A Radioisotope Thermoelectric Generator (RTG) is a device that converts heat released from the decay of radioactive isotopes into electrical energy. It uses thermoelectric materials to generate electricity from the temperature difference created by the heat of the radioactive material. RTGs are commonly used in space missions, where solar power may not be feasible. The most common radioisotope used in RTGs is Plutonium-238, which has a long half-life and produces a steady amount of heat. This makes RTGs reliable power sources for spacecraft, such as the Voyager probes and the Mars Curiosity Rover, allowing them to operate for many years without needing refueling.