Asri-unix.894 net.space utcsrgv!utzoo!decvax!ucbvax!sri-unix!Lynn.ES@PARC-MAXC@Berkeley@CCA-UNIX Thu Mar 4 14:01:37 1982 Re: Orbiting reflectors About Will Martin's reflector question: In order to double the solar radiation, the mirror would have to intercept the same amount of sun's radiation as the city does, and therefore be the same size as the city. For any continuity of service, it would have to placed in geosync orbit, would have to rotate once every two days because the sun's direction is changing as seen from geosync, and it would have to be reflective on both sides. As the mirror approached edge-on every noon, the output would drop off substantially for a few hours. Very far northern cities would have a permanent similar problem because the city lies inclined. An hour or so of eclipse would occur around midnight during fall and spring. A 20 mile mirror would appear, at geosync distance, ten times smaller than the sun, but 100 times brighter per unit area (if the full output is focussed on 20 miles of earth). This would burn a few photographers shutters and may cause instantaneous blindness if viewed. It's shadow would be spread out over 200 miles, and be hardly noticeable, and would move across almost half the equator every mid-day. The size of the light spot where it hit the earth would also spread out to 200 miles, unless the mirror were optically shaped to focus the light. In fact, I don't see how to prevent spread without a compound optical system (two or more mirrors that demagnify the image of the sun as projected onto the earth). This brings up problems of maintaining precise shape of an object miles across. Low orbit would limit the spread problem to a few miles, but then the shadow would be objectionable, the mirror would have to rotate fast and irregularly, and a given city could only get light for several minutes out of every revolution (1.5 hours). Also, no night-time operation, as the mirror would be in shadow. It might still be effective if the mirror gave as little as 10% of sunlight, so you can scale the mirror down by sqrt(10) to get a sort of lower limit. Or you could scale up the mirror until it was comparable to the spread size and get whole states rather than cities. Any way you do it, the mirror is a pretty sizable task. /Don Lynn ----------------------------------------------------------------- gopher://quux.org/ conversion by John Goerzen of http://communication.ucsd.edu/A-News/ This Usenet Oldnews Archive article may be copied and distributed freely, provided: 1. There is no money collected for the text(s) of the articles. 2. The following notice remains appended to each copy: The Usenet Oldnews Archive: Compilation Copyright (C) 1981, 1996 Bruce Jones, Henry Spencer, David Wiseman.