The Martian is already proving to be a big hit in cinemas, but Ridley Scott’s slick adaptation unfortunately glosses over some of the best bits of Andy Weir’s engaging debut novel.

Last week’s cinematic release of sci-fi blockbuster The Martian has proved an unequivocal success. It was bound to: you’ve got Ridley Scott doing another space movie, you’ve got Matt Damon as your star, you’ve got the real-world hype surrounding news from the Red Planet (Curiosity! Surface water!) and you’ve got an absolutely outstanding advertising campaign, with promo videos hyping not the movie, but the Ares 3 mission itself, as if it was really happening in our world. You’ve also got the amazing source material, the debut novel by Andy Weir, which whet a lot of appetites.

In case you’ve been living on, well, Mars, here’s the basic rundown: The Martian takes place in the not-too-distant future when mankind’s third manned mission to Mars is forced to evacuate early. Engineer and botanist Mark Watney is incorrectly presumed dead, and the story follows his determination to keep himself alive on the inhospitable planet, and NASA’s attempts to save him. It’s the biggest movie in the solar system right now, and with a Rotten Tomatoes rating of 93%, fans and critics seem to agree that The Martian is worth the hype.

One of the novel’s major differences from the film is that it opens with Watney, already stranded, beginning a written log. The reader gets Mark’s thoughts and Mark’s thoughts alone for the first few chapters, and as we join him in his isolation, we learn a lot about how he solves problems and how he’s keeping himself alive. It takes him days or weeks to make progress on anything he attempts. It’s a long, slow, tedious slog, made vivid to the reader through Watney’s humor and through Weir’s layman’s explanation of many precise scientific details.

Surprisingly, life on Mars for the movie’s Watney seems too slick, too easy. He seems to instantly have the solution to every problem, and some of those solutions seem pretty implausible. For example, the movie doesn’t mention the Earth soil samples Mark had for his planned botany experiments, containing the crucial bacteria to make plants grow. Because there’s no time for him to explain that his repair kit contains a space-grade resin glue, the cracks in his space helmet are sealed with duct tape. There’s a thousand other moments like this that gloss over Weir’s meticulous research — the book is said to be as realistic as possible based on existing technology.

It’s hard to use one scene to sum up the scope of the science lost in the name of action, just as it’s hard to pick a single moment that would have brought Watney’s strength of character through better — though the Aquaman bit, which Weir fought a losing battle to keep, comes close. Damon’s Watney is a much more emotional beast, prone to fits of introspection about his place in the universe. It can’t be easy to adapt a book where the main character doesn’t actually speak aloud to anyone else until the very end. There’s only so much a casual audience can tolerate of Watney’s log entries — changed from written to video updates — as he explains his actions to the camera. However, there are some specific scenes that we think should have made the final cut.

Houston, We Have A Problem

Not a huge thing, but the book’s fast-paced run of scenes which introduce the story’s major players on the ground — administrator of NASA Teddy Sanders, director of Mars operations Venkat Kapoor, Hermes flight director Mitch Henderson, director of media relations Annie Montrose, and SatCon employee Mindy Park, who discovers that Watney is alive – does a better job of presenting these characters, their dynamic with one another, and what their job descriptions and primary concerns are. This all weighs in to the decisions and arguments that come later on.

Also, in the novel, Mindy’s discovery of Mark’s movements is the first thing that breaks us out of his isolated viewpoint (even Mark’s original stranding is described later in the book, as a flashback) and it spurs the world to action. The tone of the movie is quite different to the tone of the book, due to this change in structure.

Mark’s Big Mistake

Admittedly, it might be boring to watch something that feels like it’s taking months and months, but the movie went a bit too far in the other direction, and Mark’s time on Mars seems to move much more quickly than it should. That’s one thing, but the elimination of the following scene marks the first true deviation from the events of the novel. During the outfitting of the rover for his cross-planet drive, Book-Mark makes what he calls “the mistake that might kill me.”

While in the process of using his drill to open up the roof of the rover, he takes a break and leans the piece of machinery against a metal table. This piece of basic human error leads to him losing contact with NASA, because on that table sat Pathfinder and the drill gave the device a power surge that completely fried it. Watney continues with the plan NASA gave him, but he’s all alone again until he actually reaches the working comms of the Ares 4 MAV.

The Plan for Johanssen

One of the book’s most fascinating but disturbing moments was a conversation between Beth Johanssen, the young computer expert on board the Hermes, and her father back on Earth, when the decision was made to extend the Ares 3 mission by over 500 days. Her parents criticize the choice, frantic with worry about her, but Johanssen (Kate Mara) reveals, rather ashamedly, that no matter what, she will survive the trip.

