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7 Programs for Study and Development In addition to the programs recommended in the preceding chapter for approval and funding during the 1980's, the Astronomy Survey Committee considered in detail many other proposed programs of comparable scientific merit. Some of the most promising of these differ from the programs already recommended primarily through being at an earlier stage of technological development or through falling at a later stage in a logically planned observational program. However, planning and development are often time-consuming, es- pecially for large projects. It is therefore important to begin, during the coming decade, study and development of programs that appear to have exceptional promise for the 1990's and beyond. The support of such programs should include funding for design studies and, where appropriate and timely, for the development and testing of instrumentation. Projects and study areas recommended by this Committee in this category include the following, in which the order of listing carries no implication of priority: A. Future x-ray observatories in space; B. Instruments for the detection of gravitational waves from astro- nomical objects; C. Long-duration spaceflights of infrared telescopes cooled to cry- ogenic temperatures; D. A very large telescope in space for ultraviolet, optical, and near- infrared observations; 157

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158 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's E. A program of advanced spatial interferometry in the radio, infrared, and optical spectral regions; F. Advanced gamma-ray experiments; and G. Astronomical observatories on the Moon. The present chapter discusses these areas of study and development and illustrates their role in addressing some of the long-range sci- entific problems that may now be foreseen. A. FUTURE X-RAY OBSERVATORIES IN SPACE The Astronomy Survey Committee recommends the study and development of space observatories beyond the Advanced X-Ray Astrophysics Facility (AXAF) for pursuit of a number of impor- tant future goals in x-ray astronomy. These investigations will either demand observational capabilities exceeding those of AXAF or will be more appropriate to observatories incorporating spe- cialized, dedicated instrumentation. The last decade has been a period of extraordinary advances in x-ray astronomy, culminating in the impressive observations re- turned by the Einstein Observatory. These studies will be accel- erated and extended to much fainter objects by AXAF, recom- mended by this Committee as the major new program of highest priority in all of astronomy for the 1980's. However, a number of important future investigations in x-ray astronomy will de- mand greater sensitivity, spectral resolution, energy coverage, or time resolution than even this powerful facility can provide. Moreover, the rich variety of objects and problems uncovered by the past decade of x-ray observations points to the need for fu- ture studies that are too specialized and diverse to be accom- plished by a single observatory. Such studies should include at least the following scientific goals: 1. An all-sky survey at x-ray wavelengths comparable in scope with the Palomar Sky Survey, to facilitate the identification and study of objects in various wavelength regions and to provide the data base for detailed studies of the statistics, distribution, and evolution of x-ray sources. 2. Comprehensive measurements of variability, over a wide range of time scales, of the x-ray emissions from faint Galactic and extragalactic sources. This work will be of crucial impor- tance for a detailed understanding of virtually every kind of

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Program for Study and Development 159 compact x-ray source, ranging from nearby flare stars to distant quasars. 3. Low- to moderate-resolution spectroscopy of faint objects, to obtain such data as x-ray red shifts of very distant clusters of galaxies and plasma diagnostics for large numbers of x-ray sources. 4. Detailed study of the large-scale x-ray structure of the Uni- verse through measurements of diffuse features as well as study of structure on smaller scales through observations of clusters of galaxies. 5. High-resolution spectroscopy and polarimetry of faint ob- jects to determine elemental and ionic abundances, temperature distributions, morphologies, and dynamical behaviors. The tar- gets of such studies will include stellar coronas, supernova rem- nants, active galactic nuclei, and galaxy clusters. 6. Comprehensive investigation of x-ray sources in the energy range above 10 keV, which will be inaccessible to AXAF. Pre- vious balloon and satellite studies have demonstrated the impor- tance of high-energy x-ray measurements in the analysis of the physical conditions in x-ray stars and active galactic nuclei and in the evaluation of the contributions of discrete and diffuse sources to the high-energy x-ray background. 7. Detection, location, and detailed study of transient x-ray sources, including sources of hard x rays associated with gamma- ray bursts and supernova explosions in external galaxies. 8. Measurements of the spectra and variability of soft x-ray and extreme-ultraviolet (EUV) sources, including stellar coronas, single hot white dwarfs, and accreting white dwarfs in binary systems (cataclysmic variables), together with detailed mapping of soft x-ray and EUV emission from the interstellar medium. The first four of these scientific goals (an all-sky survey, measurement of variability, low- to moderate-resolution spectros- copy, and study of low-surface-brightness features, all directed at faint sources) require collecting areas much larger than that of AXAF but angular resolutions sufficient only to avoid source con- fusion and to permit unambiguous identification of optical and radio counterparts. One may therefore pursue these goals through deployment of a moderate-resolution array of reflectors of large area, at a cost modest by comparison with that of a monolithic- mirror telescope. Individual modules of such arrays can be de- signed, tested, and deployed as part of the Spacelab program.

