Life had become too interesting on one world crawling across the rubble-strewn arm of a spiral galaxy, for as the system moved it swept up cosmic dust and debris. Ice ages and periods of atropical warmth followed one another very quickly. Yesterday's fabled culture might be tomorrow's interesting hole in the ground. But society had always endured. Many thought it always would. Only the brightest scientists admitted that to survive, the race would have to abandon the planet. And to do that they'd have to invent spacecraft...