question:  What caused the ship to leave its planet initially?
context: [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? 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. Instead, there was more of the blackness, and one very bright star set apart by itself in the middle of the viewport. 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. 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. There had been a revolt—again a term without any real meaning, a term that could have no reality outside of the reading machine—and the elders were overthrown. The people had decided that they did not know where they were going, or why, and that it was unfair that the elders alone had this authority. But when, in another ten years, Chuls would perish of old age, the rays would no longer suffice. You can find several of them under 'astronomy,' with a cross-reference under 'stars.' "Astronomy says some stars are variable," Crifer offered, but Rikud knew his lame-footed companion understood the word no better than he did. Something loomed there in the port, so huge that it spread out over almost the entire surface. A section of it had expanded outward and assumed the rectangular shape of the viewport, and its size as well. Rikud turned again to the port, where he could see nothing but an obscuring cloud of white vapor, murky, swirling, more confusing than ever.
output:  The reason is never disclosed.
options:  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.