Star Trek the Next Generation was on a "continuing" mission, but the original Star Trek series was on a five year mission. At least that is what William Shatner, speaking as Captain Kirk, told us at the beginning of every show. Here is the full quote.
"Space: the final frontier. These are the voyages of the star ship Enterprise. Its five-year mission: to explore strange new worlds, to seek out new life and new civilizations, to boldly go where no man has gone before."
So why was it a five year mission, especially given that the supposed 5 year mission had nothing to do with the show? I do not know for sure. I do not have inside information. However I can make a good guess.
When Star Trek was created it was the aspiration of TV producers to create series that survived for five years. This allowed them to be sold into syndication for reruns. The show could play five nights a week, Monday through Friday, for about six months before it needed to be replaced by another series, or the station began the series from beginning and went through the episodes again.
There were exceptions. Some series were rerun even though they had only survived four years. Lucky for Star Trek fans someone tried to rerun Star Trek even though it had only been on the air for three seasons, and found out that it was very popular in reruns. But five or more was considered the ideal. So that is probably why the mission was five years.
Surviving four or better five or more years was crucial to TV producers. They typically produced their shows at a small loss. If the show survived long enough to make it to reruns, they could pay off the cost of the show and make big profits. A show that survived three years like Star Trek would normally be their worst nightmare. There would be little or no syndication revenue and three years of losses to cover. But Star Trek, like the crew in so many of its stories, beat the odds, and made a fortune. Money may be unimportant to the heroes of Star Trek, but it is important in Hollywood.
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