Science Fiction and Horror are two genres that share a lot in common and, on occasion, can be interchangeable. Both use narratives to address larger thematic issues at heart - be them ethical, moral or psychological. Both often involve the fantastic, dabbling in matters and planes of understanding beyond the human mind and both involve a degree of suspension of disbelief. When thrust into such a universe, the audience is asked to take a leap of faith with the filmmaker, excepting the ground rules set and witness them invert, bend and ultimately climax in catastrophe or success.
And both can be downright terrifying. There's a reason video stores of the 90s often combined the genres in aisle shelves beyond conserving space. While it may be easy to put Halloween firmly under the "horror" banner" at your local Blockbuster, it may be a little more difficult to place something like Alien. There's plenty of space stuff, but Roger Ebert famously described it as a "haunted house in a spaceship." So its genre placement is a little more ambiguous.
There have been some truly terrifying sci-fi films, usually starting with a bang that immediately thrusts viewers into the action, letting its technical speak unfold through the narrative.
Here are the ones that gave us a jolt right out of the park.