There are other possible fission options that don't involve detonating actual bombs out the back of the rocket: https://en.wikipedia.org/wiki/Nuclear_propulsion#Spacecraft

I'm a fan of the nuclear lightbulb myself: https://en.wikipedia.org/wiki/Gas_core_reactor_rocket#Closed...

Sometimes I think that while it may be appealing to mine gold or platinum or whatever out of the solar system, what people really need to figure out how to do is mine uranium. While I could advocate with a straight face that maybe we need to freak out a bit less about lifting the occasional few dozen pounds of uranium into orbit, and point out that more radioactive material has already been launched than people realize, it is fair to observe that we probably can't afford to make lifting hundreds of pounds of fission fuel into orbit the sort of routine event it needs to be to really have a space civilization. One of the biggest major issues with any sort of space habitation is access to dense energy sources. You can smooth over a lot of other problems and get a lot more slack in the system if you have a lot of energy available to play with. Part of the challenge with current space technology is that you start out on the very edge of feasibility as it is.