the Einstein Observatory herald the rich poten- tial for discovery and detailed astrophysical measurements that AXAF will conduct. In detailed spectroscopy of bright halos of galaxies such as M87, AXAF would determine the composition, temperature, and density profiles of the halo; that would greatly
26 constrain the origin and mass profile of the galaxy halo. It would then be possible to determine whether the halo and its apparently large, dark mass component was supplied by accretion from the surrounding cluster or was intrinsic to the galaxy. Similarly, stud- ies of dark matter in other nearby galaxies and clusters of galaxies will be initiated by AXAF. More-sensitive instruments, however, will be required to extend these studies to more distant objects of cosmological interest. The overall sensitivity of AXAF is well matched to that of the major optical (HST), infrared (SIRTF), and radio (VLA and V~BA) observatory capabilities anticipated for the early l990s. AXAF will draw heavily on observations made with these facilities as well as on the moderate or Explorer-cIass missions In x-ray astronomy launched in the period from 1987 to 1994. These include the following: . ROSAT (1987~: A West German imaging soft x-ray (~2 keV) telescope, and a British XUV telescope make up this mission. ROSAT will carry out the first all-sky survey in these wavebands at high sensitivity, as well as a program of pointed observations in which U.S. observers will be directly involved. This mission should observe 100,000 x-ray sources. · X-Ray Explorers (1987 to 1992~: These are planned for the study of relatively bright compact x-ray sources. In 1987, Japan will launch a large-area (about 0.5 m2), nonimaging x-ray detector for broadband spectrophotometry and the study of rapid variability in x-ray sources. It will probe the physics of accreting neutron stars, black holes, and white dwarfs in our Galaxy and L ~ml T T ~ ~ ~ ~ _,_,,%) ~ oeyona. l ne u A. Array l lmlng Explorer (XTE), to be launched in 1992, will extend the high time-resolution studies of such compact galactic x-ray sources and permit similar studies to still higher photon energies. The internal structure of neutron stars will be probed with sustained observations of x-ray pulsars and bursters, and physical conditior~s in accretion disks will be explored. . Space Shuttle Experiments (1988 to 1990~: These will be carried out primarily on the OSS-2 mission, to develop new con- cepts tor large-area x-ray telescopes with greatly increased sensi- tivity. These short-duration flights with limited capability will be vital for the development of future x-ray missions. · XMM (1995~: This is a first-generation, high-throughput x-ray telescope now under study by the European Space Agency (ESA) as a "cornerstone" mission. It will have a large collecting , . ~
27 area (about 1 m2) for imaging and spectroscopy of soft x-ray sources at moderate spatial and spectral resolution. XMM is thus strongly complementary to AXAF and will also serve as an ideal springboard for the planning of x-ray facilities of the future (see Chapter 4~. The powerful capabilities of AXAF and the wealth of fun- damental problems it can address suggest that this facility will advance research for a long time to come. When the need arises for major refurbishment or when the technology of producing sig- nificantly larger diameter and still higher-resolution x-ray mirrors has sufficiently advanced, the entire AXAF facility could be up- graded or replaced. GAMMA-RAY ASTRONOMY Small Astronomy Satellite-2 (SAS-2) and COS-B, following earlier pioneering measurements, have provided maps of the high- energy gamma-ray emission from the galactic plane and an inter- pretation of the origin and propagation of the cosmic rays. In addition, they have made observations of pulsars as well as other galactic objects, measured the basic properties of extragalactic dif- fuse gamma radiation, and discovered gamma-ray emission from a quasar. Active galactic nuclei, the galactic center region, and various sources, including transients, have been observed in the low-energy gamma-ray region by telescopes on the High Energy Astronomical Observatory-3 (HEAO-3), the Solar Maximum Mis- sion (SMM), and other instruments. Reported observations of gamma rays at energies above 10~i eV using ground-based tech- niques bear on the physics of pulsars and active galactic nuclei. Thus, over the entire spectral range from 105 to above 10~4 eV, there are now interesting, known sources of gamma radiation. The next major advance in gamma-ray astronomy will come with the launch of the Gamma Ray Observatory (GRO), scheduled for 1988. Covering the energy range from about 105 to 3 x 10~° eV, it will have a sensitivity at least an order of magnitude greater than previous experiments over this whole energy range and a viewing program designed to scan the entire sky. Currently, the lifetime of the GRO will be from 2 to 4 years; the lifetime would be extended if refueling capabilities are provided. With GRO, gamma-ray astronomers will obtain the first comprehensive full- sky survey in moderate detail. In particular, GRO should further
