Until now only energy sources based on radioactive decay have been used in space, which have very low power, but they can provide it for many decades.
Nuclear fission reactors, similar to those used on submarines or ships, would enable very different applications.
Until now, they have not been used for fear of what would happen after a failed rocket launch, when the reactor would fall back on Earth.
This could be mitigated by sending only components of the reactor and assembling it in space.
I do not think that routine exploration of the Solar System beyond Mars will ever be possible without using at least nuclear fission reactors, because it is too slow with chemical sources of energy.
