A solution to these problems is the transfer of the power generation system to outer space. First proposed in 1968 by Peter Glaser in the journal Science, such a system could take maximum advantage of solar radiation. Sunlight converted to electrical energy by photovoltaic solar cells could be beamed to Earth on tightly directed microwaves. Stationed in geosynchronous orbit 35,800 kilometers above the Earth, Solar Power Satellites could be as large as conveniently practicable and would not be affected by either the intermittent sunlight problems of Earth's surface conditions or the ravages of atmospheric erosion and corrosion. On Earth, the power received would be clean electrical power distributed through the international power grid, or available for the electrolysis of water into hydrogen - another clean source of power that can be used for remote, portable or mobile applications where it is awkward to use electrical power. The design problem is how to construct large scale satellites in space. Even with reduced requirements for structure and protection made possible by deployment in space, great amounts of material must be amassed for each satellite - amounts beyond the logistical capacity of chemical-based rocket systems. Lifting such large masses from Earth to orbit is a forbidding and costly task - twelve times the cost of moving the same mass of materials from the moon to geosynchronous Earth orbit. There are other advantages to working in space. The construction of large, pure crystals of silicon for photovoltaic cells can be done with relative ease in a micro or zero gravity environment. Construction in space also benefits from lack of gravity - structures need only maintain form and resist inertial forces. Low-mass, omnidirectional, tension/compression structures can replace the kinds of materials- intensive, weight-supporting structures used on Earth. Space Station Over the past twenty years, planners in the United States and the Soviet Union have devoted considerable discussion time to the sequence of events to be followed in the next phases of manned space exploration. A particular point of contention in the discussions has been the value of a space station and its potential role as an Earth-oriented experimental/commercial laboratory, a way station for lunar colonization or a staging base for voyages to Mars and beyond. Project Phoenix projects a role for a space station most similar to the latter two roles, in which it serves as a transfer station between Earth to Low-Earth-Orbit (LEO) operations and LEO-to-Moon operations. Gravity-imposed costs of lifting materials from Earth to LEO require a reusable vehicle such as the space shuttles now used by NASA and soon to be deployed by the USSR. Future versions of this vehicle may be hypersonic "aircraft" able to fly to LEO, but they will continue to be specialized vehicles adapted to high-gravity/ low-gravity operations. For travel between LEO, lunar orbit and geosynchronous orbit (where the Solar Power Satellites will be positioned), another type of space ship will be more efficient - a lunar shuttle. Transfer between Earth shuttles and lunar shuttles will be best conducted at a space station in LEO.
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