Other mistake: During the storm scene in the beginning of the movie, the astronauts' faces inside the helmets are brightly lit, meaning there's a light source pointed directly in their face. That's something that would render them mostly blind and unable to see and appears to be nothing but a dramatic effect for the camera. (00:05:00 - 00:08:00)
The Martian (2015)
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Directed by: Ridley Scott
Starring: Matt Damon, Jeff Daniels, Jessica Chastain, Kristen Wiig, Kate Mara
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More mistakes in The Martian
Mark Watney: I don't want to come off as arrogant here, but I'm the best botanist on the planet.
Trivia: When discussing "Operation Elrond", the Director says he wants his codename to be "Glorfindel." This shows he has read the book, as Glorfindel was cut out of the movie and replaced with Arwen.
Question: Why would NASA decide to send a botanist on a mission to Mars? A planet where no plants can grow.
Answer: Botanists going to mars can study the ground and the dirt so they could make life on mars. Botanists are also helpful due to oxygen in space, he grows plants on the spacecraft for the oxygen that they give off.
Answer: Part of his job, aside from also being a mechanical engineer, was to use soil taken from Earth to Mars, mix it with Martian soil then grow seeds in it to see how Martian soil is for growing crops. This would be preparing for a longer term mission where growing full crops to feed the crew would be part of the mission.