Space architectures capable of supporting a permanent human presence on Mars are extraordinarily complex, with many different interdependent systems.

We have the ability, at such high fidelity, to simulate the physical world through computers. But when the spiritual world or human behavior comes into play, we don't have a very good model for that at all.

When we set out to land people on the surface of Mars, I think we should as a nation, as a world, commit ourselves to supporting a growing settlement and colonization there. To visit a few times and then withdraw would be an unforgivable waste of resources.

There's no guarantee that the United States will be around 200 years from now.

It's real easy to manufacture what you think the people want to hear. But that's not very honest.

Every couple of years, we could dispatch people from Earth to Mars.

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