Corrected entry: After they escape the planet, the nuke goes off, and they are back on the spaceship, where they should all be weightless, including the alien, and would be floating around on the ship, not walking around like they were on solid ground. This totally unscientific approach to the movie is annoying.
Correction: They are not weightless when they wake up from cryosleep at the start of the movie either. The ship obviously has some form of artificial gravity.
Correction: Well, you must just hate science fiction, then. Practically every sci-fi film set in space features a ship or station with artificial gravity. This is a fictional technology, as are faster-than-light drives, hibernation pods and androids made in human form, all of which feature in the Alien series of films, all of which are unscientific and all of which presumably annoy you as well. Unfortunately for your sense of scientific indignation, the use of a fictional technology in a science fiction film is not only not a mistake, it's practically a requirement.
Tailkinker ★