"I aim for the stars, but sometimes I hit London."

"The time will come when we can no longer afford to live on Earth."

"Don't tell me that man doesn't belong out there. Man belongs wherever he wants to go — and he'll do plenty well when he gets there."

"I'm a natural-born optimist. I believe that we're going to solve all our problems."

"It is not a question of whether we will go to Mars, but when."