Radioisotope thermoelectric generator
A power source with no moving parts that turns the heat of decaying radioactive material into electricity, for spacecraft too far from the Sun for solar panels.
A radioisotope thermoelectric generator (RTG) holds a lump of a radioactive material, usually plutonium-238. As its atoms decay they give off heat. Around it sit thermocouples: pairs of different metals or semiconductors that make a small voltage when one end is hot and the other cold. Many in a row give a steady electric current, with no moving parts at all.
Far from the Sun, sunlight is too weak for solar panels, so the Voyagers, Cassini, New Horizons and the Curiosity and Perseverance rovers all carry RTGs.
The catch is that the power fades. The fuel decays (plutonium-238 loses half its strength in about 88 years) and the thermocouples slowly wear out. Voyager's generators lose about 4 watts every year, which is why instruments are switched off one by one.
An RTG is not a nuclear reactor: nothing is split, there is no chain reaction, and it cannot explode like a bomb. It is closer to a very long-lasting, radioactive hot-water bottle wired to a thermometer that makes electricity.
