RTG

RTG stands for radioisotope thermoelectric generator, a device that generates electricity from the heat released by radioactive decay. Unlike a nuclear reactor, an RTG does not sustain a controlled chain reaction; it relies on the continuous decay heat of a radioisotope, most commonly plutonium-238, to produce electric power over long periods. The heat is converted into electricity by thermoelectric converters that exploit the Seebeck effect, typically with no moving parts and high reliability.

Most RTGs use General Purpose Heat Source GPHS modules that contain plutonium-238 in ceramic form, encapsulated in safety shells. In static RTGs, the heat flows across thermoelectric materials to produce direct current. Dynamic RTGs use a small heat engine, such as a Stirling engine, to improve efficiency. Electrical power at launch is typically tens to hundreds of watts, and the output decays slowly as the radioactive material decays, providing usable power for decades.

History and use: RTGs were developed for space missions where solar power is impractical or unreliable, such as deep-space probes and outer planets. They have powered NASA missions including Voyager, Galileo, Cassini-Huygens, and New Horizons, and the Mars rover Curiosity uses an MMRTG. RTGs have also supported missions in environments with long periods of darkness or high radiation where solar panels would be insufficient.

Safety and supply: Pu-238 is highly radioactively hazardous, and RTGs are designed with multiple containment layers to prevent release in accidents. Production and procurement of Pu-238 have been limited and subject to policy and budget constraints, leading to supply constraints for new missions.