Watching science fiction on TV can be rather depressing. A viewer gets used to the characters and setup of a particular series, begins to feel excited about what's going to happen next, and then - curses - the programme is promptly cancelled. There are so many shows that have been axed long before their natural ends that a full list would be enough to make a grown man break down and cry. There's a multitude of reasons why something would be cancelled - there weren't enough viewers, they couldn't be sold to enough markets, they weren't exactly what the network had in mind, and so on. Of course, every now and then a programme fails to return even when it's a hit: the 1998 vampire mini-series Ultraviolet is one example of such an oddity. The first and only season was actually watched by a large number of people, but the show made the error of tying up all its major plot points neatly by the end. By contrast, there are more than a few series that did stick around long after they began to whiff. Caprica never did find enough of a spark to justify its existence (sorry, Caprica lovers) and should have been put down earlier than it was. Ditto for The Event, which... um... wasn't one. But programmes can pull off the trick of lasting for what feels like pretty much the right length of time. Star Trek: The Next Generation ran for a healthy seven years and it felt right to many fans when Picard and company sailed off into the distance, destined to only return in films of variable quality. Babylon 5 also lasted its full allotted span, if not exactly in the way its creator wanted originally. Nevertheless, countless programmes end far too early and - frustratingly - usually just as viewers are getting to the good bits. Here are 15 series where the axe fell long before what viewers there were were ready.