https://www.scientificamerican.com/article/why-nasas-perseverance-mars-rover-uses-nuclear-energy/

A spacecraft is only as strong as its power source, which is why when NASA was designing its Perseverance Mars rover, the agency turned to radioactive plutonium.

The plutonium that will be blasting off the planet on Thursday (July 30) isn’t in the same form as is used for weapons, and it’s well protected in case something happens to go wrong during the launch. But these plutonium units are a respected power source for spacecraft—NASA’s Curiosity rover runs on a similar device.