Why was Enterprise on a five-year mission in TOS?

Question

There is a small difference between intro texts in TOS:

Space: the final frontier. These are the voyages of the starship 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.

and in TNG:

Space: the final frontier. These are the voyages of the starship Enterprise. Its continuing mission: to explore strange new worlds, to seek out new life and new civilizations, to boldly go where no one has gone before.

(emphasis mine)

It's pretty obvious that no man was changed to no one to include women. But, is there any explanation why was five-year changed to continuing? Also, why was it a five-year mission at all?

My only assumptions are:

• Gene Roddenberry hoped to get funding to run the show for 5 seasons.

• Gene Roddenberry just made up a number without any special meaning.

• Gene Roddenberry was a communist.

Is there any official explanation, in universe or not?

Answer 1

As Memory Alpha notes, "The Mark of Gideon" establishes the USS Enterprise had enough food and supplies to sustain its crew complement of 430 for five years.

Since Starfleet's main purpose is exploration, it's not particularly unreasonable to presume it dictated how long missions last by determining how much supplies its ships could carry. Memory Alpha also notes that by the 24th century, some missions lasted 8 years (e.g. USS Olympia), presumably due to the increases in supply capacity/efficiency.

On the other end of the spectrum, in the 22nd century, missions only lasting a year (like the Xindi expedition) heavily taxed the supplies and morale of the crew of the NX class Enterprise.

One thing to keep in mind with the USS Enterprise-D is that while its primary mission was exploration, it was the flagship of the fleet. Because of its use as a display of power and for diplomacy, it was continually in contact with Starfleet and its supply chain, indefinitely prolonging its mission lifespan.

Answer 2

With regard to why 5 years was chosen as the mission duration, I looked to some of the earliest work Gene Roddenberry did on the series to see where he may have gotten such a number from. I'll admit, this is largely speculative, but seemed interesting enough to mention...

• First, consider the second pilot they created for TOS: "Where No Man Has Gone Before" (which actually aired as the third episode of the first season). The story involved the Enterprise exploring past the edge of the galaxy, so even as early as the pilot episodes Roddenberry is thinking about space travel extending this far from Earth. How far is it exactly? Well, I don't know what estimates or galactic maps Roddenberry would have been working off of back in the '60s, so I'll just have to use a nice map of the Milky Way that I found from a 2008 issue of National Geographic. Here's a small excerpt of the image:

  

  The red line I drew shows the distance from the Sun to a point past the dense central part of the Outer Arm, which could be considered an "edge". The arc lines are spaced 10,000 light years apart, so this distance is about 17,000 light years. A round trip journey here and back would be about 34,000 light years.

• Second, the Memory Alpha entry on Warp factors says this:

  During TOS, the warp factor scale wasn't clearly defined. In his initial draft proposal, Star Trek is..., Gene Roddenberry established the maximum velocity of the starship as ".73 of one light year per hour". This would translate to a top speed of approximately 6,400 c (equivalent to TOS warp 18.57, and approximately TNG warp 9.98).

  So the top speed for space flight at this early stage was about 6,400 times the speed of light.

Taking these two estimates into consideration, a round trip to the edge of the galaxy at the given top speed would therefore result in a total duration of about 5.3 years. Round it off to the nearest year and maybe, just maybe, this was the sort of calculation that Roddenberry made to chose his mission duration. Again, it's speculative, but still food for thought. ;)

Answer 3

I doubt there is a canon explanation, so I'll take a crack at it:

Budget allocation, my friend. Starfleet had allocated funds necessary for the operation, supply and salary (don't get me started) of five years of exploration. The five-year window allows for a sunset review of the efficacy of direct, manned exploration compared with deep space probes and merely participating in joint programs with other Federation efforts. Exploration is tricky, politically - that busy with the Gorn is a perfect example of rubbing up against territorial disputes. Far simpler to explore with probes, but not nearly as effective, perhaps.

Also, after five years of deep space duty, you might be ready to spend some time teaching at the academy. Having a deep space mission of indeterminate length makes it impossible to have a family (or at least, to BE a family). If you wanted to do that, you'd have to do something crazy like take your families along the ship...hmmmm.

Answer 4

I am going to try to do this from memory. Sometime during the first and second year of TOS (1966 - 1967), William Shatner appeared as a guest on a very popular talk show hosted by Art Linkletter (The Linkletter Show, aka. Art Linkletter's House Party). Since Internet Movie Data Base only lists guests on this show sporadically, I can't find the exact date. I watched the show every day. During Mr. Shatner's appearance, the question of the five year mission came up. Mr. Linkletter asked what would happen if the show didn't last five years. Being only 13 or 14 years old at the time, I can't recall the exact answer but Mr. Shatner said that if that were the case, they would have to recall the ship and crew for some reason or another. Not canon and not an ideal answer since I can't prove any of it but I do believe that this was addressed by William Shatner himself on that show.

Answer 5

Perhaps he was inspired by Charles Darwin's voyage on the Beagle?

It was famously referred to as a "Five-Year Circumnavigation"

Answer 6

This was also addressed in the novel, Prime Directive. In the Memory Alpha transcript at the beginning, it is stated that the 5 year mission program was about exploration and expansion, it was about following the "Dream of Stars". It states that 12 of Starfleet's vessels were deployed under the 5 year mission program, and while it recounts the destruction of 5 of those ships. All 5 were Constitution class ships but what isn't clear is if all the ships assigned to 5 year missions were Constitution class starships of if other classes were also used.

Answer 7

From my best memory of the interview with William Shatner, they didn't expect the episodes to last over five years and so they put that in the opening dialog. In other words they didn't expect the show to be as big of success as it was. If they had the opening dialog would have been more along the lines of sng.