“
Tomorrow Was Yesterday” dealt with the discovery by the Enterprise of a giant “universe” or “generation” ship—that is, a slower-than-light spaceship that would take generations to reach its destination because they lacked the power to traverse the vast distances between the stars any faster. The Voyager was a colony ship that had been launched from Earth hundreds of years previously, but only now were Federation ships catching up to it, the Enterprise being the first. Unfortunately, after hundreds of years, the people inside had forgotten that they were aboard a spaceship—instead they believed their enclosed world to be the totality of existence. Part of the reason for this stemmed from a mutiny in their long forgotten past, a mutiny that had left the Voyager’s population divided into two armed camps. The elite were descendants of the well educated, and they had a high standard of living in their part of the ship. The downtrodden oppressed were descendants of the mutineers. Now, the Voyager was a giant sphere, or cylinder. Artificial gravity was provided by spinning the ship to create centrifugal force; therefore, from a shipside point of view, down was outward, up was toward the center. The upper levels in the center of the ship were where the control room was located
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David Gerrold (The Trouble with Tribbles: The Story Behind Star Trek's Most Popular Episode)