Fig. 1. Principal sketch of a solar mirror in space, rotating in a geostationary orbit around the Earth. cally the same size as the illuminated object are required. This makes necessary the use of very thin reflecting layers to keep the total mass of the mirror still in a reasonable order of magnitude. Due to the required low weight, mechanical strength and elasticity, and for easy construction in space, organic foils, coated with reflective metallic layers may be regarded as adequate. Of course, this large foil area will have to be supported by structural elements. These foils have been commercially available already for more than a decade, although of course only in smaller quantities. As an example, for the following calculations we chose a foil of polyethyleneglycolterepthalat (PETP) with the net composition (C10H8O4)n. These foils are known as “Mylar,” “Hostaphan,” etc., and are produced by companies like Bayer, Du Pont, etc. For condensor production, 2 /zm thick foils coated with (100 ± 35) A Al on one side (i.e., 190 /zg/cm2 weight and 2-4 fl/cm2 resistance) have been available since the sixties, their dimensions being already close to the requirements for space applications. (In our case, the Al coating has, however, to cover both sides of the foil.) Taking the condenser foils described above as the base of our future considerations, we see that a mirror foil of 10 km diameter, necessary to illuminate the center of an average metropolis such as Berlin, would weigh approximately 150 tons. Structural elements and installations for orbit control, stabilization and focal length adjustment may increase the mirror's total weight by at least one order of magnitude. This means that the installation of such a large space mirror could be accomplished with typically 50 flights of the Space Shuttle and the subsequent booster (for getting the material into the geostationary orbit) within one or a few years. Other applications of interest for foils in space are found in the wrapping of electronic components of spacecrafts or satellites for temperature control (11). MODIFICATION OF THE MIRROR FOIL IN SPACE — TRIM RESULTS In space, the mirror is subject not only to irradiation by electromagnetic waves (IR, visible light, UV, etc.), but also to the particle stream coming from the sun. The latter is known as “solar wind,” consisting of approximately 2x 10s H+/cm2s (96%), of about 6x 1011 a particles/cm2s (3-4%) and a few 105 ions/cm2s of higher mass with average velocities of about 400 km/h (1) (corresponding to energies of 0.85, 3.4, and
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