How do radioactive isotopes provide heat for a radioisotope thermoelectric generator?

In a radioisotope thermoelectric generator (RTG), heat is generated through the natural process of radioactive decay.1

Radioactive isotopes are inherently unstable and release energy as they decay into more stable elements. In the context of these generators, specific isotopes—particularly those that emit alpha particles—are chosen because they produce significant amounts of heat as they undergo this decay process. For example, plutonium-238 and curium-244 are utilized for their ability to generate consistent thermal power over long periods.1

This heat serves as the high-temperature source required for the thermoelectric power generator to function. The heat flows through a thermoelectric converter to a heat sink, creating a temperature differential that produces direct current electricity via the Seebeck effect. Because these isotopes provide a reliable, long-lasting heat source without the need for moving parts or external refueling, they are ideal for remote or isolated applications, such as spacecraft and deep-ocean data collection.2341