Originally Posted by JayUtah

How efficient is that - in space?

The short answer is "not very." Radiative heat transfer in general is not a very efficient means compared to forced convection, and forced convection in turn is not very effective compared to conduction. These differences can be orders of magnitude for real-world problems. We use radiative heat transfer in space because it's practically all we have available.

But when you add "in space," you bring up the point that in space radiative heat transfer is, pound for pound, more efficient there than, say, on Earth. That's because one of the many factors that affects the ability to radiate heat in practice is the incoming radiant heat from the environment, especially from nearby surfaces that radiate. Deep space is the perfect heat sink for that property. If your radiator faces deep space, there is no incoming light to fall on the radiator. If it faces a strong reflector such as the nearby Earth, or a strong emitter such as the sun, your ability to reject heat through it suffers a lot.

This was actually a problem on one of the later Apollo missions. One of the radiators on the LM faced upward, ostensibly toward relatively deep space. But in one case light reflected from a lunar mountain fell on the radiator and restricted its efficiency.