One small, simple type of radioisotope power system that is extremely versatile is known as a radioisotope heater unit, or RHU. A single RHU passively radiates about one watt of heat.
When placed carefully aboard a spacecraft or inside a rover, the heat energy from RHUs serves to keep a mission’s hardware at proper operating temperatures, compensating for the very cold temperatures often encountered in space exploration.
RHUs have a rich history of use on NASA missions. They were first used with the science experiments that were left on the surface of the Moon in 1969 by the Apollo 11 astronauts. Later NASA missions have carried them to Jupiter, Saturn, and Mars. All told, the United States has flown 300 RHUs on 32 space missions.