Behind every man now alive stand thirty ghosts, for that is the ratio by which the dead outnumber the living.

When all else failed, you had to rely on eyeball intrumentation.

Now I'm a scientific expert; that means I know nothing about absolutely everything.

Judge me by my deeds, though they are few, rather than my words, though they are many.

What was more, they had taken the first step toward genuine friendship. They had exchanged vulnerabilities.

The crossing of space ... may do much to turn men's minds outwards and away from their present tribal squabbles. In this sense, the rocket, far from being one of the destroyers of civilisation, may provide the safety-value that is needed to preserve it.

I have great faith in optimism as a guiding principle, if only because it offers us the opportunity of creating a self-fulfilling prophecy.