new electric generating capacity will be needed by 2010 to meet projected demand and to replace obsolete plants (7). There is no alternative to a technological approach to meeting this challenge. The scale of the SPS has to be such that it can meet a significant but not necessarily a major fraction of world demand, because it is unrealistic to assume that any one source of energy will meet all future global demands or that SPS development would preclude work on other alternative energy conversion methods. The required land area for the receiving antenna increases with higher latitudes. Receiving antenna sites nearer the equator require smaller areas because the microwave power beam illuminates an elliptical area whose North-South dimension increases at locations away from the equator. Third World Countries have a modest advantage over developed countries because nearly all are located at low latitudes. Because the demand for new power generation capacity is expected to increase as industrial activities, particularly in Third World countries, accelerate in the beginning of the 21st century, it will be desirable to start development of the SPS before that date. Delays in developing the SPS and resolution of known issues could force a major commitment to other energy conversion technologies which could lead to potentially undesirable consequences when applied on a global scale. Market studies to date concentrated on large SPS outputs because there may be a nearer-term demand for a power output of several GW at the receiving site on Earth if world power generation increases by a factor of 3 to 5 during the next 30 years. Already about 30 nations with 70% of the world's population consume 94% of the world power production. Therefore, about 30 nations would be in a position to use an SPS with large power output early in the next century. In addition, regional groupings of nations could share the power from one or more smaller-output SPSs to meet the foreseeable requirements of their energy economies. The SPS, as a generic concept, can be developed in versions of different power outputs on Earth to match different conditions based on a broad variety of alternative approaches. Therefore, the SPS presents a potential solution to the energy challenges faced by a large number of nations. This should facilitate the obtaining of international agreements in areas such as orbital locations in geosynchronous orbit, frequency assignments for the power beams, and standards to limit environmental effects of the power beam, the space transportation system, and SPS operations. An important impediment to support of SPS development is the substantial funds which will be required for development and demonstration and the extended time (of the order of 20 years) before returns on investments can be expected. Although the cost of demonstrating a prototype SPS may reach $25 billion, assuming that a space transportation system has been developed to meet other space mission requirements, the level of risk is in relation to its potential benefits. The scale of the implementation of an extensive SPS system is such that it may have significant macroeconomic effects in the participating countries. Most studies indicate that these effects will be beneficial in stimulating economic growth. However, investment in a global SPS system over a 50-year period may also entail opportunity costs. The SPS design objective is to minimize the use of rare materials. Although there has not yet been a systematic effort to optimize this aspect of the SPS design, many options are available. For example, argon can be used instead of cesium or mercury as a propellant in ion thruster engines and the heat exchanger pipes for cooling the microwave generators can use hydrocarbon compounds instead of mercury. A study of the industrial infrastructure required to build the SPS has indicated that the only area requiring major increases in present production capability is that asso-
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