It comes down to the work of one anti-nuke person a couple decades ago. But, it looks like this administration is not interested in listening to such a small special interest group, so the reasonable power sources for space exploration once more include nuke and isotopic modules.
We have a subcontract to work on the JIMO systems and word has it that nuclear-based power sources will continue to be developed at least as long as Bush is in office. I must admit that such appears problematic at the moment, but we have to have hope, and do what we can to help achieve that outcome.