cross-posted from: https://lemmy.world/post/19446697
It’s certainly a stupid idea if your trying to illuminate at the suns level, but if you wanted an area to have permanent moonlight? Not so unreasonable.
The moon is 400,000 times dimmer, so 1km^2 of mirror, which is ridiculous, could illuminate an area the size of Germany.
New York metro area is 12,000km^2, which would only need a mirror 173m on each side.
Actually might not be a bad tourist attraction for a crazy city, permanent artificial moonlight.
Granted, but if moonlight level is all you want I can already illuminate my surroundings to that benchmark with the flashlight in my pocket. We don’t need to park shit in space to accomplish that. And as a matter of fact, we already tried the “illuminate the entire town like the moon” model in the past as well. It turned out that even on a terrestrial scale it wasn’t actually a great idea because, you know, people in the vicinity who want to maybe turn it off… can’t. (Except in the latter case, maybe with the aid of a rifle.)
I am positive this is just an investor scam of some kind. If anyone is actually stupid enough to launch anything towards this end, it’s a mathematical certainty that they will be murdered in the street by either an amateur astronomer or a chiropterologist. It’ll be a toss-up who gets to him first.
Some calculations:
In a 1000km orbit, you’ll need a mirror about 9km across to appear 0.5° in diameter from the ground (the same size as the Sun), and therefore light up an area with the same illumination as the Sun.
Note that you can’t make due with a smaller mirror focused to a tighter area, as the brightest thing the mirror can reflect is the Sun, and so it must appear at least as large as the Sun in the sky to illuminate any point on the ground by the same amount.
With the much dimmer goal of moonlight illumination levels, the mirror shrinks to 9km / sqrt(400,000) = 14.2m in diameter, which is actually rather reasonable. However it would only illuminate an area 0.5° wide from the mirror’s point of view, or around 9km. And because the mirror is orbiting at 7.4km/s, you’d only get a second or two of illumination.
TLDR: Moonlight mirror 14m across, could light up a 9km diameter area for a little over a second.
Edit: In the case of a permanent mirror in geostationary orbit, a 500m mirror could provide moonlight illumination to an area around 300km in diameter.
While it would be cool for it to appear the size of the moon, it is not necessary with a shaped mirror.
You can keep the same size in a higher orbit, maybe even geosynchronous, then sync the rotation of the mirror to keep it pointing in the same spot on earth.
Granted a shaped mirror that size would be much harder to put into orbit than a flat mirror.
