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 (6) 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. SYNERGY WITH SPACE TECHNOLOGY DEVELOPMENTS The goal of the SPS is 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 could stimulate extensive space activities. Satellites to be built during the next decade will require more electric power than current satellites and could well get their power from a space-based power plant. Such a power plant could be modified by the addition of a microwave antenna to transmit power to other satellites. Advances in orbital assembly will increase the feasibility of constructing large structures in space. Development of thin film solar cells has the potential to reduce mass and cost and to achieve the production goals of solar cell arrays for an SPS. Orbital transfer vehicles using chemical propellants, solar electric or microwave electric propulsion are being studied. Only after such generic technologies have been developed and demonstrated would a prototype SPS be designed, constructed, and tested. Advances in space technology may also open
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