There have been thousands of satellites and dozens of space probes powered by solar electricity. Space missions out to the orbit of Jupiter can be powered by so

There have been thousands of satellites and dozens of space probes powered by solar electricity. Space missions out to the orbit of Jupiter can be powered by solar PV (Juno probe). Actual fission reactors have never gone beyond Earth orbit. Very deep space missions can't use solar power due to the inverse square fall-off of light from the sun, but even those don't use nuclear reactors. They use radioisotope thermal generators powered by radioactive decay. RTGs don't have any moving parts and can run for decades without maintenance. I think it's astonishing and delightfully living-in-the-future that solar cells -- once so expensive that only space missions could justify their cost -- are now manufactured by the square kilometer and cheap enough to "farm" electricity here on Earth. If you're pondering speculative technologies of the future, a solar collecting Dyson swarm could provide more energy than all the fissionable materials in the solar system.