28 open the Tow-energy gamma-ray astronomy window by recording nuclear emission lines and investigating the properties of gamma- ray bursts in more detail. At the same time, it will extend studies in high-energy gamma-ray astronomy to a far greater depth. Results from GRO will undoubtedly help to focus require- ments and objectives for future experiments, but the general thrust of garnrna-ray astrophysics in the post-GRO period is already rea- sonably clear. The Astronomy Survey Committee in its report Astronomy and Astrophysics for the 1980s already anticipated the need for at least two gamma-ray instruments beyond those on GRO. An advanced high-energy gamma-ray telescope of fiery large area, high sensitivity, and high angular resolution will be needed for Tong-term observations of selected sources and regions of special interest. This will be necessary to achieve the statistical accuracy in the counting of gamma-ray photons required to re- solve spatial and spectral features of sources and to analyze their variations. The field of view of the telescope need not be wide, and an appropriate goal for angular resolution is of the order of 1 to 2 arcmin. A high-resolution nuclear gamma-ray spectrometer would also be needed for the study of the gamma-ray lines from radioac- tivity in supernova remnants, positron annihilation in the galactic disk and in extragalactic sources, nuclear excitations caused by cosmic rays in dense matter, and nucleosyntheses in extragalactic supernovae. The instruments for this era are discussed in Chapter 4. It is envisioned that these instruments would be well into their development stage by 1995 and ready for a flight opportunity soon thereafter. . . ~ COSMIC-RAY ASTROPHYSICS Cosmic-ray particles provide us with the only direct sam- pling of matter of extra-solar-system origin. The great range of particle energy, covering about 15 decades, and the variety of species, encompassing the nuclei of all elements, as well as elec- trons, positrons, and antiprotons, require a number of different observational approaches. These approaches must be pursued on the ground, above the atmosphere, and in deep space. Most of the instruments flown previously on spacecraft are of relatively small size and are therefore restricted to studies at low energies (~1 GeV), where solar modulation strongly affects
29 the particles' energy spectra. Several of these spacecraft are ex- pected to remain active into the 1990s. These investigations are providing: (1) definitive measurements with very high precision of the elemental composition of galactic cosmic rays (up to iron) at Tow energies; (2) exploratory data on the isotopic abundances; (3) direct sampling of particles in interplanetary space that are accelerated in solar flares or in planetary magnetospheres; and (4) perhaps, if our current interpretation is correct, a sampling of local interstellar neutral gas through the observation of "anoma- lous" cosmic rays (i.e., helium, nitrogen, oxygen, and neon, and possibly other elements with high ionization potential that may be accelerated at the boundary of the heliosphere). The continuation of these studies through the 1990s will be essential in order to understand the spatial and time variations in the cosmic-ray flux through changing levels of solar activity. Instruments of modestly increased size will be used during this period in order to enhance the accuracy of the observations and to explore the solar system outside the ecliptic plane. Most important will be studies of the nucleosynthetic origin of the Tow-energy cosmic rays through a more precise determination of their isotopic abundances. A continuing series of observations of galactic cosmic rays at higher energies is being conducted on balloons and on the Space Shuttle. These provide important exploratory data, but definitive measurements have to await the availability of spacecraft capa- ble of carrying very large payloads for extended periods of time (years). Objectives of such measurements are to determine: the isotopic abundances at high energies; the composition of the rare ultraheavy nuclei; the energy spectra of nuclei and electrons over a wide range; and the abundances of positrons and antiprotons. The Astronomy Survey Committee had recommended carrying out a series of such observations (A stronomy and Astrophysics for the t980s), but only a few investigations are being implemented. Thus, by 1995 we will probably still be short of decisive data that could specify the character of particle acceleration in the galaxy, or that could define the nucleosynthesis processes leading to cosmic- ray matter and governing evolution of the galaxy. We will probably also lack definitive data that could further our understanding of the structure and properties of the interstellar medium, and that could address some fundamental cosmological questions such as matter/antimatter symmetry in the present universe.
