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When Dan O'Bannon and Ronald Shusett wrote the screenplay, they did not set the action in a specific year; they had only specified 2087 as the year in which humanity had several established colonies in neighboring star systems, and suggested that the film took place some time after that. The book "Aliens: Colonial Marines Technical Manual", a 1995 in-universe guide to Aliens (1986), was one of the first non-canon sources to set Aliens in 2179, based on a transmission date of 6/12/79 used in that film; by extension, Alien (1979) would take place 57 years earlier, in 2122. This date was subsequently used in a set of fictional Nostromo crew profiles as well, a bonus feature on the 1999 Alien Legacy DVD boxset based on backstories that Sir Ridley Scott had written for each character. The year 2122 was later officially canonized, and has since been used to date other media in the franchise (films, comics, games etc.).
The blue laser lights that were used in the alien ship's egg chamber were borrowed from The Who. The band was testing out the lasers for their stage show in the soundstage next door.
The chestbursting scene was NOT filmed in one take (despite the myth). The scene was filmed twice: on the first take, the chestburster was unable to cut through Kane's shirt, so the crew needed to reset and shoot it again. The failed attempt is visible in the finished film, since director Ridley Scott thought it made it look like the creature was struggling to push its way out, and made the scene more violent (see The Beast Within documentary where this is discussed). According to designer Ron Cobb, the two takes of the scene were shot from multiple angles, and about 40 minutes of footage was available for screening.
Shredded condoms were used to create tendons of the beast's ferocious jaws.
According to Yaphet Kotto, Sir Ridley Scott told him to annoy Sigourney Weaver off-camera, so that there would be genuine tension between their characters. Kotto regretted this, because he really liked Weaver.
To get Jones the cat to react fearfully to the descending Alien, a German Shepherd was placed in front of him with a screen between the two, so the cat wouldn't see it at first. The screen was then suddenly removed to make Jones stop advancing and start hissing.