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Startup Says It'll Use Huge Space Mirror to Sell Sunlight During Nighttime (futurism.com)

submitted 2 months ago by [email protected] to c/[email protected]

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cross-posted from: https://lemmy.world/post/19446697

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[–] [email protected] 4 points 2 months ago (1 children)

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.

[–] [email protected] 2 points 2 months ago (1 children)

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.

[–] [email protected] 3 points 2 months ago

The 9km mirror I'm referencing is for a sunlight level of illumination; the moonlight mirror needs only be 14m in diameter (or 500m for geostationary orbit).