Hardcore science fiction fans don't need the mainstream to validate their love of the genre, thank you very much. Still, it's always intriguing to see how a mainstream director -- for the purposes of this article I'll define "mainstream" as a filmmaker not known for his previous interest in the genre -- approaches a science fiction property.
Martin Scorsese, for example, has not tackled science fiction yet, but his knowledge and love for all types of cinema is clearly felt in his latest film, Shutter Island. The thriller reeks (in a good way) of atmosphere, mood, and tone informed by the classical brand of horror movies, the kind practiced by James Whale and Val Lewton. In a similar way, Ridley Scott's The Duellists placed the action in foreboding, menacing, gorgeous settings, allowed the story to unfold at its own pace, and displayed respect for the the genre (if Napoleonic sword fighting pictures can be termed a genre), qualities that would serve Scott well with Alien and Blade Runner.
Here, then, are the top mainstream directors gone sci-fi. These are not (all) among the best films ever made, but are fascinating nonetheless -- sometimes like that car wreck you can't tear your eyes away from.
1. Stanley Kubrick, 2001: A Space Odyssey
As brilliant as his previous pictures were, Kubrick exceeded himself with this carefully-crafted, mind-blowing masterpiece. Science fiction allowed him to let slip the surly bounds of gravity -- and narrative convention. It marked a turning point in his career, as his films became increasingly poetic and untethered to earthbound reality.