question:  Why couldn't they find a volunteer to man the big ship?
context: But with the Procyon shuttle arriving yesterday, there was no telling what might turn up. For eighty years now, they’d been sending out the little ships that vanished at greater than the speed of light, equipped with every conceivable device to make them return automatically after taking pictures of wherever they arrived. This was the first hope they’d found that the century-long trips between stars in the ponderous shuttles might be ended and he should have been filled with excitement at Jordan’s hasty preliminary report. The little ship apparently had been picked up by accident when it almost collided with a Sirius-local ship. Scientists there had puzzled over it, reset it and sent it back. He had no desire to spend ninety years cooped up with a bunch of callow young emigrants, even in one of the improved Exodus shuttles. And even if Exodus ever got the super-light drive working, there was no reason he should give up his work. The discovery that men could live practically forever had put an end to most family ties; sentiment wore thin in half a century—which wasn’t much time now, though it had once seemed long enough. Earth was becoming a backwater world; no real progress had been made in two centuries; the young men were sent out as soon as their first fifty years of education were finished, and the older men were too conservative for really new thinking. That missile reached Sirius in less than ten days. It took two accidents to get the thing back to us—and with a half-light-year miss that delayed it about twenty years before the Procyon shuttle heard its signal. Pre-setting a course may take centuries, if we can ever master it. Even with Sirius expecting the missiles and ready to cooperate. We can’t put human pilots into a ship with our drive until we’ve tested it more thoroughly, Bill, even if they could correct for errors on arrival. Mr. Giles, we’ve got to have the big ship. Heroism had belonged to those who knew their days were numbered, anyhow.
output:  there was no proof that it was safe for humans
options:  the ship wasn't going to be ready for a long time  there was no proof that it was safe for humans  the rats didn't survive, so people probably wouldn't  no one wanted to spend that much time on the ship