A satellite’s temperature is determined by the balance between the heat
that enters the satellite and the heat that leaves it. This means that
we must be concerned with how heat is transferred. Heat can be
transferred in three ways: by _conduction_, when two bodies are in
direct contact and their molecules collide; by _convection_, which
utilizes the movement of warm currents in a fluid; and by _radiation_,
in which heat energy travels as electromagnetic waves at the speed of
light. With a satellite, we are concerned only with the last of these,
since the only way energy can be gained or lost in space is by
radiation.
In the transfer of heat by radiation, the surface of the heated
body—such as a satellite—is very important. All energy gained must be
absorbed at the surface; all energy leaving must be emitted at the
surface. So the physical properties of this surface control how energy
is absorbed and how it is emitted. The origin of the radiant energy is
vitally important; most surfaces, for instance, will behave differently
when exposed to solar radiation from the sun’s temperature of 10,000°
Fahrenheit than when exposed to radiation from nearby objects at room
temperature.