"So, just as politics dictated that men should reach the Moon decades ahead of schedule, politics will also determine when men venture beyond orbit with the help of nuclear propulsion. With that first step, the solar system opens up, and is unlikely to ever be closed to us again.

I wonder, though, when that step will be taken? Four decades from now? A century from now? Two centuries? It can't happen soon enough for me, but I fear it will happen too late for most of us here to see it. "

That what happens when you when you depend on nukes as the only solution- you wait and you wait. Four decades at say 15 billion per year get you into some serious pocket change- 600 billion. Why pay 600 billion for nothing? If you don't care about 600 billion, think of all the lives you are wasting- and not just the poor saps at NASA.
Nukes are nice but they aren't going to get us there- just accept the political reality. Nukes are good because they are efficent- it requires less propellent. There are other ways of getting efficency- even better than nukes. I'll even bet we will have anti-matter proplusion in less than 4 decades- a much higher concentration of energy per mass than nukes.
But we don't need anti-matter- mass drivers for instance are more efficent than any rocket.
Could it take 4 decades or 2 centuries?
That's a tough question, I would NEVER underestimate the stupidity [and/or greed] of humans, but sheer stupidity might not be enough. And I doubt if NASA will even exist after 4 decades of nothing.
-gb (look they are going around in orbit- wow!:)