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178 productive program, which has been funded at a constant- dollar level for two decades. V. PROJECTIONS INTO THE FUTURE We have divided this part of our report into three sections: the impact of our recommendations on science management and the changes likely to result; the growth of our technical capabilities through the 1990's; and the impact this growth will have on the structure of scien- tific research and the major scientific opportunities that will be exploited during the 1990's and beyond. A. Management Considerations The 1970's witnessed the concentration of most new, ground-based observational capabilities into a few National Astronomy Centers and the decline, in absolute terms, of the health and vigor of major university and private observatories. We have made recommendations that we hope will help stem deterioration of university research. We have suggested that federal funds be used to stimulate state and private groups to increase support of their own observatory facilities. Because of technical developments, the cost of 2.5- to 5-m ground-based optical telescopes has dropped substantially. For telescopes in this size range the operating costs will, in 5-10 years, exceed the construction costs, and we urge federal help for university and private groups struggling to utilize fully the telescope capabilities that are available to them. In part, full implementation of our fifth major recommendation, for instrumentation development, will help maintain the vigor of university groups. Nonetheless, the overall impact of our major recommendations will be to continue the trend toward a concentration of major facilities into the National Centers. This trend seems unavoidable; some large universities, such as California, Texas, and Arizona, may be able to attract sufficient private support to build very large ground-based tale scopes, but even they cannot afford the largest practical ground-based facility, and no university or private group is proposing to initiate a major space effort without using federal funds. We believe that the collecting area available to ground-based optical/IA astronomy should be increased

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179 tenfold. ~ the scientific We believe that this is necessary to exploit __ ~_~ opportunities of the 1980's, as well as to back up the space and radio programs. Technological developments have made this recommendation both cost- effective and timely. Nevertheless, in the more distant future, we see an increasing development and reliance on our national space capability. Already the solar- astronomy program is overwhelmingly a space program. The gains achievable in the IR, both from the use of cooled telescopes and through the exploitation of wavelengths that do not penetrate the Earth's atmosphere, are very large. The major initiatives of the IR program in the 1980's will lie in space. ST will be one of the principal instruments for optical astronomy in the 1980's through the 1990's. there will be a shift of emphasis from ground-based to space observations. As the century ends we may well wit- ness the last major construction of optical telescopes on the ground. To be sure, these telescopes will continue to be useful well into the twenty-first century, and oper- ational support for them will be crucial, but space facil- ities will gradually take over the role of ground-based telescopes beyond the 1990's. For some decades, and per- haps indefinitely, the management of such expensive space facilities will be the province of government labora- tories. m ese facilities must be open to qualified users and, increasingly, to foreign participation. Financial support, management participation, and observational use by foreign scientists should be encouraged. m ese changes will require that careful thought be given to several management problems: It is clear that. as space technology ripens, 1. The health of university science in the face of continued erosion of competitive observational canabil- ~ , ities in state and private observatories by comparison with those of the National Astronomy Centers and gov- ernment laboratories. 2. m e need to ensure balanced programmatic approaches to observational astronomy. In the past, the existence of numerous independent observatories ensured a balance of style and programs in astronomy. As the National Centers grow in power at the expense of private and university facilities, it is important that the appro- priate review committees and time-assignment committees appreciate and support the need for diversity. 3. Adequate access to facilities established with major investments of national resources. These must be

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180 open to all qualified users. Thus, institutional staff members, outside principal investigators, and others must have their programs competitively reviewed by an impartial committee. Foreign scientists should continue to have access to major U.S. facilities and, as in the case of the STScI, foreign participation in both the management and financial underwriting of the program should be encouraged. 4. Support of survey programs and other long-term efforts. Astronomy progresses both by spectacular dis- coveries and by painstakingly slow survey programs that search for systematic relations, subtle effects, or unusual objects. Both styles of research are essential and, indeed, synergistic. As astronomy moves increasingly into a national-facility mode, with observing time on forefront facilities increasingly allocated by commit- tees, it is essential that these committees have the wisdom to support long-term survey programs in addition to those that promise immediate results. By making national facilities open to all qualified users, it should be possible to implement a program incorporating observations at the National Centers, support observations using state and private observa- tories, and instrumentation development in both the universities and at the National Centers that would ensure the health of university science while at the same time providing the powerful facilities that will be needed at the end of the century. , We see these management problems becoming acute only toward the end of the decade, when projected programs result in national observational facilities that com- pletely dominate the field. The shift from ground-based to space astronomy should raise the issue of the appro- priate roles that NASA and NSF play in astronomy. The division between ground-based and space work is becoming increasingly artificial, and it would seem appropriate for Congress to re-evaluate the roles played by these two principal support agencies for astronomy. B. Instrumentation in the 1990's It is harder to foresee the developments in instrumenta- tion than it is to recognize the problems that science management will face 10 years from now. A major issue will center around the successors to ST and the 15-m NTT.

