"In space no can hear you scream" is the iconic tagline of Ridley Scott's Alien. The original concept for Alien came from Dan O'Bannon, who wrote and co-starred in 1974's sci-fi comedy Dark Star. This was the directorial debut of John Carpenter (The Thing) and featured a scene where O'Bannon's character chases a goofy looking basketball alien around a spaceship. The writer was later inspired to create a horror movie around this idea since he felt an inbuilt issue with most haunted house movies is that the characters could just run away; in space, they had nowhere to go.
O'Bannon co-wrote the script with Ronald Shusett (Total Recall), which was initially dubbed Star Beast before being retitled Alien, based on the number of times the word appeared in the screenplay. The movie was almost sold to famed b-movie producer Roger Corman before the unexpected success of Star Wars led to movie studios greenlighting scripts set in space. Ridley Scott was hired to direct while artist H.R. Giger - who O'Bannon previously collaborated with on a cancelled movie adaptation of Dune - came onboard to design the titular monster.
They all combined to craft a horror masterpiece, with Alien regarded as one of the best sci-fi movies ever made. Everything from Scott's direction to composer Jerry Goldsmith's haunting score has been lauded, but the movie also had a fantastic marketing campaign. This included the trailer, which suggested much but showed little, and the movie's poster. This one-sheet showed an Alien egg - which was really a decorated chicken egg - against an ominous background with the tagline "In space no one can hear you scream."
This tag goes back to Dan O'Bannon's idea of space being the perfect setting for a haunted house movie and emphasizes the isolation of the main characters. The tagline is now almost as famous as the film itself and has been referenced and parodied many times, including a poster for Stranger Things season 2 directly aping the Alien poster with the tagline "In the Upside Down, no one can hear you scream." The franchise itself would come back to this tagline for Alien 3's teaser trailer, which proclaimed "On Earth, everyone can hear you scream." This of course suggested the third movie would be set on Earth, which proved not to be the case.
Alien is a rare example of a movie where everything just seemed to line up and work perfectly, which includes the poster and tagline. "In space no one can hear you scream" sums up the tone of the movie in one succinct sentence, which is what the best taglines are able to accomplish.