To send humans back to the moon would not be advancing. It would be more than 50 years after the first moon landing when we got there, and we'd probably be welcomed by the Chinese. But we should return to the moon without astronauts and build, with robots, an international lunar base, so that we know how to build a base on Mars robotically.

One of the major problems with long-term deep space human flight is the requirement for radiation shielding.

I believe that space travel will one day become as common as airline travel is today. I'm convinced, however, that the true future of space travel does not lie with government agencies -- NASA is still obsessed with the idea that the primary purpose of the space program is science -- but real progress will come from private companies competing to provide the ultimate adventure ride, and NASA will receive the trickle-down benefits.

American greatness was elevated significantly after Sputnik.

Russia perhaps is still entertaining the possibility that the moons of Mars might have access to ice or water.

The moon I see now is the same moon I saw before. Except that before, when I looked at it, it was in anticipation of what it would be like when I got there. That's behind me now.

Walking around on the moon was significantly easier than we'd thought it would be. There weren't any balance problems, so you weren't tumbling over.