One orbit, with a radius of 42,000 kilometers, has a period of exactly 24 hours. A body in such an orbit, if its plane coincided with that of the Earth's equator, would revolve with the Earth and would thus be stationary above the same spot on the planet. It would remain fixed in the sky of a whole hemisphere ... [to] provide coverage to half the globe, and for a world service three would be required, though more could be readily utilized. (1945) [Predidicting geosynchronous communication satellites]

All explorers are seeking something they have lost. It is seldom that they find it, and more seldom still that the attainment brings them greater happiness than the quest.

The fact that we have not yet found the slightest evidence for life - much less intelligence - beyond this Earth does not surprise or disappoint me in the least. Our technology must still be laughably primitive, we may be like jungle savages listening for the throbbing of tom-toms while the ether around them carries more words per second than they could utter in a lifetime.

The entire sweep of human history from the dark ages into the unknown future was considerably less important at the moment than the question of a certain girl and her feelings toward him.

We over estimate technology in the short term and under estimate technology in the long term.

I don't believe in God but I'm very interested in her.

The dinosaurs disappeared because they could not adapt to their changing environment. We shall disappear if we cannot adapt to an environment that now contains spaceships, computers - and thermonuclear weapons.