30 The investigations listed in the following subsections are under way at present or are planned by 1995. Interplanetary Spacecraft Particle detectors on Pioneer 10 and 11 and Voyager 1 and 2 will continue to explore the solar system at ever-increasing dis- tances from the Sun. Space scientists will use instruments on In- terplanetary Monitoring Platform-8 (IMP-8) and perhaps on the Wind spacecraft of the International Solar-Terrestrial Program (ISTP/Wind) as references closer to Earth. The Ulysses Mission will provide in situ measurements outside the ecliptic plane for the first time. An important time period for cosmic-ray observations at low energies will be the next minimum in solar activity, around 1988-1989. Cosmic-Ray Composition Explorer The exposure of a sensitive particle detector onboard an Explorer-cIass mission outside the Earth's magnetosphere has been highly endorsed by the scientific community, and should be im- plemented before 1995. In this instrument a large-area solid-state detector telescope, combined with trajectory measuring devices, will obtain an order-of-magnitude improvement in sensitivity and resolution over previous instrumentation. We thus expect qualita- tively new insight into the elemental and isotopic composition of a variety of particle populations at low energies (~1 GeV): galactic cosmic rays, the "anomalous" component, and nuclei accelerated in solar flares. The first two are thought to be rather contempora- neous samples of the interstellar medium, while the third is a more ancient sample that has been stored in the Sun for almost 5 billion years. The comparison between the compositions of these particle populations will therefore either reveal characteristics of the chem- ical evolution of the interstellar matter for instance, a continuing metal enrichment or indicate if the solar system composition is atypical, perhaps owing to the admixture of fresh supernova ejecta during its formation. Space Shuttle and Space Station A very large detector to measure elemental abundances and energy spectra of cosmic-ray nuclei at very high energies has been
31 developed for sortie flights on the Space Shuttle. A first flight was performed in 1985, and several reflights are expected that will yield data on the cosmic-ray composition up to energies around 10~3 to 10~4 eV. These measurements will constrain models of galactic particle acceleration by determining, for instance, the en- ergy dependence of the relative abundances of primary cosmic rays (e.g., the iron/carbon ratio). They will also provide decisive infor- mation on the interaction of particles and interstellar matter and fields by measuring the relative abundances of spalIation-produced particles (e.g., light nuclei such as lithium, beryllium, and boron) over an extended energy range. A Tong- duration flight (approxi- mately 1 year) of this same instrument could cover another decade in energy, close to the "knee" of the cosmic-ray spectrum around 10~5 eV. This may be accomplished onboard a Space Station or Space Platform in its initial operational configuration. This ex- periment would reach into an energy regime where the cosmic-ray composition is entirely unexplored at present. Instrument Development A cosmic-ray facility centered around a large superconducting magnetic spectrometer in space has been proposed by the scien- tific community and is currently under study as a high-priority new initiative. The spectrometer will be the common, central component for a succession of specific investigations, directed, for instance, toward measurements of protons and electrons and their antiparticles, antiprotons, and positrons. Other investiga- tions might include searches for heavy antimatter, and studies of isotopic composition at high energies. During the coming years, detector development for such a facility is expected to be in ac- tive progress. The first elements of the facility are to be deployed during the initial stage of the Space Station in the early 1990s. GRAVITATIONAL PHYSICS Gravity is one of the fundamental forces of nature. Its ac- tions in the neighborhood of relatively small masses like the Sun and Earth are well approximated by Newton's laws. These laws, however, have been superseded by Einstein's general relativistic description of gravity. General relativity predicts small deviations from Newtonian behavior in weak gravitational fields. Some of
32 these deviations are being measured accurately in the solar system. General relativity also predicts both new classes of phenomena in strong gravitational fields, and the generation of gravitational waves by accelerating massive objects such as compact stars in close binary orbits (e.g., the millisecond pulsar) and supernova explosive events in which enormous masses are hurled into space . . . · ~ in gigantic exp oslons.+ One test of the first of these two classes of effects will be carried out in NASA's gravity probe B (GPB). This experiment in earth orbit will involve a rapidly rotating, superconducting, nioblum- coated sphere whose pole direction will be affected a minuscule amount a precession of 0.04 arcsec/yr by the rotation of the Earth. The detection of this change should provide an important test for the magnetic-like effects of mass predicted by general relativity. The already successful solar system tests of general relativ- ~ty could be extended to second (post-Newtonian) order in the solar potential. Instruments to perform such tests could be de- veloped by the mid-199Os. NASA has also considered a mission (STARPROBE) to place a precision clock in a near-Sun orbit to measure the second-order gravitational red shift. In addition, a proposal exists to build a small astrometric optical interferome- ter (POINTS) to measure the deflection of starlight by the Sun. Each of these experiments also has many other scientific objec- tives. Should either experiment be conducted and fait to confirm the predictions of general relativity, we would be forced to re- think our understanding of a fundamental part of physics and its implications for astrophysics. Tests for the existence of at least a restricted frequency range of gravitational waves will be provided by spacecraft equipped with dual-frequency transponders in distant trajectories throughout the solar system. Passage of a gravitational wave between these space- craft and Earth should provide a signature in the arrival time of transponder response uniquely identifying the passage of such a wave. Current tests are orders of magnitude too insensitive to * A separate task group on fundamental physics and chemistry has addressed the topic of gravitational physics in the context of this study. Its findings are published in the volume Space Science in the Twenty-Fir~t Century: Fundamental Physics and Chemistry. However, this subject is of great interest to astronomers and astrophysicists and is therefore discussed briefly here.
