Radioisotope Heater Units (RHUs) have been called the "unsung heroes" of radioisotope power technology. Most spacecraft can use solar energy to provide heat for keeping their structures, systems, and instruments warm enough to operate effectively. But when other heat source technologies are not feasible, an alternate heat source is required for the spacecraft.
RHUs are small devices that use the decay of plutonium-238 to provide heat to keep spacecraft components and systems warm so that the equipment can survive long enough in the cold space environment to complete its mission. This heat is transferred to spacecraft structures, systems, and instruments directly, without moving parts or intervening electronic components.
By using RHUs, the spacecraft designer can allocate scarce spacecraft electrical power to operate the spacecraft systems and instruments. RHUs also provide the added benefit of reducing potential interference (electromagnetic interference) with instruments or electronics that might be generated by electrical heating systems.
An RHU contains a Pu-238 fuel pellet about the size of a pencil eraser and outputs about 1 Watt of heat. (The entire RHU is about the size of a C-cell battery.) Some missions employ just a few RHUs for extra heat, while others have dozens.
NASA missions enabled by radioisotope heater units
More about RHUs from the Department of Energy