Prometheus is not a direct prequel, but it IS in the Alien world.

 

 I think it deals with the "space jockey" alien more than the Alien "alien", although that one will be in it too. Ridley Scott behind the camera makes this worthwhile as this idea of going back to LV426 Before the 1979 movie was talked about for ages. I wonder whom else from the first movie is part of the crew..