“They always have a plan,” she admits, and her survival is a plan made by Commander Lewis: if the Taiyang Shen supply probe fails, the rest of the crew will immediately take pills to kill themselves. Johanssen — as the youngest and smallest, requiring the least nourishment — will bring the ship back to Earth, surviving on the meager remaining rations, and eventually, the bodies of her crewmates. This scene may have been considered a little dark for a mainstream blockbuster, but in the book it’s a realistic reminder of the risks of being stranded in space.

The Perils of the Journey

Possibly due to the fact that Watney gets to stay in contact with NASA on his journey, the movie goes on to eliminate two major obstacles that he faces on his rover road-trip. Firstly, it nixes the dust storm that Mark encounters — NASA watches horrified from afar as he drives into a storm that slowly thickens around him. It isn’t life-threatening in the traditional sense, like the storm that caused the Ares 3 mission to abort, but the dust clouds prevent his solar panels recharging, which diminishes the distance that the rover can drive per day. He risks not arriving at the MAV in time for the Hermes fly-by, but once he works out what’s going on, he “sciences the shit” out of the situation and manages to outrun the storm.

Mark also has a crisis when he’s very close to the finish line — as he begins to enter the Schiaparelli Crater, he runs into some soft ground on the slope and flips the rover and its trailer head over tail. These are huge, high-stress moments in the book, and the flipping, at least, would have still been a good moment of tension whether he was in contact with NASA or not. But in the movie, Mark preps for his drive (skimming over the finer details) and then a time jump of seven months has us meeting back up with him as the Ares 4 MAV — his rescue vehicle — comes into view. It makes the perilous drive feel like nothing at all.

The Reality of the Rescue

It makes sense that if you cast two-time Oscar nominee Jessica Chastain you’d want to give her as much to do as possible, but in The Martian movie, we ended up with a bit of a “Hermione-gets-all-of-Ron’s-best-lines” situation. Not only did the film re-assign of the duty of speaking with Watney’s parents to Commander Lewis — a task that originally fell to Martinez, Watney’s self-stated best friend — it also gave her the film’s ultimate “hero moment” of actually retrieving Watney onto the Hermes after he is blasted into space in his butchered MAV.

In the book, this scene was all Chris Beck, who, along with being the ship’s flight surgeon, was also their EVA (extravehicular activity) specialist. As the astronaut onboard with the most experience and training in “space walks,” this job fell to him as he was the most qualified for it and much of his character’s emotional depth hinged on the risks he was willing to take to get to Watney, like going out into space untethered. However, in the movie, Lewis pulls Sebastian Stan’s Beck off the job at the last minute and does it herself, purely because her emotions get the better of her. This seems to diminish her strength as a brilliant commander.

By the way, the book has no Iron Man moment — the joke is made, but it’s filed firmly in the “ridiculous” column and dismissed. Watney later describes how he didn’t reunite with his crewmates until after Beck had rushed him straight to medical, and ironically, he even comments on how unrealistic a “movie moment” ending, with all the crew rushing to him in the airlock, would have been… which, of course, is precisely how the movie does it.

But hey, it’s not all bad. The Martian is a pretty entertaining movie, and getting to witness the geography of the Martian landscape is awe-inspiring. Plus, a lot of really great character moments did survive the move from page to screen. Here’s one they nailed…

The Council of Elrond

Project Elrond is the name that Venkat gives to the meeting in which he reveals the “Rich Purnell Maneuver,” the faster rescue plan involving the Hermes crew, rather than a simple supply probe for Watney being sent straight to Mars, to NASA’s main players. All of the scientists involved immediately pick up on the implication of the name — that it’s a secret meeting where a momentous decision will be made – with Annie Montrose, head of PR, getting frustrated about not understanding. It’s one of the book’s funniest non-Watney moments, and in the movie it’s made even better.

For starters, Kristen Wiig plays Montrose, and she makes everything great. Secondly, Rich Purnell himself is present, and Donald Glover gives a charming, irreverent, motor-mouthed performance as he explains the plan he’s developed to his superiors, and finally, Mitch Henderson, the Ares 3 filght director, is played by original Council of Elrond attendee Sean Bean. The movie doesn’t linger on this subversive moment — there’s no wink-wink-nudge nudge, in fact, Ridley Scott wanted to cut the joke entirely, probably because he hates joy — but the second the audience realizes what they’re looking at, it’s pure gold.

You can read what author Andy Weir had to say about the book-to-movie changes in this interview with HitFix.