Couple of interesting things about the Space Jockey from the extended cut editions of the various Alien movies, plus some stuff I read on some fan boards....
1. The space jockey was a being from another planet who dealt the alien species as a weapon. He was making a delivery of eggs, when an egg hatched, he got infected, and the ship crash landed on LV 426, after he was killed.
2. Being who got infected by the alien species, and purposely crash landed his ship on uninhabited LV 426 to avoid infecting the rest of his species.
In any case, it interesting conjecture, there was talk of an Alien prequel showing how the craft ended up there....
The space jockey by the way is a "giant" being within the bowels of the "alien" ship (conjecture saying that the ship isn't made by THE alien species, but another alien species who fell victim to THE aliens somehow...). You will see him in wide shots of the crew inside the downed ship in the original Alien movie.....