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181 In the past, each step toward more aperture for attack- ing the perceived forefront problems of the era has led to the discovery of new classes of objects and the delin- eation of whole new arrays of problems, unimagined and unanticipated prior to the availability of those new facilities. Furthermore, in view of the new design con- cepts and lightweight materials now being tested, there is no longer a clear upper limit on telescope size. It is possible that, by the 1990's, one could contemplate the construction of a 25-m telescope on the ground within the constraints of available funding. The question of how then to proceed is complicated, however, by the possibility that a somewhat smaller diffraction-limited telescope of 7- to 15-m aperture could be put in space by that time. If the very large development costs of space platforms, extended missions, space-fabrication techniques, and cheaper (per kilogram) launch vehicles are to be totally borne by astronomy, this option is of course out of the question. Yet it seems clear that these developments will proceed for other reasons, and astronomy may benefit from these technical developments. The present experience in the astronomical community with space astronomy through IUE, Copernicus, and the HEAD series of satellites has been outstanding e The relatively high efficiency of use of observing time, absence of weather problems, and high equipment reliabil- ity, combined with the reduced background, better seeing, and lack of atmospheric absorption (thus extended wave- length coverage) available from space have been and will continue to be great attractions. me additional experi- ence to be provided to the astronomical community by ST will be critical in evaluating the advantages of large space versus (perhaps slightly larger) ground-based tele- scopes in the coming decades. At present, the technology for neither a 25-m ground-based telescope nor an extended- mission space telescope in the 7- to 15-m class is avail- able. It is, for example, not yet clear how the lack of gravity can be used to best advantage in reducing the weight (hence the cost) of a large space telescope, and how much the weight can be lowered without compromising the optical performance. We must encourage developments in both directions, so that the successors to this com- mittee may in 10 years make an intelligent, well-informed choice on a giant telescope for the 1990's. Two-dimensional detector development is being driven by commercial applications and, while that development does not place high weight on astronomical applications,

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182 nevertheless commercial demand will ensure a continued major effort that will likely produce two-dimensional detectors with nearly 100 percent quantum efficiency over the entire spectral range, from 100 ~ to 1 Em. In the IR region very great improvement over current technology should result from the military effort. High-efficiency coatings are needed to cover the wavelength interval from 100 A out to 1 mm. As optical sophistication increases in instrumentation, such coatings will allow the optical designer the freedom of increasing the number of reflections without paying a penalty in low transmission. Bare aluminum has high reflectivity down to 100 A, a soft x-ray wavelength that would allow use- ful surveys for high-red-shift quasars using the large collecting area of a normal-incidence reflector. High- efficiency, two-dimensional detectors for this wavelength region already exist and could be improved. There are numbers of suggestions for constructing very large, nonfilled optical arrays in space. A simple cross 100 m in extent, filled with 1-m mirror elements, could be used as an interferometer at optical wavelengths. The resolution of such an instrument would be about 5 X 10-9 red (10-3 arcsec). With such an instrument one would be able to image the nearer stars with about the same resolution that Galileo's telescope achieved on the Sun. Starspots could easily be seen and monitored. me struc- ture of quasars, contact binaries, and galactic nuclei could all be studied. X-ray and radio surveys have historically been used to identify interesting objects for studies at optical wave- lengths. However, there may well be other objects that are not particularly unusual at these extreme wavelengths but reveal their unusual nature at W. optical, or IR wavelengths. The Crab pulsar is not a particularly bright radio pulsar, for example. It may be possible to exploit the darkness of the sky in space to devise survey instru- ments that could automatically, using optical techniques, detect unusual objects. An objective-prism survey using a large, advanced COD array is a simple example of a pos- sible instrument. There are some beginning attempts to develop "smart" detectors. Three-dimensional CCD arrays that could per- form simple arithmetic operations before transferring the processed picture to a computer might allow the develop- ment of high-speed speckle processors. If such devices could be built, it might also be possible to build very large, diffraction-limited IR telescopes. By working at

