In the scary-looking 'Apollo 18' (in theaters Friday), astronauts find they're not alone on the moon, which is just about the last place you'd want to encounter the unknown.

Pity the poor astronaut. Even if you don't land on a hostile planet, you're still vulnerable to a great many horrifying things if you just stay inside your fragile little spaceship. As movies have shown us, when you sign up to go into space, you're signing on for claustrophobia, extreme paranoia among shipmates, and shipboard computers that may be plotting against you.

In space, no one can hear you scream -- it was the brilliant tagline for a great movie, 'Alien,' but it holds true for many other thrillers set in outer space. We rank a handful from not-so-scary to the stuff nightmares are made of.

Did we leave out your favorite? Tell us which ones we should have included.