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In outer space, the sun always shines brightly. No clouds block the solar rays, and there is no nighttime. Solar collectors mounted on an orbiting satellite would thus generate power 24 hours per day, 365 days per year. If this power could be relayed to earth, then the world’s energy problems might be solved forever.

Solar power satellites (SPS) were originally proposed as a solution to the oil crises of the 1970s by Czech-American engineer Peter Glaser, then at Arthur D. Little. Glaser imagined 50-square-kilometer arrays of solar cells deployed on satellites orbiting 36,000 kilometers above fixed points along the equator. A satellite at that “geosynchronous” altitude takes 24 hours to orbit the earth and thus remains fixed over the same point on earth all the time.

The idea was elegant. Photovoltaic cells on a satellite would convert sunlight into electrical current, which would, in turn, power an onboard microwave generator. The microwave beam would travel through space and the atmosphere. On the ground, an array of rectifying antennas, or “rectennas,” would collect these microwaves and extract electrical power, either for local use or for distribution through conventional utility grids.

The technology, as originally envisioned, posed daunting technical hurdles. Transferring electrical power efficiently from a satellite in geosynchronous orbit would require a transmitting antenna on board the satellite about one kilometer in diameter and a receiving antenna on the ground about 10 kilometers in diameter. A project of this scale boggles the mind; government funding agencies shied away from investing immense sums in a project whose viability was so unclear. NASA and the Department of Energy, which had sponsored preliminary design studies, lost interest in the late 1970s.

In the last few years, however, the communications industry has announced satellite projects that suggest the time has come to revisit the solar power satellite idea. By early in the next century, swarms of communications satellites will be orbiting the earth at low altitude, relaying voice, video, and data to the most remote spots on earth. These satellites will relay communication signals to earth on beams of microwaves. The transmission of electrical power with a beam of microwaves was demonstrated as early as 1963, and projecting power and data along the same microwave beam is well within the state of the art. Why not use the same beam to carry electrical power?

The new communications satellites will orbit at an altitude of only a few hundred miles. Instead of hovering above a spot on the equator, low-orbiting satellites zip around the globe in as little as 90 minutes, tracing paths that oscillate about the equator, rising and dipping as many as 86 degrees of latitude. Because they are closer to the earth’s surface, the solar collectors on the satellite can be a few hundred meters across rather than 10 kilometers. And because the microwave beams they generate would spread out much less than those from geosynchronous satellites, the ground rectennas could be correspondingly smaller and less expensive as well. By piggybacking onto these fleets of communications satellites-and taking advantage of their microwave transmitters and receivers, ground stations, and control systems-solar power technology can become economically viable.

Low earth orbit poses its own difficulties, though. Because they whip around the planet so quickly, low-orbiting satellites must possess sophisticated computer-controlled systems for adjusting the aim of the microwave beam so that it lands at the receiving station. These satellites will have to use sophisticated electronic systems, called phased arrays, to continuously retarget the outgoing beam.

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