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160 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's The Committee encourages the continued development of large- area modular arrays of reflectors through Spacelab flights and the eventual assembly of a large-area facility for long-duration obser- vations, perhaps upon a space platform. High-resolution x-ray spectrometry and polarimetry of faint objects will require specialized instruments with very large effec- tive areas. Preliminary design studies suggest that both kinds of measurements can be pursued in the same long-duration mis- sion, and the Committee urges continued development of the concepts for such a mission. The study of objects that are most luminous at x-ray energies above 10 keV was begun through balloon and HEAo-1 observa- tions, but a greatly expanded and more systematic study of such sources will be needed during the 1990's. Instruments providing imaging spectrophotometry up to 100-keV photon energies, with concurrent precise location and high-resolution x-ray spectros- copy of such sources, are now becoming technologically feasible; these and other instruments should be studied for possible in- corporation into such a mission, which may prove to be an at- tractive candidate for location on a space platform. A comprehensive study of the brighter members of the var- ious classes of transient and variable sources will be initiated by the X-Ray Timing Explorer (XTE) satellite. It is already evident, however, that a long-term observatory with enhanced capabili- ties will be needed during the 1990's to extend the observation of variability to a substantial fraction of the objects accessible to AXAF. Finally, the Committee believes that planning should begin for soft x-ray and EW observations beyond those anticipated from the EUV Explorer. Such observations, with improved sensitivity, an- zular resolution, and spectral resolution, will be needed for ana v . , ~ . - ~ . ~ - ~ . 1 1 . 1 ~ ~ 1~ _ 1 1 _ ~ ~ _ lytical studies of the objects detected by EUVE, as weft as to complement the optical and near-ultraviolet observations made by Space Telescope and the far-ultraviolet observations to be made by the far-ultraviolet spectrograph in space recommended as a new program in the present report. Coordinated observations of a wide variety of nearby stars at x-ray, EUV, UV, and optical wavelengths will furnish decisive tests of stellar-atmosphere models and theories of coronal activity. Measurement of the structure and physical conditions in the hot component of the interstellar medium by means of soft x-ray and EUV surveys and

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Program for Study and Development 161 spectrometry with high angular resolution will be an essential part of future studies of star formation and chemical evolution in the Galaxy. B. INSTRUMENTS FOR THE DETECTION OF GRAVITATIONAL WAVES The Astronomy Survey Committee recommends the study and development of instruments for the detection of gravitational waves from astronomical objects. Detection and measurement of gravitational waves would be of great importance to the achievement of a fundamental under- standing of the gravitational field. Success would provide infor- mation on important astronomical phenomena that is otherwise unobtainable. This includes, in particular, rapid dynamical be- havior in stars and other massive objects and advanced evolu- tion of stellar systems. The great potential value of gravitational- wave detection and the challenging technical problems it presents justify current effort on the study and development of detectors and on the theory of the generation of these waves. When this development reaches a state that allows astronomical observa- tions to become practical, instruments for such observations should be constructed. These will probably include long-baseline systems in space, which at present seem to promise the great- est sensitivity. Meanwhile, advantage should be taken of oppor- tunities for improvements in space-based efforts to detect gravi- tational waves by upgrading the quality of equipment for tracking the motions of solar-system space probes. C. LONG-DURATION SPACEFLIGHTS OF INFRARED TELESCOPES COOLED TO CRYOGENIC TEMPERATURES The Astronomy Survey Committee recommends the study and development of long-duration space observatories incorporating infrared telescopes cooled to cryogenic temperatures. In particu- lar, NASA should study the ways in which the Shuttle Infrared Telescope Facility (SIRTF) may be most effectively incorporated into such a long-duration observatory following a vigorous observa- tional program carried out through Spacelab flights. During the next decade, the most dramatic gains in astronom- ical sensitivity will probably occur in the infrared region of the

