How can a spacecraft coating reflect sunlight well but…
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How can a spacecraft coating reflect sunlight well but also emit heat well without violating Kirchhoff's law of thermal radiation?
I was reading about spacecraft thermal control and got stuck on coatings that have low solar absorptance but high thermal emittance. My understanding of Kirchhoff's law was that a good emitter must also be a good absorber. If the coating reflects most of the sunlight, I would have expected it to be poor at radiating energy away too.
Are those two quoted properties averages over seperate parts of the spectrum, rather than quantities that the law directly equates? And would the same coating absorb infrared from a warm nearby object well, even though it wouldnt absorb much sunlight? I'm trying to understand what is actually being compared.
#science
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I was reading about spacecraft thermal control and got stuck on coatings that have low solar absorptance but high thermal emittance. My understanding of Kirchhoff's law was that a good emitter must also be a good absorber. If the coating reflects most of the sunlight, I would have expected it to be poor at radiating energy away too.
Are those two quoted properties averages over seperate parts of the spectrum, rather than quantities that the law directly equates? And would the same coating absorb infrared from a warm nearby object well, even though it wouldnt absorb much sunlight? I'm trying to understand what is actually being compared.
#science
source
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