The Orphans of the Sky reference article from the English Wikipedia on 24-Apr-2004
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Orphans of the Sky

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Orphans of the Sky is a science fiction novella by Robert A. Heinlein, consisting of two parts: Universe and Common Sense.

A gigantic, cylindrical generation ship is cruising pilotless toward a distant solar system, inhabited by a crew that has forgotten their origins over time and lapsed into superstition. Most crew members lead a simple bucolic life of farming, never venturing to the upper decks where the "muties" dwell. One day, Hugh Hoyland starts exploring and finds the astonishing secret of the ship's purpose.