We have the ability, at such high fidelity, to simulate the physical world through computers. But when the spiritual world or human behavior comes into play, we don't have a very good model for that at all.

There's a historical milestone in the fact that our Apollo 11 landing on the moon took place a mere 66 years after the Wright Brothers' first flight.

History will remember the inhabitants of this century as the people who went from Kitty Hawk to the moon in 66 years, only to languish for the next 30 in low Earth orbit. At the core of the risk-free society is a self-indulgent failure of nerve.

It's been one of the greatest challenges that ever came along in my life; it was one of the more difficult things to do.

Retain the vision for space exploration. If we turn our backs on the vision again, we're going to have to live in a secondary position in human space flight for the rest of the century.

I'm in favor of changing the destination of humans. There are a lot of manned missions that can be done, but not in the direction of the moon.

The much-hyped Ares 1-X was much ado about nothing.