The average temperature in space is about 4 degrees Kelvin. If you infrared couple an object to the night sky, the sky will act as an infrared sink and the object will radiate its heat away into space. However, the object must be shielded from local things at normal temperatures, or the infrared that they radiate will be absorbed by your object and keep it warm. I have heard that the desert nomads made ice by placing water in the folds of a rumpled cloth spread out on the desert floor on a calm night. The humps in the cloth shielded the water from the infrared being emitted by the nearby sand dunes and let it cool down towards sky temperatures. A similar high school science demonstration involves putting a little water in a Dewar flask and leaving it outside on a windless night.
RE: black bodies.
Amongst my collection of useless tidbits that I have collected over the years was a short item I once saw in Popular Science or some similar publication. They said a reasonable approximation of a black body object had been made inexpensively by bolting a bunch of razor blades together side by side. Something about the way the blades narrowed in a curved manner towards the sharp edges, light would get trapped in the grooves formed by the assembly, and bounce back and forth many millions of times, thus having a high likelyhood of being absorbed and re-radiated. They said its spectra was close to the spectra of an ideal black body, although I think that they might have been talking about temperatures considerably higher than room.
Steve
