Plutonium-238 is a radioactive isotope of plutonium that is primarily used as a heat source in space missions. It has a half-life of about 87.7 years, which means it can provide a steady supply of energy over long periods. This makes it particularly useful for powering spacecraft and instruments that cannot rely on solar energy. Due to its properties, Plutonium-238 is used in radioisotope thermoelectric generators (RTGs). These devices convert the heat released from the decay of Plutonium-238 into electricity, enabling missions like those of the Mars rovers and the Voyager spacecraft to operate far from the Sun.