A number of scientific issues emerge to lessen the credibility of the otherwise excellent thriller, Alien. Some are relatively unimportant; for instance, it seems biologically impossible that all the eggs Kane discovers should be alive, as the setting indicates likely centuries or millennia having passed since they were laid. This could have been solved by having Kane come upon them in stasis chambers. More meaningful are errors regarding the alien and its gestation. In plain terms, it is unthinkable that the captain would not ask about an examination of Kane after the face-hugger is gone, or that a science officer would not volunteer the information that Kane had been medically examined. Equally irrational is that the body scan of Kane reveals the alien injecting a tube down his throat, yet nothing is seen or discussed of the implanted alien fetus in his abdomen. While the “dinner scene” impact would have been lost, the more realistic approach would be Kane’s being isolated in a chamber following the detection of the gestating alien, which would erupt in that setting. Lastly, greater explanation is needed regarding the alien blood. It melts through several metal floorings, so Ash should quickly explain some property in the face-hugger’s dermis able to contain it.
The Aliens sequel presents few issues in terms of the creatures themselves, but there are still problematic situations scientifically. To begin with, it is irrational that one colonist would be just about to “give birth,” when all the others have clearly been dead for some time; the rescue mission had not been engaged till after contact was lost with the colony, indicating that the aliens had succeeded in harvesting the humans months before the ship set out. To address this, Newt could have mentioned to Ripley later that only one other woman had survived until very recently. Another factual issue lies in the initial battle between the aliens and the marines. Ripley insists that all are directly beneath a fusion reactor, yet all the ensuing gunfire, in every direction, somehow fails to breach the reactor. This could have been addressed by having Gorman find a way to shut down that reactor, as the marines must wait in suspense. Equally unrealistic is the minimal damage to the queen after she hurls a large number of powerful grenades in the nursery chamber. Resolving this would be Ripley’s having lost her vest of ammunition before entering the chamber, through some difficulty in moving through the altered corridors. Lastly, it is scientifically unrealistic that Ripley is able to climb the ladder after the queen is ejected, and the hatch exposes her to the intense pull of space drawing out anything not stabilized. More exactly, she climbs one hand at a time with her feet basically dangling. Far more realistic would be footage of Ripley more desperately clinging to the ladder with both arms as she struggles.
Lastly, the primary scientific problem with Alien 3 exists before the story begins. Everything relies on at least one egg having been laid on the Sulaco, as both Newt and Hicks are killed, and Ripley is impregnated. This is unaccountable. At least one alien had infiltrated that ship in Aliens, but only a queen can lay eggs. Moreover, the second film makes it clear that Ripley destroyed the queen’s reproductive sack depositing the eggs. Arguably, the queen could still lay eggs, but the timing is remarkable; this would have needed to be done virtually immediately upon the queen’s sneaking into the craft. To resolve this, and when Ripley interacts with Bishop’s remains in Alien 3, the android could relate to her how the alien that had earlier infiltrated the Sulaco, killing Spunkmeyer and Dietrich, had also smuggled aboard an egg. This would be rational, and easily done within the brief exchange when Ripley seeks answers.