How and why do we control heat on a spacecraft?
Gases are a phase of matter where all the atoms or molecules are moving independently of each other all the time. All these molecules move at a different speeds but if we take the average speed of all the molecules; we get the temperature of the gas. If a particular gas has a large number of fast molecules, the gas is hot. On the other hand, if a particular gas has mostly slow molecules, then the gas is cold.
The average speed of a gas is also called the kinetic energy of the gas. When you combine the average speed for each molecule with all the molecules in the gas you get heat. Heat is basically the average speed of a single gas molecule multiplied by all the molecules in the gas. Heat can be seen as the total amount of energy of all the molecules in a certain gas.
Spacecraft designers sometimes need to cool equipment to very cold temperatures. They need to do this because they want to look at infared light. The reason why the equipment needs to be cooled to very low temperatures is that infrared radiation is generated by heat. If the camera which you are watching is warm, then it will be generating "light" and will not be able to see into space. All the camera will see is its own warmth and therefore cannot look at the warmth (infared radiation) of other objects. In other words, to look at the warmth (infared radiation) of other objects, you need to be cooler than those other objects.
There are not very many options for spacecraft designers when it comes to cooling. The refrigeration cycle like that used in your freezer is too bulky, has too many moving parts to be reliable, and does not work well in zero gravity. Therefore they sometimes use expanding gas to cool the infared cameras. When gas expands, the decrease in pressure causes the molecules to slow down. This makes the gas cold.
What could go wrong on DS1 if it gets too cold?
What could go wrong on DS1 if there's too much heat?
What is an atom?
What is heat balance?
How do active heat sinks work?
How do passive heat sinks work?
What are heat sinks in DS1?
Does heat travel differently in space than it does on Earth?
Where does energy come from and go?