from the rectenna's center would be needed as an aiming device to maintain automatic alignment between the rectenna and the transmitter. Misdirection of the bean could be precluded by circuiting which would stop transmission should alignment with the pilot signal deviate beyond an acceptable amount. Lasers have also been suggested as an alternative to microwaves for power transmission to Earth, although less investigation has been made of this possibility. INTERNATIONAL ASPECTS The SPS concept is international in character for a number of reasons. First, if the United States decides to pursue an SPS program, the resulting availability of electricity could make the United States an energy exporter, rather than importer. For example, the United States could construct and operate the SPSs, with receiving antennas located in other countries. Second, such a large-scale, expensive project could benefit from financial input from many countries. Japan and the European Space Agency (a consortium of 11 European nations plus Canada) indicated at international space program hearings held by the House Science and Technology Committee in May 1976 that they would be very interested in a cooperative SPS project. Third, the many potential problems associated with SPS (environmental effects, radio frequency interference, etc.) may require international negotiations and studies. TECHNICAL AND ECONOMIC FEASIBILITY Technical As mentioned earlier, NASA and DOE are currently in the midst of studies to determine the feasibility of SPS designs. The ultimate technical feasibility of the photovoltaic and solar thermal designs is rarely questioned, even by critics of the overall plan. Experience has already been acquired in operating both photovoltaic and solar thermal systems. Although construction of large space structures has not been demonstrated, insurmountable obstacles are not expected. Transmission of microwaves is another area in which experience already exists although not over such long distances. Objections to the SPS concept are not usually based on technical feasibility, but rather on economic and environmental concerns. Economic Factors Cost estimates for research, design, development, test, and evaluation (RDDT&E) of an SPS system can only be speculative at this early stage. Estimates for photovoltaic SPSs fall in the $60-80 billion range, while that for a solar thermal program might be somewhat higher, due primarily to the large area of concentrators (mirrors) required and attendant construction problems. These estimates include development of a special Heavy Lift Launch Vehicle (HLLV) which would be required for either SPS concept. The space shuttle, (see Issue brief 73019, Space Shuttle) which is expected to become operational in 1980, is not considered adequate by SPS advocates to support a full scale SPS program, although it might be able to serve as a transport vehicle through the prototype stage. HLLV would be fully reusable “space
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