There are always door openings. And gradually, it accumulates. The opportunities open up in front of you.

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

The way I see it, commercial interests should manage a lunar base while NASA gets on with the really important task of flying to Mars.

There's no doubt who was a leader in space after the Apollo Program. Nobody came close to us. And our education system, in science, technology, engineering and math, was at the top of the world. It's no longer there. We're descending rather rapidly.

A hybrid human-robot mission to investigate an asteroid affords a realistic opportunity to demonstrate new technological capabilities for future deep-space travel and to test spacecraft for long-duration spaceflight.

When we get there, if we don't find any life on Mars, from that point on there will be life on Mars because we'll bring it there, whether it's germs and leftover urine bags, whatever it is.

By refocusing our space program on Mars for America's future, we can restore the sense of wonder and adventure in space exploration that we knew in the summer of 1969. We won the moon race; now it's time for us to live and work on Mars, first on its moons and then on its surface.