inevitable organizational and political difficulties, the strong potential interest of energy-poor, non-U.S. participants in increased electrical supplies could help make a multinational venture more feasible than a unilateral one by the U.S. Global electricity demand may increase up to four times by 2030, and will be especially strong in developing countries. Western Europe and Japan would be likely partners for a joint project. Depending on the size and expense of the system used, a number of the more rapidly developing LDCs might also be interested in participating at lower levels of involvement. The Soviet Union is carrying on an aggressive space program which may give them an independent capacity to develop SPS, but little is known about their long- range space or energy plans. Real or perceived competition with the Soviet Union could spur a U.S. commitment to SPS. The development of fleets of launch and transfer vehicles (for SPS), as well as facilities for living and working in space, would enhance this nation's military space capabilities. Such equipment would give the possessor a large breakout potential for rapid deployment of personnel and hardware in time of crisis, though for nonemergency situations the military would prefer to use vehicles designed specifically for military purposes. The SPS itself could be used for military purposes, such as electronic warfare or providing energy to military units, hut is technically unsuited to constitute an efficient weapon. Weapons-use of SPS would be prohibited by current bilateral and multilateral treaties. The satellite portion of SPS is vulnerable to various methods of attack and interference but the likelihood of its being attacked is only slightly greater than for major terrestrial energy systems. The military effects of SPS will depend largely on the institutional framework within which it is developed; international involvement would tend to reduce the potential for use of SPS by the military sector. SYSTEMS AND COSTS The optimum SPS system has not been identified. The NASA/DOE microwave reference system, which was developed to provide a basis for review and analysis, was not intended to represent the best possible system. An optimum system should be able to deliver power in smaller units (about 1000 MW or less), use smaller terrestrial receivers, and cost less to develop than the reference system. Alternative systems may use lasers or mirrors to transmit solar energy from space to Earth. Variants of the reference system or completely different systems offer certain improvements; each will need full study before choosing a system for development. Current overall cost estimates for the SPS, as well as for its major components, are highly uncertain. The assessments of up-front costs range from $40 to $100 billion. The most detailed estimates have been made by NASA for the reference design (17). These call for a 22-year investment of $102.4 billion (1977) dollars (including transportation and factory investment costs) to produce the first 5-GW satellite, with each additional satellite costing $11.3 billion. The costs for most improvements to the reference design, or for alternative systems, are less well known due to the less-developed state of nonreference technology. Preliminary studies indicate that the total reference system costs are likely to be significantly higher (18). On the other hand, alternative systems may well be cheaper than the reference system (19). The total costs estimated by NASA include major elements, such as space transportation and photovoltaic cells, whose development is likely to proceed regardless of SPS;
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