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?
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.
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: (source)
- External Active Thermal Control System on Wikipedia
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.
There are several ways for thermal management (cooling and heating) of a satellite and in general a spacecraft. Heat can be removed from the spacecraft in space by radiation only, assuming that the spacecraft is outside the atmosphere of a planet such as the Earth or Titan (largest moon of Saturn) or Mars. A combination of one or more methods of thermal management can be used, depending on several factors such as the flight mission, allowable temperature range, heating and cooling loads, mission duration, whether mission is crewed or uncrewed, and available budget. Here is some example:
Using coatings and blankets to isolate the spacecraft from the space. This will block solar radiation coming to the spacecraft. It also keeps the spacecraft warm and additional heaters are used to maintain a desired temperature range. Excess generated heat is then rejected, for instance, by directly attaching high power equipment to the surface of a metal plate, called radiator.
Heat pipes and loop heat pipes may be also used combined with option 1. Heat pipes can help achieve a uniform temperature in the components and also can transfer the heat from high temperature interior to the radiators.
Mechanically pumped fluid loops, acting as a thermal bus, can be used to pick up heat from hot components and deliver it either to the components that need heat or to the radiator.
Phase change materials such as paraffin wax have high heat capacity and can store and release heat on demand by going through melting/freezing
Louvers are passive systems installed in front of a radiator. In high temperature conditions blades remain open to let heat radiate away, but in the cold they automatically close up instead. A bimetallic spring passively opens/closes the blades due to thermal expansion.
For cryogenic applications such as IR sensors where low temperatures are needed, cryogenic liquids such as liquid helium may be used. Liquid helium may absorb heat and vaporize and released during short missions. In long-duration mission, radiator rejects the heat or even a refrigeration cycle may be used.
Thermoelectric cooling and heating have been used as well. Radioisotope heating instead of resistance heating can be used in interplanetary flights
Here are some examples:
International space station, a huge satellite, uses an mechanically pumped fluid loops. Mars rover and Mars science lap also use mechanically pumped fluid loops
Hubble Telescope, a big satellite mainly coatings and blankets and heaters
The Defense Support Program (DSP) satellite, which has an IR sensor, in addition to coatings and blankets, uses phase change materials combined with a helium loop that rejects heat through radiators.
Balloons flown in the Earth stratosphere has used oscillating heat pipes as well as weak convection and radiation.
Loop heat pipes and variable conductance heat pipes can be used in satellites.
Morteza Eslamian, PhD