 Why were three Quest ships built?
But now unwonted excitement possessed the hundred-odd members of the Quest III's crew. The great chamber, a full cross-section of the vessel, had been at first a fuel hold. At the voyage's beginning eighty per cent of the fifteen-hundred-foot cylinder had been engines and fuel; but as the immense stores were spent and the holds became radioactively safe, the crew had spread out from its original cramped quarters. As the Quest III's fuel stores and the hope of success in man's mightiest venture had dwindled, so the strength had gone out of him. Weeks and months had passed for the Quest III in interstellar flight while years and decades had raced by on the home world.
 To have a better chance of success if any failed.
 To have a better chance of success if any failed.  So each could travel for 20,000 years.  To transport more people to the new planet.  In case the others break down.