solar energy for use on Earth (11). He suggested construction of solar power satellites (SPS) in geosynchronous orbit (GEO) about Earth. Each SPS would be approximately 12 by 20 km on a side and 100,000 tons in mass. SPS would convert sunlight to electricity, electricity to microwaves and then beam approximately 10 GW of microwave power to a few large receivers on Earth. It was suggested that many hundreds of SPSs could be placed in GEO. Both the engineering and physics of the microwave systems, termed phased array radars, are very well understood. Segmented reflectors have been used in receiving antennas and reciprocity obviously applies to their use in transmitters (31,32). Technology in this field is still advancing rapidly as solid state generators and controllers are developed for the needs of communication and radar systems. Approximately 30 million dollars was spent by NASA, the Department of Energy (DoE) and aerospace companies in the 1970s to study the SPS concept (14). It was concluded in an independent review by the National Research Council (1981) (24) and the Office of Technology Assessment (1981) (26) that the project was technically feasible but that the expense (near one trillion dollars) and time required to establish somewhat less than 1,000 GW of SPS capacity precluded the program. Additional research was recommended but not funded even though SPS would deliver net new energy to the Earth with little, if any, pollution. Studies conducted in the late 1970s at the Lunar and Planetary Institute (16), Mass. Inst, of Technology (Miller, 1979), (18) and General Dynamics-Convair (2) revealed that over 90% of an SPS could be made by factories deployed from Earth to the moon and geosynchronous orbit. Program and production costs were predicted to be competitive with SPS deployed from Earth. MIT researchers concluded that most of the factories could be made from lunar materials. SPS was a truly revolutionary concept. It would have required operations on Earth and in space thousands of times larger than our 1980s space program. On a mass basis Space Solar Power stations appear very favorable compared to terrestrial options. A 10 GWe satellite would have a mass of 100,000 tons. Compressed to the cross-section of Grand Coulee this would correspond to a strip only 3 m wide. We will argue shortly that the moon provides the 21st century “solid state” equivalent to the “Columbia River Basins” of the 20th century on which to construct solar power systems. On the moon the functional equivalents of these 3 m strips can be installed using machines with which we are relatively familiar from experience on Earth and with Apollo equipment on the moon. Each SPS was to transmit microwave power 36,000 km from geosynchronous orbit to a ground station (possibly a few) generally below it without any appreciable loss of power. The transmitting antenna in space was to be about 1 km in diameter and the receiver (rectenna) on the ground about 10 km in diameter. The mass of the rectenna could be divided into two types. Most would be formed into millions of simple concrete posts weighing 2,000,000 tons (about the same as a coal fired power plant). They would support millions of simple antennas, similar in complexity to home TV antennas, connected to diodes and power collection wires weighing a total of 5,000 tons. The ground system would be completely open and individual components could be repaired while the system was in operation. Such stations could have a very long life time, be very tough, and be simple compared to terrestrial photoconversion systems. The primary public concern regarding SPS was exposure to stray microwave energy outside the main beam and exposure of animals to energy of the main beam. Power cost from SPS was projected to be near 50 mills per kw-hr. Space solar power systems could efficiently tap into the fusion reactions of the
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