The two main heater types used in modern spacecrafts are electrical resistance heaters and radio-isotope heaters, according to the European Space Agency. The type or combination of types used depends on the individual mission.
Electrical resistance heaters utilize the energy lost as heat when an electrical current travels through a resistive conductor. Electrical resistance heating can be powered by solar energy, a resource in ample supply on many space missions. Radio-isotope heating units produce heat via the decay of certain plutonium isotopes. This method of heating is primarily used for missions that explore areas where sunlight becomes too weak or infrequent.