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It's been proposed to put optical elements in space to reduce sunlight coming to the earth: Realistic cost and feasibility of sun-shading?

That question states that the studies have all hypothesized putting these things at L1. Why? L1 is quite far from Earth, so very little of the sunlight that passes through L1 would ever have gotten to Earth anyway. Wouldn't you get more bang for the buck by putting things in low earth orbit?

[EDIT] The source of my confusion was that I'd been thinking of L1 as the earth-moon L1 point. It hadn't occurred to me that they were talking about the earth-sun L1 point.

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  • $\begingroup$ I just also found this neat picture which claims to be from Discovr which was situated at the earth-sun L1. I can't imagine the size that shield would need to be. Check out discovr/epic, if you never have. Really neat probe. $\endgroup$ – Magic Octopus Urn Oct 20 '19 at 18:45
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Because at L1, you can place a single sunshade. Sun-Earth L1 is stationary between Sun and Earth.

Any object in LEO is not stationary between Sun and Earth. It rotates around the planet in about 90 minutes. So to get continuous coverage, you can't just place one object, you have to fill the entire circle around the Earth with sunshades.

IOW, L1 is the cheapest option.

This study estimates that

For randomly distributed flyers with the design parameters established above, namely a residual on-axis transmission of 10% and 1.85 Gm of distance, the optimum cloud cross-section size is a 6,200 × 7,200-km ellipse.

A similar cloud in LEO would still be 6200 km high, but over 40,000 km wide.

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  • $\begingroup$ I see. I'd been envisioning a swarm of smaller objects, not one huge sun shade. For example, one could put a bunch of black chaff or dust in LEO, or on a slightly larger scale maybe a swarm of small, spinning mylar disks. Is there some reason that this technique inherently requires a single monolithic object? $\endgroup$ – Ben Crowell Oct 20 '19 at 17:39
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    $\begingroup$ Single object or a swarm doesn't matter. You still need a smaller area in L1 than in LEO. $\endgroup$ – Hobbes Oct 20 '19 at 17:42
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    $\begingroup$ @BenCrowell Because you'd have to coat the whole globe instead of put one sunshade. L1 would be stationary inbetween the sun and the Earth, allowing it to constantly shade. In LEO whatever shade you have is whipping around the earth at 90 minutes per second, so you'll need a shade for every possible orbit, even multiple per orbit if you want any sort of consistency as the Earth is spinning below you as you orbit. Imho, neither are feasible. $\endgroup$ – Magic Octopus Urn Oct 20 '19 at 18:36
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    $\begingroup$ @MagicOctopusUrn: Ah, I see. I'd been thinking of L1 as the earth-moon L1 point. It hadn't occurred to me that they were talking about the earth-sun L1 point. $\endgroup$ – Ben Crowell Oct 20 '19 at 18:38
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    $\begingroup$ @BenCrowell The ISS zips around the earth at 90mins/revolution. If you wanted the ISS to shade a single area constantly, you'd need it to be, well everywhere on its orbit at once, right? And, because orbits can only be around the central point of a body, you'd also need them for every possible inclination around the "sphere". To have global coverage at LEO you'd essentially have to build something like Elon Musk is trying to build with StarLink. Something that has global coverage constantly. It's a very daunting task. At the L1 we could just throw a new moon there, well, not really. $\endgroup$ – Magic Octopus Urn Oct 20 '19 at 18:48

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