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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.

The revolutionary breakthrough will come with rockets that are fully and rapidly reusable. We will never conquer Mars unless we do that. It'll be too expensive. The American colonies would never have been pioneered if the ships that crossed the ocean hadn't been reusable.

Perhaps the best motivation for going to Mars is political. It is obvious that no single nation currently has either the will or the resources to do it alone, but a consortium of nations and space agencies could achieve it within 20 years.

We're not going to get humans to Mars until at least the mid-2030s, and the world is going to change by then.

We like to talk about pioneering Mars rather than just exploring Mars, because once we get to Mars, we will set up some sort of permanent presence.

It's not going to do any good to land on Mars if we're stupid.

As we visit Mars multiple times, we will build up infrastructure on the surface to expand the capabilities and reach of humans on Mars.

Supplying fuel for a Mars expedition from the lunar surface is often suggested, but it's hard to make it pay off - Moon bases are expensive, and just buying more rockets to launch fuel from Earth is relatively cheap.

If NASA is to reach beyond the Moon and someday reach Mars, it must be relieved of the burden of launching people and cargo to low earth orbit. To do that, we must invest more in commercial spaceflight.

It would be great some day to have astronauts in a rover on Mars. But just about anyone except an oil company executive would say its more important to have 50 million solar powered vehicles in the United States.