handling problem materials, and far less need to aggressively husband the biosphere to meet energy needs. Rather than devoting a major section of the Demandite production to fueling and making the power system, the overall Demandite production rate could actually decrease, perhaps sharply (note curve C in Fig. 2). It is interesting to compare the tonnages in columns D and E Table 6 to the U.S. Demandite production rates in Table 1 (assume 2 E8 people). On a per capita basis the space units can be very low mass structures, especially considering the energy flow through them (Fig. 7). Given a materials industry off Earth with a cost distribution even remotely resembling that in Fig. 5, space power systems should be relatively inexpensive. The highly repetitive nature of the systems implies very fast growth rates. Calculations indicate the order of 10s of day's energy payback times are possible if lunar materials are used for the space segments. There exist proprietary concepts in which the solar power systems are constructed primarily on the moon. Installation rates which are significant on the planetary power level seem feasible. Investment to first returns would likely be no higher than associated with full development of the North Slope of Alaska. Power costs from such a system would likely be quite competitive with the other approaches outlined in Table 6. The Apollo and post-Apollo space programs have established most of the knowledge (scientific and programmatic) necessary to define a shortterm effort for resolution of technical questions, defining detailed implementation plans and costing the endeavor. The sun will not be depleted for several billion years whereas the North Slope will be depleted in short order. Obtaining solar power in space for use there and on Earth must receive vigorous study support. It is the only approach we have for power that provides new net gain, independent environmental control from off Earth, and, most important, can in principal be constructed very rapidly. Space systems using lunar, and eventually asteroidal materials, can be extremely efficient in the use of terrestrial matter, skills, energy, and C + C+C. Manufacturing complexes to build SPSs could be capable of producing many other products, including other manufacturing systems. SPS and space manufacturing could create many new options. The challenge is to start small, make most of our mistakes early on a small scale, and grow fast. The moon is the key. 6. THE MOON: WAYS AND MEANS It is difficult to remember how deeply ignorant we were of the moon, our nearest and highly visible companion in space, only 15 years ago. In only 10 years the lunar scientific literature has exploded by over 300,000 pages. The 30,000 pages of annual “Proceedings of the Lunar and Planetary Science Conference” (Pergamon Press; 1971) provide one plane of entry to this new knowledge. Key organizations such as NASA and the Lunar and Planetary Institute (3303 NASA Road 1, Houston, TX 77058; a part of the Universities Space Research Association) can assist in many ways to bridge the gaps in Fig. 4. Small summer studies, supported tentatively by NASA, have explored limited aspects of pushing industry off Earth. There are many references in this paper also (67). Sufficient consideration has been given to industrializing space to feel some confidence in the “Fundamentals” listed in Table 7. We hope many others will consider them and publish their own thoughts. The moon is relevant for several reasons. It appears possible both in theory and in practice to remove matter (raw or cultivated) from the moon at nearly the theoretical minimal energy cost (gravitational escape limit), which would be 0.01 $/kg for 0.05
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