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183 wavelengths for which speckle techniques are not photon limited and the required surface accuracy of the primary mirror is low, one might build a telescope using nonrigid techniques (for instance, an inflatable structure). Rubber-mirror technology with small adjustable reimaging mirrors is a different approach to the problem of constructing large, low-surface-accuracy mirrors that feed a system that is ultimately diffraction limited. Looking forward to the 1990's, we should consider novel ways of obtaining astronomical information not possible from the ground or even from Earth orbit. For example, studies suggest that it may be feasible to launch an instrumented spacecraft into an eccentric orbit about the Sun with perihelion near 4 solar radii and that the experiments on this proposed "Star Probe" mission should be able to survive encounter and transmit back data. Among the important questions that could be addressed in this way are the fine structure of the solar surface and corona (at a resolution of a few kilometers), the In situ plasma properties and wind speeds at all levels of the corona down to the temperature maximum, energetic- particle distributions, and the acceleration mechanism of the solar wind. Precise tracking of the Star Probe will also provide information on the distribution of mass and angular momentum in the Sun and should provide high- accuracy tests of General Relativity. We should keep in mind that each planetary-encounter mission has provided totally unexpected and exciting information and that the Star Probe would provide our only opportunity to study a star at close range in the foreseeable future. It seems quite certain that we are in a revolutionary period for classical astrometry. Already parallax tech- niques have undergone a tenfold improvement in precision. Electronic focal-plane systems will be greatly improved in the 1980's. The ultimate accuracy has not yet been determined, but it is clear that astrometry will make major new impacts on astrophysics and open new fields, such as the search for extrasolar planets. C. The Direction of Scientific Research in the 1990's It is clear from the developments of the past several decades that astronomy is an explosively expanding science. New discoveries pile on top of each other with bewildering frequency. That old lady Urania, the muse of astronomy, is showing us that she is divinely unpredict-

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184 able. We must not presume that we can accurately forecast future developments. But we can project the likely capa- bilities that will become available as the century ends and note the fields that will be affected by these techno- logical initiatives. The instrumental initiatives discussed earlier will lead to the following major changes in capability. 1. Large Gains in Angular Resolution During the 1980's, ST will routinely give a tenfold im- provement in angular resolution by comparison with that usually available from the ground. Large arrays in space, launched in the 1990's, will be capable of bettering the ST resolution by an additional factor of 100. To appreci- ate the significance of this improvement one must recall that the introduction of the telescope in the seventeenth century improved the resolution capability of the human eye by about the same factor of 100. For ground-based observations, angular resolution was then, as now, limited by the turbulence of the Earth's atmosphere. Above the atmosphere there seems to be no limit, except for the practical limits imposed by our ability to construct ever- larger instruments while maintaining high dimensional accuracy. Existing NASA, industry, and university studies are optimistic about our ability to achieve very high angular resolution in space. We now seem to have within our grasp the ability to end the 300-year hiatus on major improvements in optical resolution. The improvements in angular resolution can have major impact on three fundamentally different fields of astron- omy: positional astronomy for measurements of the spatial relationship of (usually unresolved) astronomical objects; the mapping of previously unresolved objects that happened to lie just below our current resolution capability; and the separation and detailed study of phenomena that are now blended into confusing background sources. It seems quite likely that high angular resolution will lead directly to exciting new discoveries and open new fields of research that are not now even imagined. We can now resolve the disk of the Sun, and we can infer the surface appearance of other stars, but we have not yet "seen" starspots on another star. Milliarcsecond resolution will resolve the disks of nearby giant stars. In the far W. the contrast between starspots and the disks of late-type stars will be large.

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185 There is no galactic nucleus of any type that is close enough for us to probe with high spatial resolution using current optical techniques. The nuclei of galaxies are bright; there is some evidence that some galactic nuclei contain black holes. An increase of a factor of 1000 in angular resolution, together with a spectroscopic radial- velocity capability, would probably settle this issue. High angular resolution will also allow us to penetrate deeply into crowded fields. Do x-ray-emitting globular clusters hide black holes in their centers? Radial- velocity studies of stars near the centers may provide an . . answer. In any case, from where does the x-ray emission originate? Deep- W photographs would do much to clarify the situation. The diameter and structure of bright planetary nebulas in the Local Group of galaxies could be studied with very high resolution. Differences in excitation class and abundances will be better understood when we can resolve the nebular envelope. High-spatial-resolution imaging and spectroscopy of the Sun during the 1980's has the potential of resolving the fundamental structures defined by the filamentary but strong magnetic fields. When this occurs, the Sun will indeed become a plasma astrophysics laboratory, in which we will see for the first time how magnetic fields and plasmas interact to yield such phenomena as heating on slow and rapid time scales, flares, and wind acceleration. The benefit of these studies to theoretical astrophysics is incalculable. High-resolution spectroscopic capability will pro- foundly affect the studies of planets in our solar system. We will be able to monitor weather patterns in their atmospheres and study structural and chemical composition changes on their surfaces. The Landsat and weather satel- lites have demonstrated the importance of remote sensing for geology and for an understanding of the Earth's global weather patterns. Their planetary counterparts will be able, for example, to monitor volcanic activity on Jupiter's satellite To. Astrometry will be revolutionized by ultrahigh angular resolution. It will be possible to measure directly the distances to all objects in our Galaxy. Planets orbiting nearby stars will be detectable from the irregular motions of these stars. It might even be possible to detect planets using direct-imaging techniques. m ese examples illustrate only the easily imagined uses of very high angular resolution. The real excitement will

