SPS EFFECTS ON OPTICAL ASTRONOMY G. J. Schuster Pacific Northwest Laboratory, P. 0. Box 999, Richland, WA 99352 Some of the passive properties of SPS are likely to have a negative impact on optical astronomy. Principally, SPS may increase the brightness of the night sky. The earth's atmosphere is the medium for the damage done to an astronomical observation by a bright object in the night sky. Although the cloudless sky is mostly transparent at optical frequencies, the atmosphere scatters some Light. Although observatories are located where meteorological conditions are such that “seeing” is good, observations of faint objects are currently limited by interfering light. When this light is collected by a telescope, it constitutes a noise signal and must be subtracted by observing a nearby section of “blank” sky. Any increase in sky brightness over the natural background results in a proportional reduction in the effective aperture of a telescope being used to observe faint objects. Each solar power satellite is oriented so that the solar cell array approximately faces an observer at the subsatellite point at local midnight. Using this situation and assuming a Lambertian pattern for the scattered light, the expected illuminance at the earth's surface, expressed as a fraction of noon sunlight is aXlJir = 1.38 x 10"8a where a is the diffuse albedo and S2s is the solid angle subtended by the satellite. Using an estimate for the diffuse albedo of 4%, a satellite would be as bright as the planet Venus ever is. SPS satellites would be the third brightest objects in the sky, only the sun or moon would be brighter. The set of 60 satellites would have the illuminance of the moon halfway between new and quarter phase. For a 60-satellite system, the sky brightness may be doubled in a region 10° in declination and 70° in hour angle. The sky brightness may be increased by 10% over a region that covers half the night sky. If this is the case, due to the geostationary orbit of the satellites, a significant number of faint objects will not be able to be observed. Finally, it must be noted that the development of the technology required for SPS should make it easier to construct and maintain space telescopes. It is widely recognized that a great deal of the future of astronomy will depend on developing space astronomy beyond current and planned levels and that some kinds of astronomy can only be done from space.
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