The radioisotope that has the longest half-life is the best to use in powering planet and space exploration vehicles because they can travel farther. Which radioisotope is the best to use?

Plutonium-239 has the longest half-life among these choices, at about 24,000 years. Carbon-14 lasts about 5,730 years, while lithium-5 and uranium-240 decay much more quickly. A longer half-life means the isotope’s radiation—and the energy it can provide—decreases more slowly, so plutonium-239 is the best choice here.

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