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Satellites are isolated systems, the only way for it to transfer body heat to outer space is thermal radiation. There are solar panels, so there is continuous energy flow to inner system. No airflow to transfer the accumulated heat outer space easily. What kind of cooling system are being used in satellites?

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    $\begingroup$ Good question. I don't know the answer, but it's noteworthy that the satellite would (inherently) radiate more as it increases in temperature, and lots of the radiation would get reflected. Some of the radiation absorbed by the solar panels get turned into electrical potential energy and not thermal energy. $\endgroup$ – Señor O Dec 1 '14 at 15:44
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    $\begingroup$ @SeñorO Eventually you use that electrical energy, and when you do it will be converted into thermal energy $\endgroup$ – Jim Dec 1 '14 at 15:54
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    $\begingroup$ @Jim unless your satellite is a death star and you use it to fire a laser and destroy Alderaan $\endgroup$ – Señor O Dec 1 '14 at 15:58
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    $\begingroup$ @SeñorO Yes, that's another effective method of radiative cooling. Alas, too bad space weapons are forbidden by international law $\endgroup$ – Jim Dec 1 '14 at 16:01
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    $\begingroup$ @Jim Did you hear that, Darth Vader? SPACE WEAPONS ARE FORBIDDEN BY INTERNATIONAL LAW. $\endgroup$ – Michael Dec 2 '14 at 3:30
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Typically, satellites use radiative cooling to maintain thermal equilibrium at a desired temperature.

How they do this depends greatly on the specifics of the satellite's orbit around Earth. For instance, sun-synchronous satellites typically always have one side in sunlight and one side in darkness. These are particularly easy to keep cool because you can apply a white coating to the Sunward side and and black coating to the dark side. The white coating has a low value for radiative absorption while the black coating has a high value for radiative emission. This means it can absorb as little light as possible while emitting more thermal radiation.

Different types of satellites have different strategies for cooling, but in general, cooling is achieved by applying functional coatings to the spacecraft that lower or raise the absorptivity/emissivity/reflectivity of its different surfaces. While designing a satellite, the space engineers perform thermal analyses and lots of calculations to determine which surfaces need to have what absorption values in order for the satellite to maintain the desired temperature.

It's hard for me to be more specific than this. But this is the reason any good space engineer knows how to find a coating with the desired absorptivity/emissivity values within a day or two.

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    $\begingroup$ Actually, it's not hard for me to be more specific. I used to work in researching functional coatings for spacecraft applications. What's hard is to give you an answer other than "it depends on what the satellite is doing and what it looks like and what temperature is ideal" $\endgroup$ – Jim Dec 1 '14 at 15:51
  • $\begingroup$ Not always. There is a new satellite in development in the EU which will go very close to the Sun. It is painted black. Why? So close to the Sun any paint will be quickly damaged, so they start with the worst case scenario from the beginning. It will radiate even in the direction of the Sun, just in wavelengths the Sun doesn't emit much in. $\endgroup$ – vsz Dec 2 '14 at 21:15
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    $\begingroup$ @vsz That wouldn't be a Sun-synchronous satellite, since those by definition are in orbit of Earth. But this example reinforces my "it depends on what the satellite does" statement $\endgroup$ – Jim Dec 2 '14 at 22:12
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As an example, the International Space Station (ISS) has external thermal radiators. They looks similar to solar panels, but instead of pointing the flat side towards the sun, they point towards empty space. An ammonia loop carries heat from various parts of the space station to the radiators.

This is a picture of a radiator: enter image description here (source)

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The satellite itself can do with radiative cooling but some instruments on board, e.g., IR sensors, require temperatures as low as than 4 K for which Helium dewars are used. Bolometers require even lower temperatures (in the mK range).

A good summary is available here.

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    $\begingroup$ Welcome to Physics! Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference. $\endgroup$ – Kyle Kanos Dec 2 '14 at 4:13

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