I think it was a piece of music from another film, also by 20th Century Fox, that they used as a temp track, and Scott liked it so much he made it happen. Goldsmith was not best pleased.
It'll probably be the music for the opening titles, because when I bought the album in the 80s I was suprised (disappointed actually) that piece wasn't on it, and leaving off the opening titles is a real omission in a movie soundtrack.