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186 result from discoveries that we cannot now expect or pre- dict. This was the case for Galileo's telescope, the extension to our vision provided by x-ray and radio tech- niques, and the improvement in resolution afforded by deep-space planetary probes. 2. Increased Light-Gathering Power m is report calls for a substantial improvement in tele- scope light-gathering power during the 1980's. me 15-m NTT will collect nine times more photons per second than the 5-m Hale telescope on Mt. Palomar. Its spectroscopic capabilities, if located on an excellent site and equipped with the most sensitive instruments and detectors, will surpass by several orders of magnitude the capabilities of the 1970's. In space it might be possible, using rubber-mirror techniques, to correct imperfections in a giant primary mirror and thus to erect very large space telescopes that would be nearly diffraction limited. While we have not yet developed the technology to deploy telescopes that would exceed by an order of magnitude the light-gathering power of a 15-m telescope, we are at a point where we could begin to think along these lines, and we might be able to construct such telescopes by the end of the 1990's. Spectroscopy is the key to understanding the physics of astronomical objects. By increasing the light- gathering power of a telescope, we are able to study sources that are increasingly faint, either because they are only weak emitters of light or because they are veiled by interstellar dust clouds. Spectroscopic studies of such objects probe the very frontiers of the physical Universe, the birth and death of stars, the evolution of galactic systems, and the physical conditions that lead to the phenomena we call quasars, pulsars, x-ray binaries, and black holes. 3. Increased Capability for Study of Objects with Low Surface Brightness Space telescopes, operating in the absence of veiling glare from atmospheric airglow, lend themselves naturally to the studies of low surface brightness phenomena. Already, ground-based telescopes, using modern fine-

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187 grained emulsions and working at dark sites, have dis- covered very low surface brightness plumes, bridges, jets, and halos associated with relatively nearby galaxies. Are these structures composed of stars, dust, or gas? Do some of them reveal a physical connection between objects of very different red shift? Are we seeing the remains of the protogalactic cloud from which the galaxy collapsed, or are we seeing material that was ejected during a period of high nuclear activity? Are they the remains of an ancient galactic collision? Since many of these sources are so faint (about magni- tude 30/arcsec2) as to be virtually undetectable, it seems a hopeless task to obtain slit spectra of them; however, it would be useful to obtain broadband colors. The spectral range free of terrestrial atmospheric emis- sion is quite narrow, extending only from about 4500 to 6500 A. In space, where there are no atmospheric prob- lems, it would be possible to obtain images in the vacuum- W, in the green, and in the near-IA, where the spectra of late-type stars reach their maximum luminosity. It would be of great interest to establish the occur- rence of such phenomena as a function of age of the object. We know that faint halos are associated with nearby Galaxies. Are they present with the same fre- , , quency and structure at a time when the Universe was only half as old as it is now? It should be possible to detect such halos at large red shift using high-sensi- tivity panoramic detectors with a telescope having high spatial resolution. We know that some of the brighter clouds are emission nebulas. From space they should be very bright at red-shifted Lyman-alpha wavelengths. Such nebulas have been found in the radio lobes of the nearby radio galaxy Centaurus A; are they also present in the much more distance source Cygnus A? Within our own Galactic system and neighborhood, there are sources that would be better understood if we had an improved ability to detect low surface brightness objects. Some supernova remnants are very faint. Old planetary nebulas expand and fade from view; high Galactic latitude clouds only shine weakly with reflected Galactic light. Interferometric techniques can study some of these sources, particularly if strong W lines are present, but they must be found and mapped. With high spatial resolu- tion these "Galactic" studies can be extended to all the galaxies of the Local Group. It would be particularly interesting to detect optical nebulosity, or faint blue stars, corresponding to the radio detection of a Magellanic Stream.