Asri-unix.894
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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
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