Inaccuracies in the book and movie
The Dust Storm
Let’s get the big one out of the way nice and early: The dust storm that sets everything in motion at the start of the movie is not accurate. Although Mars does get dust storms, the atmospheric pressure is so low that the wind is negligible, although the dust itself can be harmful.
“Dust storms certainly do occur on Mars, they get winds in excess of 100 mph (160 km/h), but a 100 mph wind on Mars, because the atmosphere is so thin, has the same inertia and dynamic pressure down at the surface as about an 11 mph (18 km/h) wind on Earth. It’s not going to have the sort of energy to move large objects the way that is portrayed in the book and the film.” ~ Dave Lavery, Program Executive for Solar System Exploration at NASA headquarters (film consultant)Andy Weir admits the dust storm was used simply to move the plot along and leave Mark Watney stranded on Mars.
Spending prolonged amounts of time in space runs the risk of getting radiation-related sickness like cancer. Most astronauts now stay within the Earth's magnetosphere and Apollo astronauts only spent a few days on the Moon but Ares astronauts spend up to a month on the surface of Mars and 8 months travelling between the two planets with nothing more than their spacesuits. It stands to reason that you could say it's inaccurate as all the astronauts should be riddled with cancer but as the book is set in 2039 Andy Weir has said they've probably invented some kind of thin layer of radiation shielding.
"I just have a paragraph or two that say 'Oh yeah the Hab, The Rovers, the EVA suit, they're all radiation shielded.' In reality, that's a huge problem and there is no thin, easy, light material that completely takes care of it. So realistically having been through what he went through and being on Mars for so long, he would have so much cancer his cancer would have cancer. But it's a magical technology that they've developed between now and 2035." ~ Andy Weir.
The MAV Takeoff
The take off from Mars that the MAV does for every Ares mission is not currently possible as Mars' gravity is only 30% of Earth's although NASA has planned a sample return mission for 2020 which will help them test out the take off technology they are developing.
Mars has a huge amount of water in the soil, for every cubic foot of water there's about 35L of water in it. This means that rather than using rocket fuel and Martinez' cross to make water he could have just gotten the soil and heated it up. This was discovered by the Curiosity Rover which only landed on Mars in August 6th 2012 and the book was written in 2011.
"Mars doesn't have a single solitary climate, just like Earth doesn't, we have the Sahara and we have like the Amazon. There are very different climates on Earth. Well, Curiosity is in Gale Crater which is about 5000 KM or so from Acidalia Planitia where The Martian takes place so I say Acidalia Planitia is a desert and no one can prove me wrong until we send a probe." ~ Andy Weir.
The Ares III landing sight
"So the university of Arizona runs the High-Rise instrument on Mars Reconnaissance Orbiter (MRO) [...] it does really really high resolution mapping of Mars. It's basically a spy satellite and it can take pictures with a resolution of 30 cm per pixel [...] like that high a resolution. And so int the book I say exactly where The Hab is, Acidalia Planitia, the exact latitude and longitude co-ordinates. And so the people who run the high rise instrument said 'Let's see what's there'. And so they took a high-res picture and they said 'This isn't anything like he described it', but it was pretty cool." ~ Andy Weir.
When Andy Weir originally wrote the book SpaceX was not as a big a thing as it is now and he has marvelled about how quickly they cemented themselves in the world of space technology.
"People have forgotten how quickly SpaceX came onto the scene. When I was writing the book, no one had heard of them... I started writing the book in 2009 and SpaceX were probably around but they weren't-they certainly didn't have contracts with NASA to supply ISS or anything like that. They have really come about very quickly, if I was writing the book again I would certainly have them be the people who provided the booster or something like that." ~ Andy WeirIn the movie's marketing material it is shown that SpaceX had a hand in building the Hermes.
Inaccuracies Only In The Movie
The Ironman thing
In the book the Ironman thing is mentioned as a joke but is dismissed. In the movie, Mark Watney pokes a hole in his thumb and uses the escaping air to propel himself towards Commander Lewis.
The Commander Lewis Rescue
In the book Chris Beck who is the mission doctor and EVA specialist is the one who goes out in the EMU suit and grabs Mark as that's his job and that was the plan but in the movie Melissa Lewis goes out as she felt guilty for leaving him behind and this was her way of redeeming herself. In real life, as she is the Commander, it would have been irresponsible of her to risk her life, go against the plan and go outside, especially as Beck is the EVA specialist.
Inaccuracies Only In The Book
In the book it talks about how Mark only has a certain amount of CO2 filters that would be saturated when he used them for space walks which meant he only had a limited amount of time outside The Hab and the Rover. Since the book has been released they've invented new PLSS (Portable/Primary/Personal Life Support System/Subsystem) and one of the functions of the new PLSSes is that they remove carbon dioxide, humidity, odors and contaminants from breathing oxygen. A PLSS is a device connected to an astronaut or cosmonaut's spacesuit, which allows extra-vehicular activity with maximum freedom, independent of a spacecraft's life support system.
"The New PLSSes, that's the backpack part of the spacesuit, they have the ability to separate carbon dioxide out of the air without any filters, without any expended materials at all and they can do this forever. Okay. Thanks NASA." ~ Andy Weir