Jeffrey Bell wrote a great opinion piece for SpaceDaily that takes on
one thread often bandied about here--the historical exploration
analogy:

http://www.spacedaily.com/news/oped-03zzx.html

Although I think we should be spending more than we currently are on
space, I agree that NASA will no more lead us to space exploration
than Cheng Ho did China to world colonization.
But Jeffrey Bell draws some questionable conclusions about what caused
Nordic and Chinese exploration to sputter. I think most would argue
that Nordic exploration might well have continued to colonize America
if the Little Ice Age hadn't arrived to kill it off at its most
vulnerable stage.
And although Chinese exploration was not accompanied by parallel
merchant activity, it is tough to tell if this followed from the
nature of the exploration or if this activity simply wasn't recorded
because it was explicitly ignored and discouraged by subsequent
government policy. I'll elaborate on these issues later...

Tom Merkle