rials which could be converted not only into construction materials to build the SPS, but also to construct industrial complexes which no longer would need to depend exclusively on Earth’s resources to meet terrestrial needs. The use of Space as an industrial site could have a revolutionary impact by making it possible to create new materials and to assemble unique structures extending over huge areas which are not subject to the destructive forces of gravity, wind, rain, and snow that act on any structure on Earth — the inevitable elements of the active terrestrial environment. .The Technical Challenges Evolution of the SPS Reference System. Preliminary studies of the SPS concept were performed at Arthur D. Little, Inc., from 1968 to 1972. During this time, the SPS concept was discussed at scientific and professional society meetings. In 1972, the Solar Energy Panel outlined a program plan for the SPS R&D program and suggested funding levels (5). In 1972, Arthur D. Little, Inc., joined with Grumman Aerospace, Raytheon Company, and the Spectrolab Division of Textron to evaluate the feasibility of the SPS concept on behalf of NASA (6). In this feasibility study, a baseline design was adopted to provide a power output of 5 GW on Earth. In addition to structural design and control, RFLavoidance techniques were investigated and key technological, environmental, and economic issues were identified. The results of this study encouraged NASA's Johnson Space Center and Marshall Space Flight Center to start extensive system definition studies with the help of Boeing Aerospace (7) and Rockwell International (8). In 1976, the Energy Research and Development Administration was assigned responsibility for the SPS program. A task group was formed; it recommended that the SPS concept be evaluated, and outlined a program for this purpose (9). In 1977, the Department of Energy and NASA approved the SPS Concept Development and Evaluation Program Plan (10) with the objective: “to develop by the end of 1980 an initial understanding of the technical feasibility, economical practicality, and the societal and environmental acceptability of the SPS concept.” Solar Energy Conversion. As originally conceived, an SPS can utilize current approaches to solar energy conversion, e.g., photovoltaic and thermal-electric, and others likely to be developed in the future. Among these conversion processes, photovoltaic conversion represents a useful starting point because solar cells are already in wide use in satellites. An added incentive is the substantial progress being made in the development of low-cost, reliable photovoltaic systems and the increasing confidence in the capabilities of achieving the required production volumes. Because the photovoltaic process is passive, it could give the SPS an operating lifetime of at least 30 years and, with scheduled maintenance, perhaps even several hundred years. Micrometeoroid impacts are projected to degrade 1% of the solar cell array area over a 30-year exposure period. Because of the lesser probability of impact, larger meteoroids are less likely to affect the solar cell array. Several photovoltaic energy conversion configurations applicable to the SPS concept are being considered (11). For purposes of comparison and assessment, two versions of an SPS reference system design have been adopted by NASA (12). For these configurations, silicon and gallium arsenide solar cells, with and without concentrators, could be used (13). For use in the SPS, the solar cells will have to be reasonably efficient, of low mass per unit area, and radiation-resistant during transit to and in operation in GEO. They
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