[akiselev]

> First of all a shadow can be cast using very very light-weight material

Not in space. Unless the material is an absolutely perfect reflector across the entire EM spectrum, which simply isn't possible, any structure would melt pretty much instantly. You'd have to make radiators that are as big or bigger than the shade structure in order to radiate that heat away and you'd have to make the entire structure much bigger than the shade part because otherwise you'd just be radiating the absorbed heat back at the Earth.

[semi-extrinsic]

> perfect reflector across the entire EM spectrum, which simply isn't possible

You don't need it to be a perfect reflector across the entire spectrum, 200 to 2000 nm is enough.

And according to "Solar Absorptance and Thermal Emittance of Some Common Spacecraft Thermal-Control Coatings", e.g. Kapton film reflects 92% of the entire solar spectrum.

[akiselev]

Sorry I meant be completely transparent or reflective. If even < 1% is absorbed in any way, the heat will steadily build up and eventually start to melt the material, which changes its properties usually causing it to absorb more and more heat (causing a runaway heating effect). You can either radiate that heat back towards the sun (not going to work) or towards the earth which defeats the purpose. This is one of the most basic design constraints in any spacecraft simply because there is no way to convect heat away like we do within our atmosphere.

Since black body radiators are very inefficient, you'd have to build a massive umbrella with an oversized "heatsink" that dumps way more heat to the sides than it does towards the earth or absorbs energy from the sun. I don't even want to imagine how massive of an engineering challenge that would be.

[semi-extrinsic]

Why can't it radiate heat towards the earth? If you reflect 85% and radiate the remaining 15% towards earth, that's a net win, right?

[akiselev]

Again, that would require perfect reflection for that 85% of spectrum and perfect transparency and/or a perfect radiator for the other 15%. If that kind of material is possible, we're capable of producing it, and the x% is capable of feeding all of the photosynthetic organisms on Earth it would be totally fine.

However, there is zero evidence that such a perfect material can even exist let alone be produced and maintained at a scale capable of shielding the earth. Any imperfections will absorb heat and steadily destroy the shielding material in runaway overheating.

[Dylan16807]

>Again, that would require perfect reflection for that 85% of spectrum and perfect transparency and/or a perfect radiator for the other 15%.

What? No! You use a semi-good reflector that reflects 85%. You use any marginally-okay radiator and it will get rid of the other 15% No need for perfect materials at all.

>and the x% is capable of feeding all of the photosynthetic organisms on Earth it would be totally fine.

What? The point is to blot out 2% of the sun. You don't need the remnant light from that area of the sky to feed anything. Plants will just have to get by on having 98% as much light.

>Any imperfections will absorb heat and steadily destroy the shielding material in runaway overheating.

What? Any imperfections will make that part heat up slightly more, which makes that part radiate more, and then it reaches equilibrium. It's a sheet of plastic/metal. It's not going to be within a hair-thin safety margin of self-destruction.

[Dylan16807]

Look into the research and tests on solar sails. It's not a difficult problem. Most of the light is reflected, and the rest is not enough to overheat it. Casting a shadow does not require 100% of photons be blocked.

[akiselev]

Solar sails are designed to move away from a large source of photons which decreases their energy input exponentially as they move. They are also attached to a spacecraft with its own heat dumping mechanism which means most of the energy can conduct away from the sails into the rest of the more massive thermal system.

[Dylan16807]

The thermal mass of the rest doesn't matter, only how much it can radiate over the long term. And an enormous film makes a good radiator.

Any solar sail capable of starting around Earth is capable of hanging around Earth indefinitely without heat problems.

Certain designs can't go very close to the sun, some can. Many designs would work just fine for a solar shade.

[akiselev]

》 The thermal mass of the rest doesn't matter, only how much it can radiate over the long term. And an enormous film makes a good radiator.

Unless you've got a perfect material that can be accelerated by radiation pressure and radiate all heat away, then the rest of the spacecraft mass very much matters because that's where 99+% of the heat will migrate to. Whether a "film" is a good raditor or not (AND a good solar sail) depends entirely on the type of material so I don't know where you're getting your assumptions from.

》 Certain designs can't go very close to the sun, some can. Many designs would work just fine for a solar shade.

Do you have a link to such a design? I would be greatly interested in learning about it as I have not heard of a single practical design that can do what you're claiming.

[Dylan16807]

If you need to deal with heat, you set up one side to be good at reflecting, and the other side to be good at radiating. That doesn't require perfect materials. It doesn't even require particularly great materials. For the job of shading the earth, you would do fine with a design that only reflected half the light. We can do much better than that with existing technology.

https://en.wikipedia.org/wiki/Solar_sail#Materials talks a bit about which designs can handle getting significantly closer to the sun than Earth. None of the designs there would have trouble with heat if they were in orbit around Earth.