Characterization of Reflected Light from the Space Power System H. B. Liemohn, D. L. Tingey, D. H. Holze, and B. R. Sperber Boeing Aerospace Company, Seattle, Washington Limited terrestrial energy sources have led to investigation of Space Power Systems that would collect solar energy and beam it via microwaves to power stations on the ground. The Baseline System^ consists of a Staging Base (SB) in low earth orbit (LEO), a fleet of Orbit Transfer Vehicles (OTVs) for movement of supplies from LEO to geosynchronous earth orbit (GEO), and assembly and operation of Solar Power Satellites (SPSs) in GEO. All of the structures would be very large in comparison with today's satellite sizes, and include large plane surfaces to collect solar energy. Due to the enormous size of these spacecraft and their assembly vehicles, they may be viewed routinely by large numbers of ground observers. The brightness of sunlight reflections off various components changes markedly as the vehicles rotate along their trajectories. Many surfaces will undoubtedly be coated with optically diffusing material, but the present baseline configurations also include large flat areas that are specular such as glass, polished metal, and smooth composites. Owing to the large size, relatively low altitude (at LEO), and/or specularity, some reflections will be exceptionally bright. The level of ground illumination and particularly the concentration of radiant energy in observer's eyes needs to be assessed. For the most part, reflections will appear to ground observers as very bright starlike points of light in relatively dark night sky. Since contraction of the iris is controlled by overall illumination levels, the eye pupil may accept more light energy than desirable from these point sources, and produce abnormally high image irradiance at the retina. If the brightness of baseline vehicles exceeds accepted limits for eye safety, certain constraints on reflectivity of surfaces and the orientation of vehicles are the most likely procedures that would lower ground illumination. This study reported in detail elsewhere^ has evaluated the components of the various Space Power System vehicles as presently defined to determine the reflectances which will significantly contribute to the ground illumination. The calculation of reflected solar intensity from the various satellite system elements requires description of the elements and description of the geometry of potential reflectance paths. To reduce the calculation to a tractable problem only the nominally flat element surfaces were considered since the curved surfaces spread the light making their contribution negligibly small at the large orbital distances. Each surface is further defined by its approximate reflectivity and an estimate of its "flatness". In addition to determining that a surface is likely to reflect a significant intensity, it is also necessary to determine the conditions under which it will illuminate a portion of the earth. The orientation of each reflecting surface is therefore also necessary, so a number of convenient coordinate systems have been used. The SPS at GEO has 55 km^ of glass covered solar cells that are oriented normal to the sun, as well as a 1 km^ microwave antenna. Transportation of construction materials from LEO to GEO requires OTVs that have 1.6 km^ solar panels oriented normal to the sun during their 6 month transits. The SB at LEO, that accommodates OTV fabrication and cargo transfer, consists of 0.5 km arms protruding from a .44 knr open grid aligned with its orbit plane. In determining possible ground illumination geometries, two cases are considered: 1) the reflecting surface rotates in orbit such that its orientation to the earth is constant (e.g., the satellite antenna), and 2) the orientation of the reflection surface to the sun is constant (e.g., the satellite solar arrays).
RkJQdWJsaXNoZXIy MTU5NjU0Mg==