cal studies, and several reviews by academic- and aerospace-contractors funded by agencies of the U.S. government. These studies have all concluded that, at least in the long run and possibly even quite early, the construction of these satellites can be accomplished at least cost and with minimum adverse environmental impact by using raw materials obtained from the surface of the Moon and refined in space using solar energy. The National Science Foundation, in close cooperation with the National Academy of Sciences, is now undertaking still another independent review of this possibility under mandate by the U.S. Congress. Still other studies during the past 10 years, particularly in the United States, recently substantiated by reviews carried out in other countries, now make it virtually certain that in the long run the vast energy and material resources of space will be of far greater importance to humanity than will all the resources available within the biosphere of Earth. It has become clear that these resources can be put to use most easily in space itself, creating and supporting orbital factories, agricultural establishments, and human habitats. Such a development, however, probably represents an ultimate rather than an initial state. Although the highly-visible American-Russian race for the Moon began as a peaceful expression of technological capability and national power, it is fortunate that the first human landing on the Moon's surface was followed by a series of flights whose purpose was scientific. These expeditions returned to scientists in many countries on Earth an enormous wealth of data, much of it still under analysis, far more abundant, quantitative, and accurate than all prior suppositions about the Moon up to that time. We learned, for example, that random lunar soil samples from the six sites visited contain an average of 30% metals by weight (including iron, aluminum, magnesium, and titanium); 20% silicon (a material from which photovoltaic solar cells can be made); and 40% oxygen (perhaps the most vital element for human survival, being the essential component of our atmosphere and making up nearly 90% of the weight of ordinary water). In retrospect, we are coming to view the greatest value of the lunar explorations to be geological prospecting for resources. Earth approaching asteroids have also been suggested as a competitive source of raw materials for space manufacturing. The available resource is emormous. In many cases, the energy required to transfer an asteroid (or a portion of an asteroid) to a manufacturing site in high orbit around the Earth is comparable to that for raw materials obtained from the lunar surface. The cost for asteroidal materials may be many times less for logistical reasons: due to the very weak gravitational pull of an asteroid, no complex propulsion engines are necessary for soft landings; and solar energy would be available continuously at an asteroid for processing and for propulsion. Studies of the light reflected from some of these asteroids reveal that their chemical compositions may be similar to those of meteorites which have landed on Earth. Some apparently contain large quantities of metallic iron and nickel, while others are rich in carbon, nitrogen, and hydrogen, elements which appear to be very scarce on the Moon. Discoveries of new asteroids over the next few years will allow selection of a variety of raw materials with numerous alternative retrieval missions possible. Techniques for retrieving asteroidal materials have been subjected to a number of engineering studies, which confirm earlier suggestions that the retrieval costs for these materials may become less than $1 per kilogram. The projected requirements for replacing losses of air and water at orbital facilities for the construction of Solar Power Satellites and the projected expansion of agricultural facilities in orbit supplying the construction workers are sufficiently large to suggest that the bulk of the raw materials for these purposes should come from the
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