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5 of The Martian’s boldest scientific twists, explained

Astronaut Mark Watney (Matt Damon) keeps track of his Martian journey.
Astronaut Mark Watney (Matt Damon) keeps track of his Martian journey.
Twentieth Century Fox

For all the zero-gravity, high-octane action sequences and breathtaking landscapes, the most remarkable thing about The Martian is how it converted scientific problems into thrilling drama. The race to get astronaut Mark Watney (Matt Damon) back from Mars depends on Watney's own ingenuity and the best and brightest scientists on Earth putting their heads together to figure out this seemingly impossible problem.

Vox reached out to scientists more familiar with Mars, in a bid to better understand the science behind five of The Martian's thrilling twists and outstanding questions.

Warning: If you haven't seen the movie yet, this post will spoil specific plot points and, perhaps, their miraculous cinematic luster.

1) The sudden and catastrophic storm that forces Watney's crew to leave him behind

The inciting incident is a horrific storm that whips up almost out of nowhere, taking the Ares 3 crew by surprise. They scramble to get back to their ship for an emergency departure — and an errant piece of equipment sends Watney flying into the distance, never to be recovered.

This is a particularly contentious point in the general science of The Martian. Even Andy Weir, the author of the original e-book that the movie is based on, acknowledged as much in a recent interview with NPR:

In reality, Mars' atmosphere is 1/200th the density of Earth's. So while they do get 150 kilometers an hour sandstorms, the inertia behind them — because their air is so thin — it would feel like a gentle breeze on Earth. A Martian sandstorm can't do any damage. And I knew that at the time I wrote it.

Jack Mustard, professor of earth environmental and planetary sciences at Brown University, agrees. "Our atmosphere can be ferocious and whip things around," he wrote via email, "[but that's] not likely on Mars."

But Michael Mumma, the founding director at NASA's Goddard Center for Astrobiology, gives the incident a little more room. "They can come up very quickly on Mars," he told me, "because the atmosphere is so bloody thin that it doesn’t take much. In some regions that have high winds, the winds are as high as 100 meters a second." (That translates to 224 miles per hour, a wind speed well within the range of a Category 5 hurricane on Earth.) He also pointed out that some Martian dust storms can envelop the entire planet, citing the 1971 Mariner 9 mission that couldn't take pictures of Mars's surface for a full month due to an enormous storm that rose 25 miles above the surface, obfuscating nearly everything.

"Of course you really wouldn’t want to be exposed to such a storm when you’re on the surface," he said. Martian dust is smaller and smoother than lunar particles, but if the storm is fierce enough, "a strong wind will drive the tiny particles into every possible crevice, including any seals on space suits, habitats, and accessible machinery." Furthermore, Mumma said, "if they impact at 100m/sec speed, the larger particles will sandblast and erode surfaces ... if a rare sharp particle strikes your spacesuit at a high speed, it could just snap a hole right through it.”

Still, The Martian's storm was so intense that it tore heavy equipment to shreds. That, at least, seems unlikely. But the ensuing storms, sudden and fierce, are still very real considerations for any mission that goes to Mars.

2) Watney grows potatoes in the Martian soil by using human excrement as fertilizer

Watney and his miraculous potato friends.

Twentieth Century Fox

Watney's botany knowledge comes in handy when he realizes that the crew left behind a drawer of potatoes, meant to be a Thanksgiving treat. He uses the stored packages of human waste from the habitat's toilet as fertilizer and cultivates himself a little potato farm as an invaluable extra food source while he figures out a way to get home.

While this detail sounds like the most ludicrous, it is in fact one of the more plausible parts of the movie. The human waste fertilizer would provide some of the microbial content the Martian soil would need in order to sustain life. Otherwise, Dr. Mumma said, Mars has "very hostile soil" because of its high level of perchlorates, oxidant reactive chemicals that can be highly toxic to humans. "You can’t just take a seed potato and ... bring it into a habitat with some oxygen and Mars soil and expect it to grow."

Good thing Watney has an unlimited supply of human waste.

