The whole point of science is that most of it is uncertain. That's why science is exciting--because we don't know. Science is all about things we don't understand. The public, of course, imagines science is just a set of facts. But it's not. Science is a process of exploring, which is always partial. We explore, and we find out things that we understand. We find out things we thought we understood were wrong. That's how it makes progress.

It is our task, both in science and in society at large, to prove the conventional wisdom wrong and to make our unpredictable dreams come true

There are three reasons, . . . apart from scientific considerations, mankind needs to travel in space. The first . . . is garbage disposal; we need to transfer industrial processes into space so that the earth may remain a green and pleasant place for our grandchildren to live in. The second . . . to escape material impoverishment: the resources of this planet are finite, and we shall not forego forever the abundance of solar energy and minerals and living space that are spread out all around us. The third . . . our spiritual need for an open frontier.

There is a great satisfaction in building good tools for other people to use.

If we want to go to space with humans, that's for fun not for science. Human adventures in space are just sporting events.

We cannot hope to either understand or to manage the carbon in the atmosphere unless we understand and manage the trees and the soil too.

Aviation is the branch of engineering that is least forgiving of mistakes.