they will be IR visible, but what do you mean they don't exist? Apollo used them to bleed off excess heat.

"A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence." [0] Black bodies are theoretical, they do not exist. Apollo had radiators, they might have been black but they were not a black body. Vantablack comes close to a black body for the visible spectrum which is p cool tho.

[0]https://en.wikipedia.org/wiki/Black_body

> Black bodies are theoretical, they do not exist. Apollo had radiators, they might have been black but they were not a black body.

they were, as I said, "black body radiators" which are not theoretically ideal black bodies, but still manage to radiate black body radiation.

I think the term you're looking for is simply "radiator."

yes that is more parsimonious however in non-technical language the term "radiator" is associated with heat exchangers. if you put heat exchangers on the sides of your spacecraft they would function as radiators so I suppose it doesn't matter anyway, arguing semantics is a waste of time.

a "radiator" is a heat exchanger and "radiator" is the industry standard term. these semantics are required for effective communication about technical topics.