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9 III. SUMMARY OF PRINCIPAL RECOMMENDATIONS A. Major Programs 1. Advanced X-Ray Astrophysics Facility X-ray astronomy is a vigorous and mature science. Its results provide critical and unique information about nearly every subject under active investigation in astronomy. Millions of Galactic and extragalactic objects are accessible to x-ray observations with existing techniques. The Einstein x-ray observatory (HEAD-2) proved the power of image-forming x-ray tele- scopes and demonstrated the feasibility of a major advance in observational x-ray astronomy. We therefore give highest priority to the construction of a Permanent orbiting x-raY observatory, the Advanced X-Ray Astral physics Facility (AXAF), with capabilities on a Par with the Very Large Array for radio astronomy and Space Tele- scope for optical/ultraviolet astronomy. 2. Cosmic-RaY Studies Cosmic rays carry in the details of their composition and energy spectra unique information about the origin of the elements, the mechanism of supernova explosions, the nature of cosmic particle accelerators, and the properties of the interstellar environment. Progress in detector technology achieved during the past decade and the avail- ability of the means for placing very heavy payloads in orbit now make it feasible to carry out definitive measurements of the elemental and isotopic abundances and energy spectra of cosmic-ray nuclei and of the spectra of electrons and positrons over a broad range of energies. Therefore we recommend that instruments required to perform definitive cosmic-raY measurements be developed and placed in near-Earth orbit for lonq-duration exposures. 3. Facility-Class Instruments for Solar Astronomy within the Spacelab Program Progress in understanding the Sun, the nearest star, is essential to achieving the goal of understanding stars in

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10 general. Solar astronomy now requires simultaneous observations by a variety of instruments covering the entire electromagnetic spectrum with the highest attain- able angular, spectral, and temporal resolution. We therefore recommend the development of facility-class instruments for solar astronomy in the Spacelab Program with the aim of incorporating them in an Advanced Solar Observatory toward the latter part of the 1980's. 4. Augmentation of Funding for the Explorer Program Important and well-defined problems in observational high- energy astronomy will remain unsuited to or outside the capabilities of the projected major facilities and short- term Shuttle missions. These include a variety of specialized investigations that require Explorer-class space missions. We recommend that the funding level of the Explorer satellite program be augmented to support a sequence of specialized high-energy astronomy missions at a rate of one every two Year S and that the Extreme Ultra- violet and the X-Ray Timing Explorer missions be started at the earliest possible times. 5. Development of Advanced Instrumentation through the Spacelab Program The long-range scientific objectives of x-ray astronomy require further major improvements in the sensitivity and precision of measurement. We recommend that advanced instrumentation for sPectrometrY, nolarimetrY, and imaging be developed through the Spacelab program with the ultimate goal of deployment on the Large Area Modular Array of Reflectors (LAMAR) and the X-Ray Observatory (XRO) to be constructed as major facilities near the end of the decade. B. Smaller Programs 1. Solar Neutrino Detectors Observations of neutrinos produced in the interior of the Sun are our only direct source of information about the

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11 processes of stellar energy generation that are of funda- mental importance in the structure and evolution of the Sun and other stars. Recent technological developments have opened new ways to achieve substantial improvements in the measurement of solar neutrinos. We therefore recommend that the ongoing program of solar neutrino research be adequately supported and, in particular, that international collaboration on development of a gallium detector be supported with the aim of placing such a detector in operation by 1983 or 1984. We also recommend a continuation of support for efforts directed at the detection of neutrinos produced in episodes of stellar collapse. 2. Development and Deployment of Detectors for Gravitational Waves The detection of gravitational waves from cosmic events may be within reach during the 1980's if instruments currently in development or under consideration are placed in operation. The detection of gravitational waves would have profound significance for our under- standing of gravity and would open a new and unique channel of information about phenomena in collapsing stars and galactic nuclei under relativistic conditions that are inaccessible to other means of observation. We therefore recommend that strong support be given to the development and deployment of detector systems for gravitational waves. 3. Increased Support for Theoretical High-Energy Astrophysics Supporting research and technology, analysis of data from ongoing or recently completed missions, and theoretical research related to but not necessarily tied to specific projects are essential components of a productive national program in high energy astronomy. We urge that these components be strengthened. We recommend, in Particular, that the National Aeronautics and Space Administration establish a program for theoretical high-energy astro- physics at a level of effort that is appropriately related in scale to the total effort in observational high-energy astronomy.