I'm convinced that sending people to Mars is so expensive that if you go once and bring the people back and then go again and bring the people back, we're eventually going to run out of money. But what if we send people the first time and they don't come back? What if they stay there?

Most people never believed in the real possibility of going to the moon, and neither did I until I was in my twenties.

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

You need propellants to accelerate toward Mars, then to decelerate at Mars, again to re-accelerate from Mars to Earth, and finally to decelerate back at Earth. Accordingly, the mass of these required propellants, in short, drives our need for innovative launch vehicles.

You can never tell when a commercial space venture will suddenly become viable.

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

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