Nuclear pulse propulsion is the only currently viable technology that could be used to make humans an interstellar species. It would also allow us to practically ship up enough materials to build self-sustaining habitats in near space. It is extremely unfortunate that this kind of research is over.

Fun fact: the background radiation levels introduced by nuclear propulsion would actually have a very slight positive health effect on humans according to more accurate radiation hormesis models, rather than the very small negative effects suggested by more naive no threshold models.