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On Friday, the movie "The Martian" opens in theaters across the country. It's about an astronaut who gets marooned on Mars. Space experts say it is Hollywood's most realistic depiction ever of the challenges of surviving on Mars. NPR science correspondent Geoff Brumfiel found out why.
GEOFF BRUMFIEL, BYLINE: If you watch "The Martian," you'll see plenty of Hollywood explosions and special effects, but you're also going to hear actor Matt Damon, who plays the astronaut, sit down and calculate how much food he has left.
(SOUNDBITE OF FILM, "THE MARTIAN")
MATT DAMON: (As Mark Watney) For redundancy, they sent 68 soles worth of food. That's for six people. So for just me, that's going to last 300 soles, which I figure I can stretch to 400 if I ration. So I've got to figure out a way to grow three years' worth of food here on a planet where nothing grows.
BRUMFIEL: The numbers are a matter of life and death for Damon's astronaut, and the estimates for food and water rations are all for real.
ANDY WEIR: If you care to double-check the results from what you see in the film, then you'll find out it's accurate.
BRUMFIEL: That's Andy Weir, the author of the book "The Martian" which the film is based on. After figuring out how long everything would last, the stranded astronaut realizes he will have to grow potatoes by planting some the crew brought along. But how many potatoes can grow in his enclosed habitat? Here's Weir reading from the book.
WEIR: (Reading) In 62 square meters, I could grow maybe 150 kilograms of potatoes in 400 days. That's a grand total of 115,500 calories, a sustainable average of 288 calories per day.
BRUMFIEL: There are no quick Hollywood fixes to the challenges faced by Weir's astronaut. Weir himself is a longtime computer programmer who got the idea for the book while thinking about what a mission to Mars might actually be like and what could go wrong.
WEIR: What happens if this thing breaks? What happens if that breaks? What's the backup plan for this? How do you make sure the crew doesn't die at the first sign of trouble? And I finally thought, huh, this is actually - might make for a pretty interesting story.
BRUMFIEL: Weir put the first draft of his novel up on his personal website. He guesses it was read by about 3,000 or so hardcore fans, readers who really knew the science.
WEIR: Chemistry professor, physicists and science and space enthusiasts.
BRUMFIEL: These were the kinds of readers who love to detail.
WEIR: These are nerds like me. These are my people. And so I was writing something for them. I was writing a story where I show my work.
BRUMFIEL: Watching Weir's astronauts solve life or death problems without sci-fi teleporters or magical aliens is what drives "The Martian" forward. And it takes the plot in some unexpected directions. For example, those potatoes - when Weir worked out the details, he realized they would cause a water shortage. That's because the mission brought along enough to drink but not enough to grow crops. The astronaut has to start making water by burning rocket fuel, a chemistry trick that is scientifically accurate and, as the astronaut finds out, really dangerous.
WEIR: It's, like, one of the best parts of the book, in my opinion. And so all of that plot - that cool plot thing that just came about because I did the math on potato growth. I wouldn't have thought of it on my own.
BRUMFIEL: Weir's super-nerd readers pushed his realism even further. Whenever he made a mistake, they let him know.
WEIR: There is nothing a nerd likes more than finding a math error. I speak from experience. When I see it, I'm like, oh.
BRUMFIEL: And because the book was initially posted online, he just rewrote it to make it right.
WEIR: I fixed it. I corrected it (laughter).
BRUMFIEL: The result was a novel - and now a film - that even real Mars experts can enjoy.
SUZANNE SMREKAR: To be honest, at times, it felt a little bit like I was at work.
BRUMFIEL: Suzanne Smrekar is a planetary scientist at NASA's Jet Propulsion Laboratory who's helping put together a real mission to the red planet, though it's robotic. She read the book and says it gets tons of things right - one of the big ones - all that problem-solving. That's the way real scientists see the world.
SMREKAR: Every scientist and engineer has sort of had that buzz of figuring something out. And the way I read it, it's really about the thrill of trying to figure things out on your own.
BRUMFIEL: Of course, being a scientist, it didn't take long for her to find a hole in the plot. Right at the start, the astronaut is stranded on Mars by a powerful dust storm that separates him from his crew.
(SOUNDBITE OF FILM, "THE MARTIAN")
DAMON: (As Mark Watney) Commander, are you OK?
JESSICA CHASTAIN: (As Melissa Lewis) I'm OK.
BRUMFIEL: (Imitating buzzer). Wrong.
SMREKAR: The atmosphere of Mars is one-one-hundredth the density of our own atmosphere, so it really cannot provide much of a force. That's the one big artistic license in the whole book.
BRUMFIEL: Actually, Andy Weir knew dust storms on Mars wouldn't be that fierce. But he wanted a thrilling way to start his novel, so he allowed himself some literary leeway. Another problem is that the science keeps changing. Earlier this week, researchers announced the discovery of small quantities of saltwater in some regions of Mars. Maybe the astronaut could've used that water to grow potatoes. Obviously it's too late to update the plot now that the story is a blockbuster film. But you know what? Experts like Smrekar are willing to go easy on "The Martian" when it's time to give the final grade for accuracy.
SMREKAR: I would give it an A-minus, B-plus.
BRUMFIEL: After all, it is science fiction. Geoff Brumfiel, NPR News.
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