ELI5: How does the Voyager still have power after all the years it's been in space?

Take two pieces of metal and put them very close to one another. Then heat one of the pieces of metal. The resulting temperature difference between the two pieces of metal generates a very small electric current.

Voyager 1, 2, Cassini, Galileo, Curiosity, and New Horizons use a small pellet of Plutonium to create this heat difference. The radioactive decay is what generates the heat. It doesn’t provide very much electricity but is extremely long lasting.

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