Who is the Engineer in Alien?

I just posted a thread regarding this very same question! Yes, I was so confused as that same engineer was killed by the giant face hugger and could not have been the space jockey that the Nostromo crew found. When he sat on that seat - when he was trying to get the ship to Earth, I was like "oooh, that's cool how they tied that up with what the Nostromo crew found in 'Alien'" but when David tells Shaw that the engineer is after her, it definitely threw me off.

the one in Alien is obviously larger, by a great deal....

there are more than one ship - when you've got bio weapons on board it's not improbable that on other planets/moons the same thing happened. I expand in another post about another explanation regarding this - that the engineers themselves are engaged in a civil war of sorts... [url=http://www.prometheus-movie.com/community/forums/topic/7452]there are hints to back this up....[/url]