In the early days of the US Space program there was a parallel nuclear power program to develop the nuclear power technologies needed for spaceflight. The Space Nuclear Auxiliary Power (SNAP) program worked on both isotope decay generators and fully operational fission reactors. And while several spacecraft have been launched to other planets using radio isotope generators, the US only launched one fully operational test reactor - SNAP 10A which operated according to predictions from ground tests. Until the host spacecraft failed 40 days into the mission.

While the concept was proven, no mission could be matched to the capabilities and no other test reactors have been flown by the USA.

(The Soviet Union on the other hand flew several)

Follow me on Twitter for more updates:
  / djsnm  

I have a discord server where I regularly turn up:
  / discord  

If you really like what I do you can support me directly through Patreon
  / scottmanley