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After climbing a great hill, one only finds that there are many more hills to climb.

Back in the days of Apollo, sending humans to the moon was the only viable way to get the scientific data we wanted. But now, with our computer and robotics technology, there's very little an astronaut can do on Mars that a well-designed rover can't.

Humans can actually read a landscape, go through a lot of rocks - crack them open, throw them, pick up the next one. Rovers are great - they do amazing science - but it is a lot more tedious process; they go much less far than a human can cover in a day.

We knew it was going to be difficult to get to the moon. We didn't know how difficult.

It is easier to go down a hill than up, but the view is from the top.

I remember it was hard to believe that I was taking a step onto the lunar surface.

To unambiguously settle the questions of whether there was life on Mars, it will take scientists down on the surface.

Concerned that others were not coming onto the summit and because I had no radio link to those below me, I began to wonder if there were difficulties down the mountain. I made the decision to descend.

Climbing is a journey without culmination.

When President Kennedy challenged Americans to go to the moon, the question wasn't whether we'd get there, it was only when we'd get there.