ciated with the solar cell arrays. There are several promising candidate photovoltaic materials to replace the single-crystal silicon now used to supply power for satellites. The energy required for SPS construction, deployment, and orbital assembly is a function of specific design approaches and the materials employed, e.g., photovoltaic materials, structural supports for the receiving antenna arrays, propellants for launch to LEO, and electric propulsion for transfer from LEO to GEO. The energy payback period for the SPS ranges from one to three years, and the energy ratio for the SPS compared with coal and nuclear power plants is positive. ENVIRONMENTAL IMPACTS While the SPS may be the most benign large-scale energy conversion technology yet conceived, a number of areas of potential environmental concern have been identified, including microwave biological effects, heating of the ionosphere, deposition of rocket engine exhaust products in the upper atmosphere and interactions of ion engine propellants with the magnetosphere. Several of these effects are not well understood and a few may have long-term consequences, therefore, steps to mitigate them should be explored. Research is needed so that data can be available at appropriate decision points in the SPS program, and to guide development efforts. No environmental effects have been identified which preclude continued consideration of the SPS. It is obviously necessary to trade off environmental risks and costs against perceived benefits. The decision not to develop the SPS could also have ecological consequences if the SPS energy option is foreclosed prematurely and intractable problems surface which may stop the large-scale applications of remaining known energy options. If the SPS is not developed, some other route which is clearly more economically attractive, environmentally benign, and socially desirable must be found. It must be accepted that risk-free developments of energy conversion technologies are impossible. The best that can be done is to choose a course which maximizes benefits while minimizing undesirable side effects. The environmental impacts of the SPS must therefore be considered in comparison with those alternative technologies which might be used to meet world energy demands. SOCIETAL EFFECTS The SPS is an enterprise whose scale compares with the largest engineering programs ever undertaken. The implications for society could be profound, but whether or not such effects would be beneficial will depend on political, organizational, and institutional factors. Proponents of the SPS believe that it could be a major contributor to solving the global energy supply challenges, that it would stimulate economic growth, and that it would open space for use by man. Opponents hold that the SPS is a pernicious example of “megatechnics,” the preoccupation with technical feats which is largely responsible for the unsatisfactory predicament of society. It has even been claimed that the SPS would require creation of an authoritarian state to protect massive capital investments and to force people to accept exploitation by utilities or other monolithic corporations. It seems clear that the objective of a just and rewarding society is more readily attainable by maximizing the options and opportunities available to all people than by curtailing them to coerce the people into adopting a particular (and perhaps less
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