A radioisotope thermoelectrical generator (RTG) is an electrical generator that obtains its power from radioactive decay.
RTGs have been used on Earth to supply electrical power in remote and inaccessible facilities, such as unmanned lighthouses and remote sensing stations inside the Arctic circle, as well as satellites and space probes.
Radioactive decay is the spontaneous breakdown of an atomic nucleus, which releases energy and matter.
This is a photo of a 'hot' pellet of radioactive material (plutonium-238 dioxide). The pellet glows because of the heat generated by the radioactive decay.
The released heat energy is converted into electricity and used to warm the Rover's internal components, freeing electrical power for the operation of the vehicle and instruments.
While RTG's are designed to operate for several years, the radioactive material remains hazardous to biological life for many decades. To be safe, the radioisotopes must be contained behind shielding for long after the productive life of the unit.
There have been incidents reported in the Soviet Union; in one instance, the radioactive compartments were opened by a thief, and in another three woodsmen came across two RTG heat sources that had been stripped of their shielding; two of the three were hospitalised with severe radiation burns after carrying the sources on their backs.
An abandoned RTG in North-West Russia.