 Why couldn't they find a volunteer to man the big ship?
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. The little ship apparently had been picked up by accident when it almost collided with a Sirius-local ship. 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.
 there was no proof that it was safe for humans
 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