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“It’s a brand new box of tricks,” said Sir Ridley. “We know what the road map is, and the screenplay is now being put on paper. The prequel will be a while ago. It’s very difficult to put a year on Alien, but [for example] if Alien was towards the end of this century, then the prequel story will take place thirty years prior.”

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http://www.iwatchstuff.com/2009/10/alien_prequel_to_take_place_pr.php