vehicles, (2) breakthrough in SPS solar power components, particularly solar cells, (3) resolution of the nuclear power question, (4) further study and experimental work in the microwave transmission of power, (5) a considerable advance in current capabilities for the fabrication and assembly of large space structures, and (6) substantial technological development to achieve the necessary power conversion processes and devices. In determining SPS feasibility the economic question is almost certainly the pivotal one. It is reasonably clear that the technological problems can be solved, given sufficient resources; it is not so clear that these problems can be solved economically. Section V discusses the economic issues and presents a coherent economic analysis of the SPS concept. Section VI, Energy Balance, calculates the energy requirements of an SPS system, compares the results with the energy requirements of several alternative systems, and summarizes the basic material resources required for a nominal SPS program. The environmental inpact associated with the SPS can be considered under three headings: (1) social impacts, (2) environmental pollution, and (3) microwave transmission. Social inpacts include radio frequency interference with communications systems, land use for microwave receiving and launch facilities, and the general issues of public safety. Environmental pollution stems from the massive manufacturing processes and the extremely large number of launch vehicles required. The microwave transmission issue revolves primarily about the acceptable levels of such radiation that can be permitted. Section VII contains a detailed summary of the potential SPS environmental inpact. There are basically three key issues in the undertaking of an SPS program. These issues, which are discussed at some length in Sections IV through VII of this report are: (1) technology, (2) economics— including the question of energy balance, and (3) environmental impact. Section VIII, Key Issues, highlights several of the more pressing problems that must be solved in these three areas. Three SPS-related topics are then treated in Sections IX, X, and XI. These are, respectively, the O'Neill concept characterized by space
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