to the delivered cost of power from the SPS so that one can assess whether the SPS has any chance of being competitive; identify the cost drivers in the SPS system so that R&D efforts can be properly focused to reduce the projected costs; develop a consistent framework for evaluation of different technological options for the SPS system; determine the impacts of SPS requirements on raw materials availability and costs; study the effects of an SPS development program on labor costs and capital markets; and assess the cost risks for the SPS reference system and alternative SPS technologies. The wide range of costs for a specific energy conversion technology indicates the difficulty of assessing energy technologies unless consistent accounting approaches are utilized. On the basis of present value, an analysis of the cost of supplying nonrenewable fuels such as coal to a power-generating plant compared with the cost of the SPS demonstrates that SPS costs are within the competitive range. Cost comparisons among different energy technologies are further complicated unless external costs, particularly those due to environmental impacts and societal effects, are appropriately recognized and included as part of the total costs of a specific energy technology. Environmental Issues Among new technological endeavors, the SPS is unique because for the first time a major program is being evaluated before proceeding with the next steps in the development effort not only on the basis of its technical feasibility, but on its potential environmental effects, comparative economic factors, and societal issues as well as program risks and uncertainties. The latter considerations are appropriate at this time when public skepticism of complex, large-scale technologies appears to have been justified by well-publicized failures and untrustworthy assurances by industry and government that implementation of specific systems will not contribute to involuntary exposure to health and safety hazards. Among the key environmental issues which are being explored so that they can be quantified and ways found to adapt design approaches to reduce or eliminate environmental effects are the following: Health and Ecological Effects of Microwave Power Transmission. These effects are of greatest concern because of the possibility that large populations will be exposed to very low levels of microwave energy. The results of investigations indicate that properly protected workers could safely maintain the SPS in orbit and the receiving antenna on Earth even while the system is in operation. According to present scientific evidence, any effects on the public beyond the receiving antenna site are most unlikely. However, continued research on microwave biological effects is indicated to establish whether long-term exposure to very low levels of microwave energy will be acceptable. The fact that large populations have been exposed to microwave energy for communications, medical, radar, and for industrial purposes for many decades, and more recently to microwave cooking without demonstrated adverse effects indicates that if such effects exist at all, and there is no certainty that they do, they have not been in evidence. Rocket Exhaust Effluents from Space Vehicles. Liquid hydrogen and liquid oxygen which are the most likely propellants for the space transportation system, will interact with the upper atmosphere. For example, they could change the radiation charac-
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