there is no synodic period problem, i.e. the tanks are available each time the shuttle flies to a desirable inclination orbit. In January 1988, SSI conducted a workshop to examine early economic opportunities for the use of lunar materials. The study groups concluded that minimally processed products such as iron, glass, raw soil for shielding and oxygen were likely to produce the earliest economic benefits [28], In late 1988, SSI contracted an additional study to Space Research Associates to look at the construction of space power assets from the sorts of nonterrestrial materials likely to be first available as a result of operations in cis-lunar space. Specifically, scavenged external tanks and other spent stages, lunar glass, iron and oxygen will be considered as feedstocks for space power applications. Study results are expected by the third quarter of 1989. Another general line of inquiry involves looking beyond present assumptions for use of nonterrestrial materials in space power applications. In general, it has been assumed that minimizing use of Earth launched materials in finished products is desirable. There has been an implied assumption that this strategy results in the lowest cost system. However, it is possible that under certain conditions it may be less expensive to implement an SPS (or precursor), which uses a somewhat higher percentage of terrestrial materials, but which minimizes the cost of processing, manufacturing or construction. The present Space Studies Institute/Space Research Associates study is beginning to address this issue. Additional Research Directions A wide range of investigations should be undertaken in support of solar power satellites. The following is a brief non-exhaustive list of possibilities: (1) In general, we must continue to ‘prospect’ for nonterrestrial materials. (2) In particular, we need to have a complete geochemical map of the lunar surface. Of special interest is the possibility of frozen volatiles which may be trapped in the polar regions of the moon. SSI is presently engaged in studies of a low-cost lunar polar prospector, and is considering options for a non-governmental flight of such a probe. (3) The search for possible asteroid candidates for research recovery requires additional support. Space-based observations should be initiated where appropriate. (4) Continued investigation into relatively simple robust processing methods, such as electrolytic techniques for the production of silicon, aluminum and oxygen from lunar ore, proposed by Keller [29], should be continued. (5) Alternative system configurations to the ‘canonical’ SPS should be explored. Examples include Freeman’s Earth polar orbiting SPS [30], and Criswell & Waldron’s idea of using the lunar surface itself as a natural SPS [31]. (6) Economic precursors to solar power satellites should be explored. These could include platforms to provide power to space station(s), space shuttles or satellites with high-power requirements, such as future direct broadcast platforms. The use of beamed power for spacecraft propulsion, as discussed by Sercel [32] could provide a market for transmitted energy. (7) The development of tuned laser/photovoltaic cell arrays and high-frequency small aperture microwave transmission systems for space-to-space applications should be examined. (8) The transmission of power from the surface of the Earth to a spacecraft in
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