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162 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's spectrum as a result of observations carried out in space with improved detectors and cryogenically cooled optical systems. Programs currently under development, such as the Infrared As- tronomy Satellite (IRAS) and SIRTF, will exploit these advantages in part; by the end of the decade, infrared astronomy should ri- val radio, optical, and x-ray astronomy in the depth and rich- ness of its observable sky. It is therefore important to begin planning now for the long-duration infrared observatories em- ploying cryogenic optics that will be needed to exploit these op- portunities during the 1990's and beyond. As emphasized in Chapter 4, STRTF will be the cornerstone of space infrared astronomy during the coming decade. This facil- ity will be able to carry out a powerful observational program even within the relatively brief, 7-day length of an early Shuttle sortie mission. Frequent Shuttle flights of SIRTF, with appropriate refurbishment between flights, thus constitute one of the high- est priorities for U.S. astronomy during the 1980's. However, the scientific return from SIRTF will almost certainly provide an overwhelming case for long-duration spaceflights of cryogenically cooled infrared telescopes. A possible way to achieve this capability is to develop SIRTF itself as a facility for eventual placement onto a space platform, to be refurbished at intervals of 6 to 12 months. Alternatively, development of a free-flying ob- servatory for SIRTF may prove to be more effective. We urge NASA to pursue the general development and design of detectors and facilities for infrared astronomy with the aim of achieving long- duration spaceflights of cryogenic infrared telescopes by 1990. D. VERY LARGE TELESCOPE IN SPACE FOR ULTRAVIOLET, OPTICAL, AND NEAR-INFRARED OBSERVATIONS The Astronomy Survey Committee recommends the study and development of the technology required to place a very large telescope in space early in the next century. The advances in ultraviolet, optical, and infrared astronomy expected in the 1980's are extremely impressive and will cer- tainly lead to great advances in our understanding of diverse astrophysical phenomena. Yet, just as certainly, the instruments of the 1980's will discover new- phenomena that will require new and even more powerful facilities for their systematic investiga- tion. By the turn of the century it may be possible to place in orbit a very large telescope-perhaps 30 m in diameter with

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Program for Study and Development 163 diffraction-limited performance from the far-infrared to the near- ultraviolet regions. The capabilities of such an instrument would be awesome by today's standards. At visual wavelengths it would have a re- solving power of about 4 rnilliarcseconds, a limiting magrutude for medium-resolution spectroscopy of about 30, and a limiting magnitude for imaging of about 3Wmore than a hundred times fainter than the faintest object accessible to Space Telescope. A telescope of such power could, for example, observe the planets with resolutions ranging from 12 km at Jupiter to 90 km at Neptune, permitting long-term studies of the atmospheric dy- namics of the major planets; observe the star-forming regions in the Orion nebula with a resolution of 10 astronomical units at near-infrared wavelengths, permitting direct observation of the process of star formation and detailed studies of preplanetary nebulae; observe the main sequence in the Andromeda Galaxy tour magnitudes Below tne oldest turn-off point and obtain spectra one magnitude below this point; perform spectroscopy of solar-type stars in nearby galaxies, permitting direct chemical- abundance and abundance-history determinations for these gal- axies; with suitable care and stability, obtain relative positions of , . ~ ~ ~ ~ - . . ~ ~ ~ . ~ ~ ~ . stars and background quasars to a precision of about 100 mi- croarcseconds-enough to provide accurate parallaxes out to a distance of 1000 parsecs and to measure proper motions of stars in globular clusters and in nearby spiral galaxies, permitting studies of the structure and dynamics of these stellar systems; obtain high-resolution ultraviolet spectra of the brightest stars in galaxies of the Virgo cluster of galaxies, making possible de- tailed studies of the intergalactic medium over long path lengths; and observe the nearest quasars with a resolution of 10 parsecs and the nearest active galaxies with a resolution of 0.2 parsec, fine enough to reveal fundamentally new structural details. These examples are only a few from a long list of important problems that a very large space telescope could pursue. Such an instrument could be the most powerful too! of astronomy at the start of the next millenium. We therefore recommend that NASA begin exploratory studies of this project and encourage the de- velopment of the requisite technologies. Development of tech- nology for the Large Deployable Reflector in space (recom- mended in Chapter 6) should provide a strong impetus for the further advances needed for this even larger, more powerful in- strument.

