A radioisotope thermoelectric generator (RTG) is a device that converts heat released from the decay of radioactive isotopes into electrical energy. It uses materials that generate electricity when exposed to temperature differences, allowing it to produce power continuously for many years without needing fuel. RTGs are commonly used in space missions, such as those involving NASA spacecraft, because they can operate in environments where solar power is not feasible. They provide a reliable energy source for instruments and systems, enabling long-term exploration of distant planets and moons, like Mars and Titan.