terns resulted in prohibitive cost projections. An independent study, prepared by the Congressional Office of Technology Assessment, arrived at conclusions considerably more optimistic than those of the NRC (5). European technical studies of the SPS have also been performed. The British Department of Industry funded a study of the SPS, completed in 1979, which has led to growing interest by the British aerospace industry. The European Space Agency began assessments of the SPS in 1977, resulting in a number of papers in the ESA Journal in 1978. Interest in the SPS was expressed by ESA in 1978, and the agency has supported studies under the auspices of the International Astronautical Federation. In June 1980, an international symposium on the SPS was held in Toulouse, France, attracting representatives from many European nations and agencies (6). To date, most European studies have concentrated on developing European requirements for the SPS. Apart from innovative but generalized systems analyses (focussed particularly on techniques for reducing the area required for a receiving antenna), there has as yet been little attention to detailed engineering aspects of the system. In general, European analysts have relied on the results of the CDEP for technical information. Work also is or has been underway in Japan, Canada, Czechoslovakia, and India. There is also evidence that the U.S.S.R. has an interest in the SPS, based on its contribution to the international dialogue on the project. At the U.N. Conference on New and Renewable Sources of Energy, Nairobi, 1981, Soviet speakers implied that the U.S.S.R. intended to develop the SPS and use it to supply energy to Third World nations. ALTERNATIVE CONCEPTS FOR ENERGY FROM SPACE The SPS concept has motivated the consideration of alternative concepts for obtaining energy from space. A number of concepts have been proposed which use satellites to generate or transmit energy for use on Earth. While these concepts exhibit considerable differences in specific technologies required for their operation, in unit power output, and in projected costs, they all utilize space as an ideal medium for transmission of electromagnetic energy in the form of microwaves, laser light, or sunlight. The technology options which have been explored could use optical reflectors in space to provide continuous insolation at specified points on Earth; reflectors of microwave or laser beams to transmit power from point to point on Earth as an alternative to long transmission lines; and transmit power from satellites in orbit where either solar energy or nuclear energy is converted and beamed to a receiver at a desired location on Earth. In the foreseeable future, space systems requiring power supplies with continuous megawatt outputs will be developed. For some space power applications, nuclear reactors will be preferred, but solar energy conversions will also be used extensively. Beamed energy may be useful for supplying power to remote space systems, such as free-flyer carriers, or for laser propulsion. Therefore, it is reasonable to expect that technologies will be developed for generating significant quantities of electrical power in space and for transmitting power over long distances. There is considerable commonality between the projected space power applications over the nearer term and the conversion of energy in space for use on Earth. Development costs will most likely be spread over several potential applications and technologies which will reduce investment requirements for future space power applications will be preferred.
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