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164 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's E. PROGRAM OF ADVANCED SPATIAL INTERFEROMETRY IN THE RADIO, INFRARED, AND OPTICAL SPECTRAL REGIONS The Astronomy Survey Committee recommends the study and development of advanced spatial interferometers for the radio, infrared, and optical spectral regions. Exploration of the structural details of astronomical objects from planets to distant quasars requires ever higher angular resolu- tion. Radio astronomers have pioneered in such studies with re- fined interferometric techniques. The Very Large Array produces radio images with resolutions of an arcsecond or better, and continent-spanning very-long-baseline interferometry currently re- solves detail down to a few milliarcseconds. Increases in base lines for radio telescopes and extension of interferometric tech- niques to infrared and optical wavelengths can achieve even higher resolution. Important next steps are the Very-Long-Baseline (VERB) Array, the placement of a radio antenna in low-Earth orbit to complement the ground-based VERB Array, and the development of a spatial interferometer for the mid-infrared region, as presented in the recommendations for new programs (Chapter 6~. These steps would be followed by the placement of radio antennas in highly elliptical orbits (increasing VERB} baselines by a factor of 10) and by the construction of a ground-based infrared interferometric array with baselines of a few kilometers. These instruments would achieve angular resolutions of a few hundred microarcseconds in the radio and a few milliarcseconds in the infrared region. In both cases, detailed image reconstruction would be possible since they would provide good coverage in the Fourier-transform plane, a necessary condition for the accurate mapping of complex sources. Further improvements of infrared interferometric capabilities and significant developments in optical interferometry will probably require interferometers in space to escape the deleterious effects of the Earth's atmosphere. A desirable goal is a program of space optical and infrared interferometry leading, by the early part of the next century, to an interferometer with baselines of a few tens of kilometers and resolutions of 1 to 10 microarcseconds at optical wavelengths. Development of interferometric techniques to this extent will require the capability to place large structures in space, significant strides in optical technology, and extensive ad- vances in systems for space-vehicle control, communication, and

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Program for Study and Development 165 information processing. With space-based infrared and optical interferometers it would be possible to measure trigonometric parallaxes of objects throughout the Galaxy, to detect Earth-like planets orbiting nearby stars, and to resolve significant structure in nearby quasars, to name only a few of the many important projects that would be advanced with such powerful instru- ments. Advanced interferometry on the ground, and then in space, will be among the most important and exciting areas of astronomy in the coming decades. We therefore strongly recommend that NSF and NASA begin preliminary planning, exploratory studies, and technological development for such a program. F. ADVANCED GAMMA-RAY EXPERIMENTS The Astronomy Survey Committee recommends the study and development of advanced gamma-ray experiments to follow the program to be carried out by the Gamma Ray Observatory (GRO). Subsequent to GRO, an advanced high-energy gamma-ray tele- scope of very large area, high sensitivity, and high angular res- olution will be needed for long-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 resolve spatial and spectral features of the sources and to analyze their variations. The field of view of the telescope need not be wide, and an appropriate goal for angu- lar resolution is the order of 1 to 2 arcmin. A high-resolution nuclear gamma-ray spectrometer should be included in the mis- sion for the study of the gamma-ray lines from radioactivity in supernova remnants, positron annihilation in the Galactic disk and in extragalactic sources, nuclear excitations caused by cosmic rays in dense matter, and nucleosynthesis in extragalactic superno- vae; energy resolution sufficient to study line profiles will be desirable. Development of such instruments, possibly for de- ployment upon a space platform, should begin as soon as there emerges a clear understanding of the observational requirements from analysis of results from GRO. G. ASTRONOMICAL OBSERVATORIES ON THE MOON The Astronomy Survey Committee recommends that agencies of the U.S. Government, working in concert with those of other

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166 ASTRONOMY AND ASTROPHYSICS FOR THE 1980's nations, take steps to ensure the preservation of sites on the Moon (particularly on the far side) for astronomical observa- tions. In addition, the Committee urges NASA to set aside re- sources for the establishment of lunar astronomical observatories as an important corollary to the establishment of large-scale in- dustrial or power-generation facilities in Earth orbit. The Moon offers certain decisive advantages as a base for as- tronomical observations. In particular, the far side of the Moon provides protection from radio interference from sources on or near the Earth and therefore has great potential value for radio astronomy. Shielded at all times from earthlight, sites on the far side of the Moon are also shielded from sunlight for substantial parts of each month and thus offer advantages for optical and infrared observations requiring the darkest possible sky. These considerations become compelling if large military, industrial, or power-generation facilities are constructed in Earth orbit, for the electromagnetic pollution arising from the operation of such fa- cilities may well make it difficult or impossible to observe faint astronomical objects from the ground or from Earth orbit. The utility of the Moon for astronomical observations must therefore be protected as a unique resource for future generations on Earth. The preservation of sites on the Moon for astronomical obser- vations is clearly a task of international scope. However, the Committee recommends that agencies of the U.S. Government take the lead in such an effort and begin planning in the near future for the establishment of lunar observatories early in the next century. In addition, the U.S. Government should consider carefully the potentially disruptive effects on Earth-based astron- omy of the construction of large-scale military, industrial, or power-generation facilities in Earth orbit.