In spite of technological optimism, there are significant environmental issues which have to be resolved before a deliberate SPS development program can be undertaken. The most significant environmental issue is the effect of the long-term exposure to low-level microwaves. There are predictable problems with the land use associated with receiving antennas. To some extent, the public attitudes towards receiving antenna locations will depend on locally perceived costs and benefits, for example, secondary uses of the antenna site for agricultural purposes. Therefore, offshore antenna designs deserve much more attention and secondary uses for mariculture, wave energy extraction, and location of port and industrial facilities should be evaluated. The effects of space transportation system rocket-exhaust products on the Earth's upper atmosphere will have to be quantified and experimental data gathered so that mitigating strategies can be considered, such as trajectory control or, after the SPS has been shown to be an effective method for providing power to Earth, the eventual use of extraterrestrial materials. The economic justification for proceeding with an SPS development program should be based on a classical risk/decision analysis which acknowledges that it is not possible to know the cost of a technology which will not be fully developed for at least 10 years and commercialized, i.e., produced, operated and maintained, in not less than 20 years. Such justification, of course, is equally difficult to provide for other advanced energy technology projects. This justification, therefore, requires an appreciation of the competitive cost of alternative energy sources for the generation of electrical power which would be available in the same period. For the SPS reference system which utilizes demonstratable technology, the cost estimates, rough as they are and subjected to criticism as they may be, fall within a potentially interesting range—clearly sufficient to justify a continued research and technology verification program. Projections of SPS construction and operational costs between the years 2000 and 2030 are speculative. Forecasts of future costs presume a knowledge of future technology which, when contrasted with revolutionary advances of technology during the past few decades, makes such a projection of doubtful validity. Even though cost projections may imply a trillion dollar capital investment in a system consisting of 60-5 GW SPS constructed over a 30-year period, it should be acknowledged that introduction of any alternative advanced energy technology on a similar scale will require comparable levels of investment. For example, the capital investment in terrestrial solar energy conversion technologies to produce heat and generate electricity to meet 20% of United States1 energy requirements is projected to reach one trillion dollars by the year 2000.11 F. GLOBAL IMPLICATIONS AND POTENTIAL One of the most significant aspects of the SPS concept is its global implication. Once the feasibility of the SPS concept has been established, other countries may be interested in joining in the development and demonstration phases of the SPS program, and in space experiments which will need to be conducted on future space missions. International participation in the SPS program would permit the sharing of the significant development cost of the SPS by the countries which could also be expected to benefit from the power which would be
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