That was over 42 years ago in 1979.
Today she is 71, and has starred in 94 movies in total, 87 since Alien was released.
In Alien, I think Sigourney Weaver looks:
Alien scores 8 out of 10 on The Movie Database.
Sigourney Weaver's first acting role was as 'Alvy's Date Outside Theatre' in Annie Hall, released in 1977 when she was 27.