I don't think we're going to build a 50-person spacecraft or a 100-person spacecraft.

My first biography written in '73 was not 'Journey To The Moon.' It was 'Return To Earth.' Because for me, that was the more difficult task - disappointment.

Unless we are able to commit to a permanent growing settlement [on Mars], then I don't think just going there with humans and coming back is worth doing. The expense of planning to come back is like the people who left Europe to come to America and then to turn around and go back to Europe, it really doesn't make any sense at all.

Exploration is wired into our brains. If we can see the horizon, we want to know what's beyond.

The way I see it, what is going to come out of the moon activities is a respect for U.S. leadership.

To move forward, what's required is a unified space agenda based on exploration, science, development, commerce, and security.

Climate has been changing for billions of years.