liftoff weight were proposed to lift the components into orbit. Very large engineering would be required. The scale of this engineering effort encouraged O'Neill (27) to suggest obtaining the raw materials for SPS from the moon and building the units in space. A much smaller scale of engineering is involved. This can be realized by noting that if hydrogen and oxygen were electrolytically combined in a fuel cell and the energy used to eject 100,000 tons from the moon, only 13,000 tons of water would be produced. This corresponds to a volume of water 10 by 36 by 36 m on a side or 150 times less than for launch from Earth. If components of the SPS are made on Earth then it is necessary to design them so they can survive the environment of Earth prior to launch (humidity, oxygen, packaging), launch from Earth to space (vibration, changes in conditions), assembly in space (more handling) and slow transport through the radiation belts to geosynchronous orbit. All these steps add greatly to the costs of the system. In both the MIT (18) and General Dynamics-Convair (2) studies it was found that a much lower level of industrial activity was required on Earth and in space if the production of most components (90%) were done in space. Parts and their production could be simplified. Bock (2) concluded that over 90% of an SPS could be constructed from lunar derived materials with no major changes to the SPS reference design. MIT contended that most of the factory to make SPS could be made of lunar derived materials. Waldron et al. (36) concluded that chemical engineering approaches are reasonable to develop for providing a wide suite of engineering materials from the raw lunar soils. Most of Grand Coulee dam was constructed of local materials. It was not necessary to construct the Columbia River Basin before building Grand Coulee. We argue next that the moon is the “solid state'' equivalent in the 21st century for directly obtaining solar power as were the river valleys of Earth for indirect solar power in the 19th and the early 20th centuries. We argue further that most of the materials of construction can be found on the moon if a modicum of design flexibility is utilized. II. LUNAR POWER CONCEPT As noted earlier, the moon contains most of the materials required to build an SPS. In addition, the moon always turns the same face toward the Earth. Small fractions of the lunar surface on the east and west limbs (visible edges) of the moon can be transformed into solar collectors and transmitters. Therefore, power can be transmitted from one or the other of the stations, except for a definable 1 to 2 day period about new moon, to stations on the moonward side of the Earth. Figure 2 schematically illustrates power beams from the moon being directed to rectennas on Earth. Rectennas could be 6 to 15 times smaller in area than a terrestrial photovoltaic array of equal average output. At first, power would be received from the moon approximately 8 hr a day and excess energy stored. However, as the EPS grows, microwave reflectors can be placed in high inclination orbits about Earth. In this manner power can be distributed about Earth with the same efficiency that communications satellites redirect telephone calls. LPS with reflectors permits the worldwide distribution of power while also following the diurnal power needs of any given location. SPS at best could only service the base load power needs of along a line of longitude. SPS could service equatorward regions better than high latitude regions where most of the world power needs are located. LPS reflectors can service all the world.
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