imately a factor of 2 to 3 at vertical illumination. Remember that most terrestrial power is presently consumed above 30° latitude. Typically SPS would view its rectenna at a considerable angle off its local vertical. The intensity distribution of a LPS beam can be electronically rotated about its propagation axis so as to follow the changing aspect of a rectenna on Earth as the Earth rotates. As will be seen later the costs of power will essentially be determined by the cost of building the rectennas on Earth and the intensity at which power is received. Power costs would decrease as beam intensity is increased. A given rectenna can receive power at low or high intensities within wide limits. The larger the antenna, the more power received. Rectennas in remote areas (offshore, in deserts) would be able to provide several GWs per km2. As will be discussed later LPS provides more latitude than SPS to minimize the costs of rectennas by increasing the frequency of the microwave beam. LPS should be able to provide power from a rectenna at significantly lower costs than the nominal 50 mills/kw-hr estimated for SPS. It is conceivable that the stray microwave power of a LPS beaming over 10,000 GWe could be less than a few billionths of a watt per square centimeter (few nw/cm2) at Earth. This is significant because the human body radiates incoherent power in the microwave region at a level of several hundred nw/cm2 (35). If such low stray power levels can be attained then there is no biological risk from exposure to low level radiation outside the power beams. It may be possible to provide economic power via LPS at beam intensities so low that heating of animals is negligible. These possibilities require careful study and are certainly amenable to analysis. There are several ways to insure that an excessive power density cannot be focused at any point on Earth. This could be via construction restrictions on the moon (limit aperture size, limit beam slew, many power limited stations, build restrictions into the electronics) and possibly by natural limits due to ionospheric interactions. Power collection can be efficient on the moon because of several fundamental factors. The natural lunar environment is appropriate to the construction and longterm operation of very thin devices to convert diffuse sunlight into electricity. There is no air to produce storms or degrade solid state circuits. There is no water to hit the devices as rain or as a vapor to dissolve and corrode films only a few tens of microns thick. The native soil has the consistency of face powder, is easy to work, and can be formed into glasses, ceramics and agglutinates [23], Lunar glasses are stable in the lunar environment for millions of years. The soil is an excellent electrical resistor, thus power is not drained away from the collection circuits. The soil can protect circuits from impacts, seismic vibrations (a big moonquake produces ground motions of only millionths of a cm), extremes in temperature between night and day and against radiation. The native lunar dirt can be formed into ceramic or glassy substrates upon or within which to form thin film and very large scale integrated circuits. By converting several areas totaling about 10,000 km2 into advanced power conversion devices, the entire Earth could be supplied with far more power than it now uses. A lunar system would require very little maintenance. All components could be progressively upgraded as technology improves without shutting down the power flows. Figure 3 is from a recent NASA study of a return to the moon. The figure is taken directly from work by Drs. Waldron and Criswell. It shows an astronaut inspecting the operation of an automatic tractor about the size of the rovers used by the Apollo astronauts on the moon. The tractor is plowing up the top 10 cm of the lunar soil. The lunar soil has about 0.4% by weight of tiny blebs of metallic iron left over from
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