an object is to be cooled down in outer space, then its side facing the sun must be made reflective and its shadow side dull black or it must be protected against the sun by means of a mirror The object will lose more and more of its heat into space as a result of radiation because the heat can no longer be constantly replaced by conduction from the environment, as happens on Earth as a result of contact with the surrounding air, while replenishing its heat through incident radiation would be decreased to a minimum as a result of the indicated screening. In this manner, it should be possible to cool down an object to nearly absolute zero (273° Celsius). This temperature could not be reached completely, however, because a certain amount of heat is radiated by fixed stars to the object on the shadow side; also, the mirrors could not completely protect against the sun.
Okay, I didn't want it to come to this, but here it goes (if you don't like technical explanations stop reading)...
The density of space is approximately 1 atom per cm^3. This makes the thermal conductivity and the convection coefficient (which is simply conduction with fluid motion) effectively zero. So the only method of heat transfer is radiation which is given by the equation
q_dot = F*A*epsilon*sigma*(T1^4-T2^4) where
q_dot is the heat transfer [W]
F is the view factor [-]
A is the area of the heated surface [m^2]
epsilon is the emissivity [-]
sigma is the Boltzman constant [W/m^2-K^4]
T1 is temperature of the heated surface [K]
T2 is the temperature of the temperature being radiated to [K]
A typical CPU is about a square inch in area (6.453 cm^2). Let's say for simplicity it has a view factor and epsilon are both 1 (it only sees one uniform temperature, and is a perfect emitter/black body). We'll say it's emitting to space which is at 3 K (although a more realistic average temperature is ~40 K) and the CPU is producing 70 W of heat. This leaves one unknown, the temperature of the CPU, which comes out to be 1176 K or 903 C - much too hot for any CPU. Now, if we add more area - a radiator - the temperature will drop. A 1 m^2 radiator (assuming perfect conditions) will reduce the temperature to 187.5 K or -85.7 C. So basically, like I said before, a radiator is essential, and this is what is done in practice in space applications... I can say that from experience.
? You would need a fairly large radiator to dissipate that much heat at a low temperature. This is the reason that most cooling on spacecraft (ie for IR sensors) is done by cryocoolers instead of a plain radiator.
