cave reflector, continuous or faceted, focussing the sun’s rays at the mouth of a cavity absorber about 70m in diameter. The turbines and generators are mounted outside the absorber, together with the radiator, which is arranged edge-on to the sun. The most important feature of this type of system from a visibility standpoint is that the bulk of the area (the reflector) has an extremely low diffuse reflectance and that the specular reflection, being directed to the absorber, cannot strike the earth. The next largest component, the radiator, is located so as to present minimum area to the sun, coincidentally minimizing reflected sunlight. The absorber and turbomachinery are relatively small in overall dimensions and will probably be sufficiently irregular in shape to reduce substantially any reflection. It has already been seen that the truss-type structure has very low total luminosity because of its small effective area. It may be concluded, then, that a thermal cycle SPS can be expected to have considerably less luminosity than the photovoltaic reference systems. This does not consider the transmitting antenna, which would be the same in either case. 8. CONCLUDING REMARKS Diffuse reflection of sunlight will give each SPS the visibility of a bright planet for most of the night. With the reference configurations, this appears to be unavoidable. With the exception of optical astronomy, the diffuse reflection does not seem to present any major problems. Specular reflection can also occur from two sources (three for the CR2 reference configuration) for brief periods. These may have a potential for retinal damage to telescopic observers. However, all of the specular reflections can be prevented from striking the earth by one means or another if further research should show it to be necessary. In short, although the visibility of the SPS may be an inconvenience, it will not be a hazard. Even the inconvenience could, if necessary, be mitigated by use of a thermal conversion system instead of photovoltaic. Acknowledgement — The editor wishes to thank Dr. Harold Liemohn and Professor Harlan J. Smith for their assistance in reviewing this paper. REFERENCES 1. Satellite Power System Reference System Report, U.S. Department of Energy and National Aeronautics and Space Administration, Washington, D.C., DOE/ER-0023, pp. 12-37, October 1978. 2. J.L. Cioni, Johnson Space Center, private communication, February 9, 1979. 3. H.H. Koelle, Ed., Handbook of Astronautical Engineering, McGraw-Hill, New York, pp. 8-11 and 8-12, 1961. 4. P.A. Ekstrom, Battelle Pacific Northwest Laboratories, private communication, April 26, 1979. 5. G.R. Woodcock, Solar Power Satellite System Definition Study — Final Report, Boeing Aerospace Co., Seattle, Washington, D180-22876, December 1977, contract NAS9-15196. 6. G. Hanley, Satellite Power Systems Concept Definition Study — Final Report, Rockwell International, SD78-AP-0023, April 1978, contract NAS8-32475.
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