After two years of highly secret training, on 12 Apr 1961 Yuri Gagarin climbed into his Vostok ('East') spacecraft. As the engines fired on the Baikonur launchpad in Kazakhstan, he shouted:
Poyekhali!
Let's go!

A terrible wilderness of mountainous country constitutes the immediate environment of St. Paul's. It is a precipitous cliff into the abyss, a gate of hell, more horrible than the fantasy of Dante could express it.

I have ever been prone to seek adventure and to investigate and experiment where wiser men would have left well enough alone.

I wanted to see what no one had yet observed, even if I had to pay for this curiosity with my life.

If we knew exactly what to expect throughout the Solar System, we would have no reason to explore it.

Once human beings realize something can be done, they're not satisfied until they've done it.

Some gifted adventurer is always sailing round the world of art and science, to bring home costly merchandise from every port.

The world was full of locked doors, and he had to get his hand on every key.

There ought not to be anything in the whole universe that man can't poke his nose into—that's the way we're built and I assume that there's some reason for that.

This is a day we have managed to avoid for a quarter of a century. We've talked about it before and speculated about it, and it finally has occurred. We hoped we could push this day back forever.
Comment on the explosion of Space Shuttle Challenger and the loss of the lives of all seven crew.

Tis Man's to explore up and down, inch by inch, with the taper his reason.

Today there remain but a few small areas on the world's map unmarked by explorers' trails. Human courage and endurance have conquered the Poles; the secrets of the tropical jungles have been revealed. The highest mountains of the earth have heard the voice of man. But this does not mean that the youth of the future has no new worlds to vanquish. It means only that the explorer must change his methods.

We shall not cease from exploration
And the end of all our exploring
Will be to arrive where we started
And to know the place for the first time.
This was a favorite quotation of John Bahcall, who used it in his presentation at the Neutrino 2000 conference.

When we think how narrow and devious this path of nature is, how dimly we can trace it, for all our lamps of science, and how from the darkness which girds it round great and terrible possibilities loom ever shadowly upwards, it is a bold and a confident man who will put a limit to the strange by-oaths into which the human spirit may wander.

Why had we come to the moon?
The thing presented itself to me as a perplexing problem. What is this spirit in man that urges him for ever to depart from happiness and security, to toil, to place himself in danger, to risk an even a reasonable certainty of death? It dawned upon me that there in the moon as a thing I ought always to have known, that man is not made to go about safe and comfortable and well fed and amused. ... against his interest, against his happiness, he is constantly being driven to do unreasonable things. Some force not himself impels him, and he must go.