Which radioisotope is used in Voyager's RTGs?

Voyager’s radioisotope thermoelectric generators use plutonium-238. The isotope’s natural radioactive decay produces heat, and thermocouples convert part of that heat into electricity for the spacecraft’s instruments, computer, heaters and communications system.

Voyager carries three MHW-RTGs mounted on a boom. At launch, they produced about 470 watts of electrical power, but their output gradually declines as plutonium-238 decays and the thermocouples become less efficient. Engineers have therefore switched off instruments and heaters over time to keep the mission operating.

Plutonium-238 is not the same as plutonium-239, the isotope commonly associated with nuclear weapons. Voyager’s RTGs do not generate electricity through a chain reaction like a reactor; they are long-lived heat sources. This makes them valuable for deep-space missions, where sunlight is too weak for practical solar panels. NASA has used the same basic power approach on several distant planetary and interstellar spacecraft.