The way I see it , it ' s similar to how bomber pilots in WWII had to survive a certain number of missions , which it was assumed most wouldn ' t survive especially those ball turret gunnersSince Star Trek tos takes place a little after the Federation Romulan war , it can be assumed that the 5 year mission was just a carryover from a time in which it wasn ' t expected for a ship to survive that long in space I didn ' t see Star Trek Enterprise , so idk if they have a better explanation . By the time the EnterpriseC rolls around , space is considered safe enough for everyone to bring entire families . Iirc , early in the show several planets Kirk visits already had a Starfleet ship visit them to which they ' re destroyed in some way on leaving . If it wasn ' t for this first event , surely Kirks enterprise would have met the same fate . E Wrote this early in the morning whilst on my phone and for some reason auto correct made me say Star Wars Enterprise . Oopsie .