Just to restir the hornet's nest:
What is the point of continued manned exploration in th intermediate (say 50 years or two generations) future? This puts it in the range of vision of the (vanishing) middle class--"what is in it for my kids or my grandkids?" Note this is still a huge stride compared to the American political time frame, which must demonstrate results in 2, 4, or 6 years depending on whether you are a Congressman, President, or Senator. Kennedy set the goal "by the end of this decade" if memory serves. We rocket junkies may have great motives, we may even be "right" in saying exploration of space is "worth" the cost, but what argument will convince the majority of voters to select political representatives who will either redirect (or increase) tax money to pay for the program?
Interesting comment about public freaking out about "nuclear energy." The science of Magnetic Resonance Imaging (MRI) was based on Nuclear Magnetic Resonance. The "Nuclear" was based on the physics of the atomic nucleus, most commonly the single proton of the hydrogen atom. There was (and isn't and never will be)any IONIZING radiation associated with NMR or MRI. But the word "NUCLEAR" was dropped.
My gut feeling is that the "breakthrough" required (if it occurs) will be the practical development of cheap (REEEEAALLLLY CHEAP) energy. If/when this occurs, it will provide the economic and technical leap to proceed with exploration beyond lunar orbit. The other factor will be population stabilization as the currently lrojected course is to keep breeding until disease, famine, or war act as involuntary population controls.
I know, here come the hornets!
It is amazing what you can do when you don't have a choice.
Smart people learn from their mistakes.
REALLY SMART PEOPLE learn from OTHERS' mistakes.