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

Globalisation means many other countries are asserting themselves and trying to take over leadership. Please don't ask Americans to let others assume the leadership of human exploration. We can do wonderful science on the Moon, and wonderful commercial things. Then we can pack up and move on to Mars.

At the core of the risk-free society is a self-indulgent failure of nerve.

The urge to explore has propelled evolution since the first water creatures reconnoitered the land. Like all living systems, cultures cannot remain static; they evolve or decline. They explore or expire. . . . Beyond all rationales, space flight is a spiritual quest in the broadest sense, one promising a revitalization of humanity and a rebirth of hope no less profound than the great opening out of mind and spirit at the dawn of our modern age.

We must still think of ourselves as pioneers to understand the importance of space.

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

I think there would be no shortage of applicants to the government astronaut corps to be settlers on the planet Mars. And I think this would be very inspiring.