Some potential problems associated with microwave energy transmission might be obviated by using lasers to transmit power to earth. Potential benefits of the laser transmission system currently under study include [ 1 ] smaller land area requirements for the receiver (the diameter of the laser beam is about 10 m), [2] elimination of radio frequency interference, and [3] reduction of biological and ecological impacts. The feasibility of the SPS concept is enhanced because of two or more options for each of the critical subsystems including • Conversion of solar energy to electrical energy (photovoltaic and thermodynamic cycles); • Conversion of electrical energy to radio frequency energy (klystrons, magnetrons, and solid-state); • Power transmission to earth (microwaves and lasers); • Transportation (shuttle derivatives, heavy lift launch vehicles, chemical and electrical orbit transfer vehicles); and • Space construction and manufacturing (earth-based, non-earth-based, or combinations thereof). Environmental Assessment Exposure of humans to SPS microwaves at higher power densities approaching existing and projected standards, should they occur, will be infrequent and will involve small numbers of individuals. The major uncertainty regarding human health involves the potential long-term or late effects of continuous exposure of large general populations to very low-power densities. Relevant experimental data are scant, but definitive experiments to reduce the uncertainty are being designed. The main impacts of SPS construction and operation are likely to arise from the required large-scale extraction, transportation, manufacturing, and construction activities. The greatest uncertainty regarding potential human health impacts is in the area of space operations. Data supporting assessments, now in progress, appear to be adequate to both define the potential hazards of ionizing radiation in space and zero gravity as well as to provide the bases for developing the requisite space construction strategies. Existing information appears to exclude the possibility that effluents from the SPS space transportation systems will cause significant modifications of local or regional climate. The data indicate that compliance with existing laws and regulations regarding air and noise pollution is possible. One of the major uncertainties is the magnitude of ionospheric disturbances produced by effluents from the SPS transportation systems and the magnitude and nature of potential telecommunications and climatic impacts associated with such disturbances. Additional data to support an adequate assessment of the impact of ionospheric disturbances will be obtained prior to June 1980. Because of the power density of the SPS microwave power beam and the non- negligible power in the side lobes, the potential exists for impacting other users of the radio frequency spectrum, either through ionospheric disruptions caused by microwave heating or by direct coupling with SPS microwave energy in the main beam and in the side lobes. Direct testing of the impacts of ionospheric heating on telecommunication systems is underway. Preliminary experiments, that heat the D- and E-layers for long periods to ensure steady-state ionospheric temperatures, show that telecommunication impacts are minimal for power densities of 23 mW/cm2, the presently prescribed upper limit for the SPS power beam.
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