very attractive option. Figure 7 shows an artist's conception of astronauts unrolling a thin-film solar array to provide power for a manned base on the surface of Mars. For a long-term manned lunar base, transportation costs are moderately high. However, the mass of the solar array for a lunar base is negligible compared to the storage capacity required for the 14-day lunar night, so specific power of the array is not an issue. Important uses for thin-film cells may be for intermediate (14 day) staytime missions where the array is brought with the spacecraft, and for manufacturing power, e.g. lunar oxygen extraction, that require large amounts of power but could be run only during the sunlit periods. In the long term, it may be economically feasible to manufacture solar cells on the moon from available lunar materials. In this case, the only practical cell material is silicon, and the much smaller materials requirement for amorphous cells makes this the preferred technology. This is discussed in more detail by reference [51]. 7. Conclusions Thin-film solar cells show a potential for making extremely lightweight solar arrays. Thin-film photovoltaic materials being developed for terrestrial use which may be able to be adapted to space solar arrays include CuInSe2, CdTe, and amorphous silicon. While the efficiencies are low compared to current technology space cells, the projected specific power levels are still extremely good. Development of multibandgap cascades raises the possibility that the efficiencies can be considerably improved. Ultra-lightweight space arrays will require that the materials can be deposited on
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