 Why had the children never seen Earth?
They had grown sated with the sight of wonders—of multiple systems of giant stars, of nebulae that sprawled in empty flame across light years. It was a subdued excitement; men and women, they came and stood quietly gazing into the big vision screens that showed the oncoming star, and there were wide-eyed children who had been born in the ship and had never seen a planet. The grownups talked in low voices, in tones of mingled eagerness and apprehension, of what might lie at the long journey's end. 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. People pressed round the screens, peering into them as if with the naked eye they could pick out the atom of reflected light that was Earth, home. Earth's still there." He said, with a touch of tolerant amusement, "What did you think might have happened to Earth? Someone switched off the lights, and a charmed whisper ran through the big room as the people saw each other's faces by the pale golden light of Earth's own Sun, mirrored and multiplied by the screens. And we don't know what changes have taken place on Earth.
 They were born while on Quest III.
 The ship didn’t have enough fuel to return to Earth.  Earth was destroyed before they were born.  They weren’t able to see through the portholes.  They were born while on Quest III.