In Alien,the crew gets woken up halfway the trip back home they dont know were they are and why they are so far off course.
When mother calls Dallas you see Ash pushing him with "mother wants to talk to you"
etc etc etc
They move on to the derelict Ripley finds out of the beacon's real signal purpose and Ash waves it off in a very dissrespectfull way to the lives of the crew.
So the crew is brought there on purpose knowing they cant defend themselves and probably will serve as carriers for the Xeno's only so later on the Nostromo could be boarded by the people who want the Xeno's.
Offcourse not knowing Ripley has the same fighting spirit as the Xeno's
That makes me come to mind that Cameron fully ignored the mysterie effect and the part that its all about the origin and were the derelict ship came from but then again maybe Ridley didnt allow it to mess around with his and Gyger's mystery and kill it.
Or the trip the marines took with Ripley was only for the climate station and the colony's,
so therefore they could make a movie in between that some people were going for the jockey and the ship before ripley was found etc.
But a prequel is also good but i hate the mystery and the possibility Scott will not make another movie of Alien/Prometheus,and only a handfull people can pull of such a mystery.