of Mars has been proposed, as has generation of a carbon monoxide-oxygen propellant load from CO2 in Mars' atmosphere. Water on or beneath the surface of Mars might also be dissociated for propellant use. Studies have also shown that resources obtained from the Moon or the asteroids could be delivered to Earth orbital locations at much less energy cost than required to lift them out of Earth's deep gravity well. Liquid oxygen, worth a few cents per kilogram on Earth, is worth thousands of dollars per kilogram once delivered to low Earth orbit at today's space transportation costs. Transport energy costs works against the idea of economic import of extraterrestrial products to Earth. Scientific data is of course an exception, and past studies have considered another commodity that can be transported by electromagnetic means: energy. The specific value per unit mass of electromagnetic energy (mass obtained by m^E/c1) is very high: a billion dollars per kilogram, and transport cost is not an issue. Special products made more easily or better in the low gravity and high vacuum of space—electronic crystals—or products having high value because of their origin— space-manufactured jewelry—are candidates. One cannot entirely discount the possibility that space settlers will eventually develop special high-value products exportable to Earth because the technology to make them does not exist on Earth. There is a possibility that some high-value raw materials (noble metals are comparatively plentiful in asteroidal nickel-iron) may eventually be imported economically from space. Economic Analyses To make economic analyses, we need economic models appropriate to the different ways of using extraterrestrial resources. There are three models of interest: (1) cost reduction for space mission activities being carried out for reasons other than obtaining extraterrestrial resources; (2) economic payback where a principal reason for the mission is obtaining extraterrestrial resources, e.g. for use in a space transportation operation; and (3) economic payback where export/import trade is necessary to achieve economic self-sufficiency of a space settlement. In the remainder of this paper I present a case study using each model. Economic Case Studies Case Study 1: Use of Lunar Resources to Enhance a Lunar Base. Lunar base logistics operations are described in Woodcock (1986), for base sizes from 6 to 1000 people. Use of lunar oxygen for refuelling transportation vehicles was shown significantly to reduce Earth launch logistics requirements. In the same reference, it is shown that buildup of large bases is mainly constrained by the magnitude of the logistics task of delivering capital facilities from Earth. Production of lunar oxygen can be evaluated on a simple payback basis, while capitalizing on indigeneous resources may be regarded as more or less enabling for the establishment of permanent human presences of more than a few tens of people off Earth. A summary of needs and estimating factors is presented in Table I. Use of lunar oxygen in a simple refuelling mode reduces Earth launch requirements for lunar base logistics by about half. For a small base (6 to 30 people) in a buildup phase, the lunar oxygen production requirements are roughly 250 T/yr, and the savings somewhat greater. From Table I, the mass of a production facility for 1000 T/yr, including nuclear power supplies, is about 200T. The mass for 250 T/yr was not estimated but would be about half that for 1000 T/yr. The payback period is very
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