"It's set in 2085, about 30 years before Sigourney [Weaver's character Ellen Ripley]. It's fundamentally about going out to find out 'Who the hell was that Space Jockey?' The guy who was sitting in the chair in the alien vehicle — there was a giant fellow sitting in a seat on what looked to be either a piece of technology or an astronomer's chair ... Our man [Tom Skerritt as Captain Dallas] climbs up and says "There's been an explosion in his chest from the inside out — what was that?" I'm basically explaining who that Space Jockey — we call him the Space Jockey — I'm explaining who the space jockeys were ... [The film] is about the discussion of terraforming — taking planets and planetoids and balls of earth and trying to terraform, seed them with the possibilities of future life."
Read it all Here