33 detect such waves, but future observations of this kind may be suf- ficiently improved to provide a useful complement of ground-based instruments developed to detect gravitational waves of far higher frequencies. To date, the only indication that we have for the existence of gravitational waves comes from the secular decrease in the orbital period of the binary radio pulsar. NASA OPERATIONAL STATUS The post-1995 program for astrophysics is strongly dependent on the state of NASA technologies. The task group anticipates routine Shuttle operation and the ability to transport large masses to optimum scientific orbits. With regard to the state of astrophysics projects, the task group assumes the following: . HST will be operational with at least one, and probably two, instrument upgrades completed. . GRO will be nearing the end of its operating lifetime with about 50 percent of the scientific instrumentation operating. . AXAF will be in orbit and nearing the time for its first · ~ servicing. . SIRTF will be operating with the first cryogen servicing planned for the near term. LDR will be undergoing intensive design study. . Among the moderate-scale projects, FUSE, QUASAT, and HTM will also be operating. INTERNATIONAL PROGRAMS There are a number of programs originating outside the United States in Europe, Japan, and the Soviet Union. These should be listed here as well. Some have already been approved; others are planned or only under discussion. Further, we must understand that some of the missions both within and outside the United States will be launched before 1995, while some might be delayed. Others might be logical continuations of missions started before 1995. A clear-cut demarcation therefore cannot be drawn at 1995. A further complication is the difference between international Eu- ropean missions as distinct from autonomous national European missions. Lumping all these varied efforts together, we then have the following array of instruments:
34 . Radio: There is a Japanese version of QUASAT (space VLBI) under study by a working group. The plan is to launch a 5- to 10~m diameter antenna at 20 GHz into a 500- to 40,000-km orbit around the year 1995. There also is an approved Soviet plan, RADIOASTRON, to launch several radio telescopes into orbit for VERB! purposes, possibly coordinated with QUASAT. . Infrared and Submillimeter: ESA is constructing an In- frared Space Observatory (ISO) to be launched around 1992. It will have the capability for producing maps and obtaining low- resolution (R = 1000) spectra over a wavelength range of 2 to 200 ,um. There will also be polarization sensing capabilities aboard. A further effort is the Far Infrared Space Telescope (FIRST), a het- erodyne spectroscopy mission planned for the mid-199Os. It will have high-precision capability for determining chemical composi- tion and velocity structure within coo! clouds, at submillimeter wavelengths. · Optical Astronomy: The ESA astrometric mission HIP- PARCOS will improve optical astrometric accuracy by an order of magnitude and survey about 100,000 stars. It will provide parallaxes for a variety of astrophysical studies including a re- fined color-luminosity relationship and an improved distance scale based on direct measurements of a few Cepheid and RR Lyrae variables. HIPPARCOS will also provide proper motions with an uncertainty of about 1.5 milliarcsec/yr. These will find use in the study of the dynamics of the Hyades and in the determina- tion of the birthplaces of young stars. In Japan, a survey-type ultraviolet telescope, UVSAT, is on the menu of the series of small-to-moderate satellite missions. The hope is that UVSAT will complement HST with a sub-Lyman-alpha, moderate spatial- and spectral-resolution capability. Currently, it is intended for launch in 1995; the exact schedule will depend on that of Japan's next generation of ground-based optical telescopes. . High-Energy Astrophysics: Japan's Explorer-ciass x-ray satellite, ASTRO-C, is being prepared for launch in 1987 in a col- laborative venture with the United Kingdom. Whenever possible, Japanese satellites will carry a small gamma-ray burst monitor as well, as a matter of policy. The gamma burst monitor for ASTRO-C will be provided by Los Alamos National Laboratory. ASTRO-C will be nonimaging with an area of 0.5 m2. ASTRO-C
35 may be followed by ASTRO-D. The plan is to carry an imaging de- vice with an area in excess of 500 cm2 and with moderate angular resolution (about 1 arcmin). The West German ROSAT mission of 1987 will be followed by a hard x-ray mission (SAX) being prepared in Italy and by a gamma-ray imaging facility, SIGMA, being prepared in France for flight on a Soviet mission. In addition, ESA is planning a high-throughput x-ray spec- trometer, XMM, within their future space science plan (Horizon 2000 Program).
