Go nukes. When I was taking my minor in nuclear chemistry, one of the faculty was a nuclear engineer who had worked on the Nerva project. Shielding specialist--really knew his stuff.
I visited the Cape in the summer of '68. We saw the Apollo 8 being stacked and were given a very thorough tour, (buncha AF Junior Officer!). I remember our tour guide showing us the building attached to the Vertical Assembly Building which was to house the Nerva upper stage before it was mated to the Saturn V.
The tour guide said the NERVA upper stage would be the foundation for a future Mars mission. Unfortunately, NERVA was cancelled and an opportunity was postponed. Hopefully we're ready to pick up the baton and continue the race!
Was it Jimmy Carter? He was a nucyulur engineer. At least that's what he said.