cycle, terrestrial solar arrays require at least four times more area than the SPS solar arrays for baseload power generation on Earth as well as extensive energy storage systems. The terrestrial obstacles to solar energy conversion systems can be overcome by placing the SPS in GEO, where, except for short-duration occultations of the Sun by the Earth during the equinoxes, 24-hour operation is possible. GEO provides an ideal environment for the construction of large structures such as solar arrays. Such arrays need not be braced against gravity and wind loads, permitting the construction of lightweight contiguous solar arrays covering areas more extensive than any which would be possible on Earth. Studies indicate that the savings in solar array costs could be sufficient to offset those incurred in transportation to and construction in space, and in transmitting power from the satellite to a load center on Earth in desired locations on land or off shore. To date, analyses of the SPS have shown that the concept is technically feasible and that estimated costs overlap those for competitive non-solar power generation technologies. The economic competitiveness of the SPS in comparison with alternative energy conversion technologies requires further resolution. Assessments of the SPS concept indicate that, when operational, the SPS can be an economically attractive option for power generation on a global scale. However, for most advanced technologies, e.g., fusion and SPS, the uncertainties and projections of costs for up to several decades considerably exceed the cost differential which in practice will determine their relative competitiveness. Therefore, both the SPS and fusion are promising technologies, but further research is needed to provide data for economic analyses to justify decisions about their development and deployment. The SPS has the potential to provide an economically viable and environmentally and socially acceptable option for power generation on a scale substantial enough to meet a significant portion of future world energy demands. The concept is based on the extension of known technologies and on further development of an effective space transportation system. There are no foreseeable technical impediments to the development of launch vehicles of adequate cost and performance. OTHER SPS DEVELOPMENT CONSIDERATIONS Development of the SPS would require a significant commitment of national and international resources. Decisions regarding directions for the SPS program can be based on a set of principles, goals, and value judgments for which there is broad but not universal consensus. It is important to address the basic premise for the SPS concept, to permit a meaningful dialogue with those who have rational doubts about the concept. Opposition to the SPS arises primarily from disagreement about basic principles rather than about technical details, and often the objections are of a philosophical or ideological character. The fundamental assumption on which the SPS concept is based is that it is feasible and desirable to find technological solutions to the energy challenges confronting society. This assumption is not generally shared, and many major energy projects arouse controversy, motivated by a wide variety of concerns, some rational, others not. The debate is frequently between specialized interest groups which seek to shape public policy according to views that may or may not represent broad segments of society. Worldwide projections indicate that at least 2500 GW and as much as 7500 GW of
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