toward the end of the period. It also predicts that solar electric or laser electric propulsion should show advantages for LEO to GEO and return missions. Reference A6 (NASA) outlines a lengthy, and apparently complete, list of tasks and task objectives for the development of the orbit transfer stage. c. Large Cost Reduction in Solar Arrays The most widely discussed energy conversion processes for solar satellite power generation are those designated thermal electric and photovoltaic. The photovoltaic concept utilizes a very large expanse of 2 solar cells (on the order of 20 square miles - 50 km ) in a thin array directed toward the sun to convert solar energy directly into electrical energy. The array in this concept is fabricated, launched into low earth orbit, assembled and deployed there, and then transported to its final geosynchronous position. The solar array includes the cells, the lamination (blanket), the mirrors (concentrators), and the bus and support; it excludes the basic satellite structure, although these could be integral. Primary efforts to date have been based on silicon photovoltaic devices. This is because of the predictable performance of such cells demonstrated in the space program and because of extensive development efforts being conducted and planned to improve the performance and reduce the cost of silicon solar cell arrays. Alternative solar cell types, including Ga As, Cd S and multilayer or hybrid cells may also be suitable for this application. Ga As is particularly attractive since it is a natural thin cell and operates well at high solar concentration ratios. Generally, however, these cells are either in a rather early state of development relative to the silicon cell or exhibit unfavorable performance/ cost characteristics. In order for the photovoltaic concept to become viable, a number of problems regarding the cells and arrays must be resolved. These can be conveniently grouped into five general categories: Reduced solar cell cost Improved solar cell performance Reduced solar array weight
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