QUESTION:
My curiosity was peaked when I read in a previous question how cooling of
the electronics was accomplished.  Freon was circulated and heat rejected
into space.  This is hard to understand since the density of space is
negligble compared to the enviroment I operate in.

If the space craft reaches an average space temp of 10 deg. C., why do all
celestial bodies not also?

ANSWER from Charles Whetsel on July 17, 1997:
Thermal control of spacecraft (especially high power electronics like
microprocessors or radio frequency equipment) is a very challenging task,
much more so than on the earth. In general, most spacecraft are open to free
space and must rely on radiative cooling to keep parts from getting too hot. 
The use of freon or other liquids for thermal control during space flight is
primarily used to convect heat from a hot part of the spacecraft to a large,
cooler radiator plate where the heat can be rejected into space by
radiation.

All bodies in space which are not generating heat from some internal source
(e.g. spacecraft, most planets, etc. but not the Sun or other stars which
generate heat from nuclear reactions) will reach some equilibirum
temperature which is a function of how much heat input they recieve (from
the Sun or some other star), how much they reflect (based on the "color" of
the surface facing the heat source) and how much they radiate out in all
directions (based on the "emissivity" of the other surfaces.  The poorer a
radiator something is, the hotter it gets before it is eventually able to
radiate all of the power that it absorbs.  Ultimately, these laws of
thermodynamics (based on the surface properties of the body) determine what
the "average" steady-state temperature will eventually settle out to.  If
they body is inhomogeneous (i.e. part metal, part rubber), it may have local
hot spots or cold spots above or below the average, but it is impossible to
cheat on what the thermodynamic average will be, once the radiative
properties of the covering material are decided (Although some small
percentage of the energy might also be rejected by the radio beams leaving a
spacecraft).