We were making excellent progress in nuclear propulsion, and politics was really the only thing that brought that to a halt. NERVA met or exceeded all expectations and NASA had big plans for it. What really irks me is that people are so lackadaisical in their attitudes towards things like space exploration, but billions are thrown seemingly indiscriminately at the military. Scientists and engineers from pretty much any field that you can think of are finding creative military applications for their work w/ fecundity because it's an easy path to funding(if there really is such a thing). I would like to try offering those kinds of incentives for contributing to the space program.
Space exploration is a matter of great importance and I don't know why so many people seem to have a hard time grasping this. Overpopulation is ultimately one of the greatest problems facing humanity. Everyone thinks it's their right to have kids, but honestly, without expanding away from Earth, this is clearly not an option. As draconian as it sounds the only real choice would be to keep people from reproducing. Not enough people will make that choice on their own.
And then there's the threat of an asteroid or what have you. The probability that something like this will threaten Earth is for our purposes...inevitable. It might not happen real soon, but it very likely will eventually.
I'm sorry to veer off into politics and policy when this thread is primarily about the engineering aspects, but the point I'm trying to make is that the things I'm talking about are the biggest obstacles to contend with in this endeavor and NOT the technical feasibility. I really felt the need to establish that. I think things will actually go much better now that the private sector is being allowed to pursue this.