3) Watney figures out a way to make more water for his potato crops

Watney takes the risk of creating his own water by using his oxygenator (a device that creates oxygen from carbon dioxide), extracting hydrogen from the crew's unused hydrazine fuel (from the MDV, or Mars Descent Vehicle), and burning them to create the extra water he needs to nourish his potatoes. The process is painstaking and risky, since burning hydrogen has a tendency to go poorly.

This method also passed the scientific test, by and large. Mumma confirmed that you can extract oxygen and hydrogen from those sources, even if collecting a useful amount of carbon dioxide from the thin Martian air is tough.

Mustard pointed out that "hydrazine is too important to expend on water when other methods are available," but conceded that Watney was in enough of a bind to warrant its use. He also wrote that the uncrewed Mars rover that NASA plans to launch in 2020 will likely include an instrument to begin similar "in situ resource utilization."

4) How did Watney survive on Mars for so long with all of the planet's radiation?

Watney looks out at the Martian desert.

Twentieth Century Fox

The film does not make much mention of the fact that Mars has a greater exposure to space radiation and would require some serious preparation to survive for as long as Watney does (500 sols, or roughly 1.5 Earth years).

"This is a serious problem," Mumma told me. Since Earth's atmosphere is vastly more dense than Mars's, it also has "a shielding equivalent of 2 meters of solid rock" against radiation. Mars, however, only has about an inch of shielding, which won't stop much.

Not only that, Mumma told me, but "if the sun has a major X-ray flare, it can send out huge quantities of radiation and they will cook you — even if you’re on a spaceship, by the way. But if you’re on the ground, you don’t get the extra shielding." He was unsure how Watney could survive a full 500 sols, though suggested that he perhaps could have driven a rover into the mouth of a lava tube to protect him.

To give an idea of just how much radiation we're talking about, consider that one of the takeaways from the Mars Curiosity Rover that's currently on the red planet in real life is that "in just getting to Mars, an explorer would be exposed to more than 15 times an annual radiation limit for a worker in a nuclear power plant."

To be generous to The Martian, we don't know exactly how far in the future it takes place (though that didn't stop Reddit from trying to guess), so NASA might have figured out the radiation issue by then. But the movie certainly made no mention of it, likely because there is no real solution to it right now, anyway.

5) The final Hail Mary slingshot tactic that gets Watney home

Mars orbit versus Earth orbit. / NASA

The late-breaking solution to ensure that a rescue attempt can reach Mars faster is when an astrodynamicist (Donald Glover) proposes that Watney's Ares 3 crew use their gravitational momentum upon approaching Earth to whip back around toward Mars as a sort of slingshot.

Apollo 13 author Jeffrey Kluger points out in Time that a similar maneuver has been used before:

Such a slingshot maneuver was what guaranteed the first few Apollo lunar crews a free ride home if their engine failed as they were approaching the moon, and it has regularly been used in interplanetary explorations, as unmanned probes swing close to, say, Jupiter, to pick up some extra gravitational speed on their way to, say, Saturn.

Mumma concedes that such a gravity assist could work between Earth and Mars, but only in a very specific instance — and even then, its success is unlikely. "When we send a probe to Mars from Earth, we have to wait until the two planets are in a special position with respect to one another," he said. With Mars moving in a different orbit from Earth, their positions line up for a slingshot maneuver very infrequently.

"Now imagine," Mumma said, "that you’ve now sent someone from Mars back toward Earth. You have to launch at the right time to get close to Earth so you can land when it’s time, and Earth will be there. So if you decide instead that, ‘Oh I’ve changed my mind, I’m not gonna land, I’m going to actually slingshot around Earth and then whip round again,’ the trouble is that Mars, meanwhile, has been moving in its own orbit. It's not gonna be in the right place for you to slingshot and catch up with Mars."

He paused. "There may be some special time when you could do that," he said, "but it’s very rare."

So if you're going to try a slingshot maneuver with a multibillion-dollar spaceship, it might as well be when you've got the help of some serious movie magic.

Corrected to reflect that the unused hydrazine fuel came from the MDV, not a rover. This article also initially overstated the severity of Martian dust, and has since been modified to reflect a follow-up with Dr. Mumma.

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