 What caused the ship to leave its planet initially?
[Transcriber's Note: This etext was produced from Galaxy Science Fiction September 1951. When nobody aboard ship remembers where it's going, how can they tell when it has arrived? Every day for a week now, Rikud had come to the viewport to watch the great changeless sweep of space. But ever since the engines somewhere in the rear of the world had changed their tone, from the steady whining Rikud had heard all twenty-five years of his life, to the sullen roar that came to his ears now, the feelings had grown. The stars—it was a meaningless concept to Rikud, but that was what everyone called the bright pinpoints of light on the black backdrop in the viewport—were not apparent in the speckled profusion Rikud had always known. Instead, there was more of the blackness, and one very bright star set apart by itself in the middle of the viewport. Someone was clomping up the companionway behind Rikud. "In five more years," the older man chided, "you'll be ready to sire children. But it came again, and each time it left him with a feeling he could not explain. Rikud knew that something which he could not explain had entered the elder man's head for a moment, but it had departed almost before Chuls knew of its existence. Perhaps two score men lay about, naked, under the ray tubes. Rikud, for his part, wanted to get back to the viewport and watch the one new bright star. There was a strange thought, and Rikud's brain whirled once more down the tortuous course of half-formed questions and unsatisfactory answers. Once, here in this same ray room, he had had the impulse to hurl himself head-first against the wall, just to see what would happen. But something soft had cushioned the impact—something which had come into being just for the moment and then abruptly passed into non-being again, something which was as impalpable as air. Rikud had been stopped in this action, although there was no real authority to stop him.
 The reason is never disclosed.
 There was a shortage of women, and the main characters were sent to find mates.  The reason is never disclosed.  The planet they were from ran out of viable resources.  They are explorers who got lost, and their fate was to drift the universe.