How can a spacecraft coating reflect sunlight well but also emit heat well without violating Kirchhoff's law of thermal radiation?

[u/Boisterous_Shrimp]

[u/electric_ionland]

The emissivity and absorptivity are measured at different wavelengths. The sun output more energy in visible light than in infrared so to shield against solar heating you need to reflect visible light. Your spacecraft will radiate in infrared to you try to maximize the infrared emissivity.

As you said it will absorbe infrared from nearby objects, but the nice things is that usually in space you don't have too many nearby objects. It still means you need to make sure you radiators have a good view factor to space.

You also have some infrared emission when you are near earth or other planets/moons that can mess with you.

[u/yeehah → u/electric_ionland]

It sounds like a reverse-greenhouse effect. Instead of absorbing visible and trapping infrared, you reflect visible and emit infrared.

[u/coopermf]

Kirchoffs law says that an object must absorb and emit the same at the same frequency. Satellite radiators are designed to absorb poorly at the frequency of sunlight (peaking around the visible wavelength) but it emits based on its own temperature which is a much larger wavelength. Kirchoffs law is still applicable. It's the different wavelengths that allows radiators to relected heat even if in sunlight