5 December 1975, Volume 190, Number 4218 SCIENCE Space Colonies and Energy Supply to the Earth Manufacturing facilities in high orbit could be used to build satellite solar power stations from lunar materials. Within this century it may be feasible to establish manufacturing facilities in space, possibly in the vicinity of one of the Lagrange libration points of the earth-moon system (/. 2). Near two of these points, called L4 and LS, there are orbits which are stable under the combined gravitational effects of the earth, the moon, and the sun. A space manufacturing facility (SMF; the terms space community and space colony have also been used to describe such a facility) would be a self-sustaining habitat for a large number of people (of the order of 10' to 105). Its energy needs would be met by solar power, used directly as sunlight for agriculture, as process heat for industry when concentrated by mirrors, or indirectly as electricity. The SM F may be economically more effective than alternative industries on the earth for the construction of products whose end use would be in geosynchronous or higher orbits. Such products, if made on the earth, would have to be lifted by rockets out of the earth's gravitational potential well, which is about 6500 km deep. In contrast, the SM F would obtain the raw materials for its products from the surface of the moon, whose gravitational well is only I/ 20 as deep. As a consequence of the moon's vacuum environment, and of that factor of 20 in energy, a launching device located on the moon could transport material to the SM Fat low cost relative to shipment from the earth. In this article I suggest that solar power stations may be con5 DECEMBER 1975 Gerard K. O'Neill structed at a space colony, and relocated in geosynchronous orbit to supply energy to the earth, at a lower cost than if such stations were to be built on and lifted from the earth. Energy Needs The increasing demand for electricity, the shortage of fuels on the earth, and concern about widespread use of nuclear energy have led to consideration of satellite solar power stations (SSPS's). Glaser (3) has studied the SSPS concept, which is the location in geosynchronous orbit of stations converting solar into electrical energy, to be sent down as microwave power for conversion to direct current or to a power line frequency at the earth's surface. In 1975 the Energy Forecast Working Group of the Institute of Electrical and Electronics Engineers (IEEE) summarized forecasts, by 12 organizations, of the electric generating capacity which will be required by the United States during the years 1975 to 2000 (4). The IEEE summary estimated an increase from about 500 Gw in 1975 (5) to a required capacity of 781 to I070 Gw in 1985, and to a capacity of 1880 to 2250 Gw in 2000. The IEEE estimates therefore correspond to an average construction rate of new generator capacity of about 65,000 Mw/year in 1990 and 115,000 Mw/year in 2000. A study by Associated Universities, Inc. (AUi), predicted a demand for 85,000 Mw/year of new capacity at the turn of the century (6). The discussion that follows is not sensitive to such differences in the estimates. At current prices [typically $450 per kilowatt installed for a coal-fired plant (7)] the forecasts therefore correspond to a market of $30 billion per year in the United States alone in the year 1990, and $40 billion to $50 billion per year a decade later. [The dollar figure may be conservative; the AUi study (6) was based on the assumption that most of the increased capacity during 1985 to 2020 would be powered by nuclear reactors, with higher installed costs (7) of $600 to $1800 per kilowatt in 1972 dollars.] Economic selfinterest would tend to enlarge the market to the wider range beyond national borders; assistance to developing nations in the form of electrical energy would also increase the total production requirement for new power plants. Environmental Effects Each method so far considered for power generation has characteristics which are potentially damaging. Nuclear power produces radioactive wastes and materials convertible for use in nuclear weapons. Coal-fired plants require extensive stripmining to keep them supplied. In the year 2000, electric generation for the United States alone will require the mining of more than 2 x I 09 tons of coal per year, unless alternative sources provide most of the energy needed at that time. Transmission to the earth of the energy generated by an SSPS would require a microwave beam to a central antenna. That may be less desirable environmentally than the high-voltage lines used conventionally at the surface of the earth for the interconnection of large generator plants. Microwave transmission may, though, be more acceptable than the alternatives of nuclear power or strip-mining, and that is an important issue which should be studied carefully. Glaser (3. 8) has stated that the microwave beam intensity outside the antenna site would be low enough to satisfy stringent environmental requirements. Because the conversion of microwave energy The author is professor of physics at Princeton University. Princeton, New Jersey 08540. 943
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