Sunshine is an option. It tends to be a fairly divisive film, but it still does a lot of things right and would be good for discussion.
Event Horizon was the first movie that crossed my mind. Goosebumps just writing that...Where we're going... we don't need eyes to see!
It's very pretty as a movie. Kind of predictable. I tried to read Alex Garland's Tesseract and found myself woefully unimpressed. Then I read his draft of Halo and was even less impressed. I'd put it this way: The Beach is the last Danny Boyle worth watching, which contains the best work Alex Garland ever did. Which isn't saying much.
Sunshine is a tough one, still so disappointed with how it ended but the fact that the great (Brian Cox)[http://en.wikipedia.org/wiki/Brian_Cox_(physicist)] was a consultant on it helps a little.