[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.