The SPS concept is firmly founded on the premise that international participation in all phases of implementation will assure that the power generation and transmission technologies will only be used for peaceful purposes, and that the SPS will contribute to meeting future global energy demands. EVOLUTION OF THE SPS CONCEPT Since the SPS concept was first proposed, no other future energy macroengineering project has been studied as comprehensively from technical, economic, environmental and societal perspectives. Preliminary studies of the SPS concept were performed from 1968 to 1972. During that period, the SPS concept was discussed at many scientific and professional society meetings. In 1972, the Solar Energy Panel of the U.S. National Science Foundation outlined a plan for the SPS R&D program. Also in that year, NASA started a program to evaluate the feasibility of the SPS concept. In that feasibility study, a reference system design was adopted to provide a power output of 5 GW on Earth. In addition to structural design and control, RFI-avoidance techniques were investigated, and technological, environmental and economic issues were identified. In 1975, extensive system definition studies were started. In 1976, the principal responsibility for the SPS program was transferred to the Energy Research and Development Administration (ERDA). On the basis of its own appraisal, ERDA recommended that a more complete evaluation be made. At about that time, ERDA was reorganized and absorbed within the new U.S. Department of Energy (DOE). In 1977, DOE, working closely with NASA, started a three-year Concept Development and Evaluation Program (CDEP) (2), with a total budget of $19 million, to develop an initial understanding of the technical feasibility, economical practicality, and societal and environmental acceptability of the SPS concept. The CDEP provides a useful model for preliminary assessment of other energy conversion technologies. Although the depth of analysis was limited by the available funding, the studies supporting the program examined an unprecedented variety of issues which may influence development of the SPS. An explicit objective was to involve public interest groups in discussions about the SPS so that future decisions concerning the project could be based on broad consensus rather than on narrowly defined expert opinion. Many of the participants in the CDEP were initially skeptical about the SPS. However, no serious impediments to feasibility (other than uncertainties about cost) were identified and areas for continuing research were suggested (3). Since the conclusion of the CDEP, work related specifically to the SPS by U.S. Government agencies has not resumed, pending policy decisions about the future directions of the space program. There are, however, continuing studies, mostly funded by NASA, of technologies important to the SPS, including especially the areas of space transportation, large-scale construction in orbit, ion propulsion, space solar power conversion systems, and a permanent manned space station. When it became clear that the CDEP would produce results generally favorable to the SPS, a separate study was undertaken by a committee of the National Research Council (NRC), an agency of the National Academy of Sciences (4). Although several of the working groups involved urged continued research on the SPS, the resulting report is negative, primarily because overly conservative assumptions regarding the advances in solar energy conversion technology and space transportation sys-
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