was limited by the available funding, the studies supporting the program examined an unprecedented variety of issues that might 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 a broad consensus rather than on narrowly defined expert opinion. SPS designs ranging from 10 MW to 5000 MW have been studied in Canada, Czechoslovakia, England, France, Japan, the Soviet Union, and West Germany, indicating the wide interest in the power generation potential. One reason for the increasing confidence in the technical feasibility of the SPS is that alternative technologies have been identified for nearly all components of the system (6). Most studies have been concerned with the SPS reference system which was chosen during the CDEP to provide a common basis for assessments. This “reference system,” based on assumed guidelines, was established by NASA to evaluate environmental effects, explore societal concerns, and perform comparative assessments. It is a design concept based on known technologies of the early 1970s; it does not represent a system that was expected to be actually constructed. An operational SPS, which could be developed during the next 20 years, would use some of the many alternative technologies that already have been identified for advanced SPS designs and would thus be quite different from the SPS reference system. Just as one aircraft design does not meet all of the requirements of the air transportation industry, SPS designs for different purposes will have to be developed. Although the SPS does not produce the undesirable environmental effects associated with large-scale fossil and nuclear energy conversion technologies, several areas of potential environmental concern have been identified. These concerns include low level microwave biological effects, heating of the ionosphere by the microwave beam, deposition of rocket engine exhaust products in the upper atmosphere, and interactions of ion thruster propellants with the magnetosphere. Several of these effects are not yet well understood, and a few may have long-term consequences. Therefore, steps to reduce them will have to be explored. Scientific investigations already have indicated that exposure to microwaves at the flux densities estimated to be received at the ground antenna site will not pose hazards to bees and birds. No environmental effects have been identified that a priori preclude continued consideration of the SPS. The National Academy of Sciences (3), in its critique of the SPS concluded: “Some type of SPS would be technically possible if costs were not a consideration.” This conclusion was reached by focusing on the SPS reference system, which was evolved as a tool for inquiry and included an implementation scenario for 60 satellites placed in operation between 2000 and 2030. This scenario was not a plan of what would or should happen, but was a basis for studies pertaining to space transportation, materials resources, and manufacturing requirements. Both the SPS reference system and the scenario for its implementation assumed a “stand-alone” SPS development program. The Office of Technology Assessment, Congress of the United States in its report (7) indicated that, when operational, the SPS could be an economically attractive option for power generation. For most advanced technologies, such as fusion and the SPS, uncertainties in cost projections extending several decades into the future considerably exceed the cost differentials that, in practice, will determine their relative competitiveness. For example, 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 implementation.
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