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.

Had we lived, I should have had a tale to tell of the hardihood, endurance and courage of my companions which would have stirred the heart of every Englishman. These rough notes and our dead bodies must tell the tale, but surely, a great rich country like ours will see that those who are dependent on us are properly provided for.
[Final words in a 'Message to the Public' left written in his diary dated 25 March 1912, shortly before he died on the Ross Ice Barrier, Antarctica. When searchers found his body, on 12 Nov 1912, Scott was discovered sitting upright against the pole of the tent with the diary behind his head, as if for a pillow.]

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

I shall collect plants and fossils, and with the best of instruments make astronomic observations. Yet this is not the main purpose of my journey. I shall endeavor to find out how nature's forces act upon one another, and in what manner the geographic environment exerts its influence on animals and plants. In short, I must find out about the harmony in nature.

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

I'm sure we would not have had men on the Moon if it had not been for Wells and Verne and the people who write about this and made people think about it. I'm rather proud of the fact that I know several astronauts who became astronauts through reading my books.

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

In the infancy of physical science, it was hoped that some discovery might be made that would enable us to emancipate ourselves from the bondage of gravity, and, at least, pay a visit to our neighbour the moon. The poor attempts of the aeronaut have shewn the hopelessness of the enterprise. The success of his achievement depends on the buoyant power of the atmosphere, but the atmosphere extends only a few miles above the earth, and its action cannot reach beyond its own limits. The only machine, independent of the atmosphere, we can conceive of, would be one on the principle of the rocket. The rocket rises in the air, not from the resistance offered by the atmosphere to its fiery stream, but from the internal reaction. The velocity would, indeed, be greater in a vacuum than in the atmosphere, and could we dispense with the comfort of breathing air, we might, with such a machine, transcend the boundaries of our globe, and visit other orbs.

It is difficult to conceive a grander mass of vegetation:—the straight shafts of the timber-trees shooting aloft, some naked and clean, with grey, pale, or brown bark; others literally clothed for yards with a continuous garment of epiphytes, one mass of blossoms, especially the white Orchids Caelogynes, which bloom in a profuse manner, whitening their trunks like snow. More bulky trunks were masses of interlacing climbers, Araliaceae, Leguminosae, Vines, and Menispermeae, Hydrangea, and Peppers, enclosing a hollow, once filled by the now strangled supporting tree, which has long ago decayed away. From the sides and summit of these, supple branches hung forth, either leafy or naked; the latter resembling cables flung from one tree to another, swinging in the breeze, their rocking motion increased by the weight of great bunches of ferns or Orchids, which were perched aloft in the loops. Perpetual moisture nourishes this dripping forest: and pendulous mosses and lichens are met with in profusion.

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 Columbia is lost; there are no survivors. … In an age when space flight has come to seem almost routine, it is easy to overlook the dangers of travel by rocket, and the difficulties of navigating the fierce outer atmosphere of the Earth. These astronauts knew the dangers, and they faced them willingly, knowing they had a high and noble purpose in life. Because of their courage and daring idealism, we will miss them all the more. ... The cause in which they died will continue. Mankind is led into the darness beyond our world by the inspiration of discovery and the longing to understand. Our journey into space will go on.

The more experiences and experiments accumulate in the exploration of nature, the more precarious the theories become. But it is not always good to discard them immediately on this account. For every hypothesis which once was sound was useful for thinking of previous phenomena in the proper interrelations and for keeping them in context. We ought to set down contradictory experiences separately, until enough have accumulated to make building a new structure worthwhile.

The progress of science has always been the result of a close interplay between our concepts of the universe and our observations on nature. The former can only evolve out of the latter and yet the latter is also conditioned greatly by the former. Thus in our exploration of nature, the interplay between our concepts and our observations may sometimes lead to totally unexpected aspects among already familiar phenomena.

The scientific method is a potentiation of common sense, exercised with a specially firm determination not to persist in error if any exertion of hand or mind can deliver us from it. Like other exploratory processes, it can be resolved into a dialogue between fact and fancy, the actual and the possible; between what could be true and what is in fact the case. The purpose of scientific enquiry is not to compile an inventory of factual information, nor to build up a totalitarian world picture of Natural Laws in which every event that is not compulsory is forbidden. We should think of it rather as a logically articulated structure of justifiable beliefs about nature. It begins as a story about a Possible World–a story which we invent and criticise and modify as we go along, so that it ends by being, as nearly as we can make it, a story about real life.

The truth is that the scientific value of Polar exploration is greatly exaggerated. The thing that takes men on such hazardous trips is really not any thirst for knowledge, but simply a yearning for adventure. ... A Polar explorer always talks grandly of sacrificing his fingers and toes to science. It is an amiable pretention, but there is no need to take it seriously.

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.