Star Trek may be a cultural touchstone, but every iteration of the final frontier has eventually met its end. Beginning in 1966, Star Trek has been a science fiction institution for the majority of its existence. The franchise's overall popularity has ebbed and flowed over the decades, but rarely has there been a time where a Star Trek film or TV series wasn't at least in some stage of development.

While the cancellation of Star Trek: The Original Series - and its eventual revival as a film series - is a key part of the franchise's mythos, the circumstances around the endings of some of the latter day Star Trek series are less well known. Some of the more popular series got to wind down on their own terms, while a handful of others were cut down before their time, the victims of low ratings or fan ambivalence.

Related: Every Failed Star Trek Show (& Why They Didn't Happen)

In many ways, the health of the overall franchise can be gauged by the circumstances under which each series ended. The franchise was perhaps at the height of its popularity in the 1990s, which featured three series that all ended of their own accord, while the 21st century has been less kind. Here's why each series ended, and what it meant for Star Trek as a whole.

Star Trek: The Original Series

Kirk and Spock Star Trek

Star Trek: The Original Series is of course where it all began. After NBC rejected Gene Roddenberry's Star Trek initial pilot for being too dull, Jeffrey Hunter declined to return as Captain Christopher Pike and was replaced with William Shatner's swashbuckling James T. Kirk, and the show's dynamics almost instantly snapped into place. Kirk and Spock - played with subtle charm by the late, great Leonard Nimoy - became cultural icons, and a fandom was born.

TOS, however, was never a ratings behemoth. It faced the specter of cancellation after season 2, but an unprecedented letter writing campaign from fans helped NBC see the potential in the little sci-fi series. But due to network politics and a burned out Gene Roddenberry, season 3 ended up in a horrific time slot and suffered a creative slump, resulting in its premature end. That was not the end of the story, of course. The show would go on to be an unexpected monster hit in syndication throughout the 1970s, and was eventually revived with the 1979 big screen adventure Star Trek: The Motion Picture, and the franchise never looked back.

Star Trek: The Animated Series

Star Trek The Animated Series - Enterprise, Spock, Scotty, Kirk, McCoy

Sometimes forgotten or dismissed even among Star Trek fans, Star Trek: The Animated Series featured the first new Star Trek adventures on fans' televisions four years after TOS's demise. With the exception of Walter Koenig, the entire cast from TOS returned to voice their respective characters, and the show was so well received it won an Emmy for Outstanding Children's Series. The series consisted of 22 episodes split over two seasons. In that era animated shows - even high profile ones like Star Trek: The Animated Series - were only designed to run for a finite amount of time, so it wasn't exactly cancelled, it just fulfilled its intended run.

Related: Why Star Trek’s Animated Shows Are Always Controversial

Star Trek: The Next Generation

The cast of Star Trek The Next Generation pose for a promotional image

The franchise's first great leap forward, Star Trek: The Next Generation featured a new crew and new Enterprise, set about a century after the adventures of Kirk and Spock. The show aired in first run syndication, meaning it didn't have to abide by notes and direction from any single television network, like TOS had with NBC. The result was one of the most popular and financially successful shows of its era. It ran for seven seasons, with a huge, consistent viewership that went far beyond hardcore Trekkies.

TNG's much celebrated series finale, "All Good Things," was watched by over 17 million people, putting it on par with contemporary network hits like Seinfeld and Home Improvement. As might be evident by that number, the show could have easily kept going, and an eighth season was original planned, though ultimately scrapped in favor of making TNG movies, a decidedly questionable decision in retrospect.

Star Trek: Deep Space Nine

Star Trek Deep Space Nine

Star Trek: Deep Space Nine was a darker, more morally complex take on Star Trek than virtually anything that had come before. It dealt with religious themes, the costs of war, and some decidedly imperfect characters, none more so than Captain Benjamin Sisko, perhaps still the most complex lead character in Star Trek's history.

It was also Star Trek's first foray into serialized storytelling, which worked against it in first run syndication; in the era before streaming and DVRs, if you missed an episode, you had no idea what was happening. This resulted in the show having something of a ratings ceiling, never approaching the popularity of TNG during its initial run; DS9 even added Worf from TNG in season 4 in an attempt to boost viewership. The show ended on its own terms with season 7 and was only considered a modest success in its day, though it's only grown in acclaim over the years as one of the first shows to utilize what audiences now think of as modern storytelling devices.

Related: Star Trek: All 15 TNG Characters Who Returned In DS9

Star Trek: Voyager

The cast of the television series Star Trek Voyager.

Star Trek: Voyager should have been a home run. It had a great premise, a strong cast, and was the flagship show of the brand new UPN network. Despite bringing in a massive audience for its series premiere, the show almost immediately began bleeding viewers; by season 4 Seven of Nine, a former Borg drone, was brought on in an effort to boost the show's lagging ratings, to little success. The show could never quite commit to its premise, the cast was uneven, and the writing was at times dire. Star Trek: Voyager ended on its own terms after seven seasons, but it limped to the finish line, rather than going out on top like The Next Generation. Voyager wasn't cancelled, but it was the first sign that there were serious cracks in the franchise's foundation.

Star Trek: Enterprise

T'Pol in Star Trek Enterprise

Prequels were all the rage at the dawn of the 21st century, and Star Trek decided to get in on the fad. It would prove to be a poor decision - Star Trek: Enterprise was greeted largely with ambivalence from regular TV viewers and scorn from Star Trek fans. The show's series premiere ratings were about half of what Voyager's brought in, and the ratings promptly fell off a cliff. Beyond its relatively low quality, audiences had simply tired of Star Trek by 2005. The show was almost constantly in danger of cancellation by UPN, and despite a fairly well regarded creative reboot in season four, the show was unceremoniously axed, the first Star Trek show to be outright cancelled by its network since TOS in 1969.

Discovery, Picard, and Lower Decks

Star Trek Picard and Star Trek Discovery Season 3

The future looks bright for the current crop of Star Trek series. It's unknown exactly how popular the current slate of Trek shows are, as CBS All Access doesn't release viewing figures, but Star Trek is the backbone of that streaming service. Current Star Trek executive producer Alex Kurtzman has confirmed there are multiple more seasons planned for Star Trek: Discovery, which just began production on season 4. Likewise, Star Trek: Lower Decks is already in production on season 2 and is likely to go beyond that.

Arguably CBS All Access' most popular series, Star Trek: Picard is in a curious position. The show brought in a massive new wave of subscribers to CBS All Access and generally garnered better reviews from both critics and fans than Discovery, but the entire show is contingent on how long Patrick Stewart is interested in participating. Kurtzman has suggested there's a three year plan and a five year plan, depending on how Sir Patrick feels down the line. Even if Picard were to end after three seasons, there's still Strange New Worlds, Prodigy, and the Emperor Georgiou spinoff in the pipeline. Star Trek is in a healthy place, and there's currently no end in sight for the final frontier.

Next: The Complete Star Trek Timeline Explained