At least three unrelated "nuclear" things are involved: * Radioisotope power sources (like this, Cassini, New Horizons, etc.) * Critical fission reactors (pop

At least three unrelated "nuclear" things are involved: * Radioisotope power sources (like this, Cassini, New Horizons, etc.) * Critical fission reactors (popular in the early space race before solar PV, still the best option for high-power deep-space missions like (cancelled) Jupiter Icy Moons Orbiter/Project Prometheus) * Nuclear pulse propulsion -- setting off nuclear weapons, riding the shockwaves (e.g. Project Orion) Pulse prolusion is banned, quite pointlessly, under the Partial Test Ban Treaty of 1963. (Note it extends not only to atmospheric explosions, but ones in deep space). The others aren't. Contained, critical nuclear reactions are legal -- NASA chose a nuclear reactor a few years ago for the Jupiter Icy Moons Orbiter, before they cancelled it. And no one cares about radioisotope sources except confused hippies. http://en.wikipedia.org/wiki/Partial_Nuclear_Test_Ban_Treaty http://en.wikipedia.org/wiki/Partial_Nuclear_Test_Ban_Treaty Obscure history: a running Soviet nuclear reactor crashed from space into the Canadian wilderness in 1977. In case anyone's curious of the consequences. http://en.wikipedia.org/wiki/Kosmos_954 http://en.wikipedia.org/wiki/Kosmos_954