Hi there, new to the forum. Just finished my Electrical engg. undergrad at the University of Calgary. Anyway I had a question regarding thermal transfer in a vacuum. Over my internship I did a substantial amount of work with cooling electronics using heat sinks and/ liquid, so I have a fairly basic understanding of the nature of thermal transfer in atmosphere. On with the question: So far as I understand in space, since there is no (very, very little) atmosphere, cooling based on convection wouldn't work, obviously conduction wouldn't be of use once apart from moving the heat around the satellite, leaving radiation as the only mechanism that could be used to cool the device. Would an arbitrary "heat sink" placed in space naturally cool to the 4K or so of space? What kind of thermal resistance could be expected in this situation? What would change depending on whether the device was in the sun or shade? I can only assume that the cooling must be in the shade. And, if anyone knows: how do they actually cool devices in space? Whew, that was a little more long winded that I expected. Thanks. B Edit: also, why isn't radiant cooling used terrestrially?