“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.”
Even if it sounds a little shaky I'm confident to say that I rekkon they've got the right man for the job.