commercial applications such as communications satellites. A significant boost would be identification of near-term, large-scale commercial applications of space (space tourism has been suggested as one such application). Pending such an as-yet unknown commercial application, however, I see little prospect for commercial space enterprise to develop transportation on the scale required. The SPS infrastructure is thus dependent on development of the required space infrastructure by space exploration missions conducted by the various national governments of Earth. Any of the various manned missions proposed in the near term (Space Station Freedom, return to the moon, manned Mars mission, ‘Mission to Planet Earth’) could provide elements of the necessary experience. Various unmanned missions, such as planetary probes (e.g. Cassini) and exploratory missions to the smaller bodies of the solar system such as asteroids and comets (CRAF) contribute little to the transportation infrastructure needed, although they are important preliminary elements to the long term exploitation of space resources. An aggressive planetary exploration policy has additional long-term applications to SPS. The projected cost of an SPS could be considerably reduced if extraterrestrial resources are employed in the construction [17]. One often-discussed road to lunar resource utilization is to start with the mining and refining of lunar oxygen, the most abundant element in the Moon’s crust, for use as a component of rocket fuel to support the lunar base as well as exploration missions. Once the mining and refining process is in place to produce oxygen, the next-most abundant elements, aluminum and silicon, can be refined to produce solar arrays [18]. Such lunar-manufactured solar arrays could have many applications (Fig. 2): not just to support growth of manufacturing capabilities on the moon, but also in LEO, GEO, and to support planetary missions, as well as to support solar-electric inter-orbital transportation and to serve as primary power supplies for the beamed power transportation systems discussed in the previous section.
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