How and why do we control heat on a spacecraft?
Most spacecraft depend on the combination of the onboard thermal control devices plus pointing constraints to survive thermally. Using a pointing constraint means controlling the orientation (position compared to the Sun) of the ship so that certain parts are kept in shadow.
Different parts of the ship play different roles in thermal regulation. Some parts are designed to capture sunlight and must be oriented towards the Sun--they also make shadows that other parts can sit in. Some parts are specifically designed to release heat into space--those need to be kept in shadow so that they are surrounded by very cold space for the heat to escape into. If the ship changes orientation by rolling from side to side or changing attitude, the parts that used to be in shadow will be exposed to solar heating, and the parts that used to be in the Sun will not be warmed. To reduce cost and weight and power, most spacecraft are designed and built with sides intended to point toward the Sun and sides intended for shade.
Devices to actively cool and heat all parts of a spacecraft in all attitudes would draw a lot of power, weigh a lot, and cost a lot to launch and fly. The compromise is to consciously group subsystems on a spacecraft so that some sides of the spacecraft tolerate solar heating better than others. For DS1, the DS1 flight team will try to avoid pointing 3 sides toward the Sun: the "-x" side with the imaging instrument; the "+z" side with the imaging instrument and the Power Processing Unit; and the "-z" side with the ion thruster (during ion thruster use). The "+x" side is the one we intend to keep toward the Sun, in general. The two y-axes need to be kept at right-angles to the Sun so that the solar arrays capture all the energy they can get.
Why do we have to control heat on a spacecraft if there are no people on board?
How is DS1's heat balance maintained?
How do solar panels supply energy for spacecraft?
Does heat travel differently in space than it does on Earth?
Will DS1 get heated directly by the Sun?
What is heat balance?
What is attitude control?
What components are where on DS1?
Why do solar arrays need to be oriented towards the Sun?
Why do solar panels become less useful the farther they get from the Sun?
What makes EM radiation?
Where does energy come from and go?