“The Martian,” both the book and the film, has always been generously praised for its realism. The Wall Street Journal noted that the novel “read like a detailed survival manual,” and Jim Green, NASA’s director of planetary sciences, told CBS earlier this month that the film “is as close to science fact, as any science fiction I have seen in going to Mars.”
Scientific accuracy runs in the blood of the novel’s author, Andy Weir, who was the child of a particle physicist and an electrical engineer. Weir had no intentions of writing a best-seller at the start, and instead concentrated on getting the details right.
But for the premise of the story to work, Weir had to introduce several elements that don’t exist in reality, and laid out his workarounds in a Q&A on the popular discussion board Reddit, where earnest fans grilled him on miscellaneous details in his novel.
The story of “The Martian” revolves around NASA’s Mark Watney trying to survive on the red planet after he gets cut off from the rest of his crew in the middle of a dust storm. One fan asked Weir if such withering storms were possible on Mars.
“No. Mars’s atmosphere is too thin. This was a deliberate concession to drama that I made because it’s a man-versus-nature story and I wanted nature to get the first punch in,” Weir replied.
The fastest winds on Mars reach speeds of up to 100 miles per hour, according to NASA research, the same as hurricane winds on Earth, but on Mars they’re fairly harmless because of their low mass. The atmospheric density of Mars is around 1/100 that of Earth.
“Even if you were stood right in the middle of a dust devil, you’d feel roughly the same strength of wind blowing past you that you’d feel on a gentle cycle ride on a calm day,” Robert Walker, a programmer and inventor, wrote on Quora.
Once stranded on Mars, Watney makes his way toward the Schiaparelli crater, where another manned mission will arrive in four years. Spending that much time on the planet would expose him to a dangerous, and probably fatal, amount of cosmic rays, if not for his handy (and fictional) radiation blocker.
“The book has a completely fictional material that blocks radiation. No such thin, flexible, light radiation shielding exists in the real world,” Weir told the AMA audience.
In the real world, cosmic ray radiation sets an upper limit on how long astronauts can stay in outer space. NASA’s Curiosity rover calculates that a manned mission to Mars—180 days to get there, a 500 day stay on the planet, and 180 days to get back—would cumulatively expose astronauts to 1.01 sieverts, above the 1.0 sievert lifetime limit that European Space Agency imposes on its astronauts.
NASA limits its astronauts to between .8 and 1.2 sieverts of lifetime radiation exposure, which would raise the lifetime risk of cancer by 3 percent. Because the sample size of astronauts who have spent a considerable time in space is so small, the exact health effects of cosmic ray exposure is clouded with uncertainty. A 2014 report from the Public Library of Science suggests that cancer risks were higher than previously estimated, meaning that NASA and other space agencies should lower the lifetime exposure limit.
The most effective shields against cosmic rays, like water or liquid hydrogen, tend to be heavy and not portable—meaning they’re suitable inside the walls of a space capsule, but impractical in a spacesuit. Researchers are working on plastic cosmic ray shields, but haven’t gotten there yet.
There were also inaccuracies that Weir didn’t plan on. On Mars, Watney mixes hydrazine, a toxic substance usually handled by people in hazmat suits, and oxygen to make drinking water. The admixture also produces ammonia, which as one fan points out, “isn’t very healthy either.”
“Yes, very toxic. I didn’t know that at the time. Had I known, I would have had him wear his EVA suit during the process,” Weir replied.
But Weir doesn’t mind these minor blemishes; relative to other science fiction novels, his book is as real as they come.
“I’m not worried about my work coming under the eye of experts. It’s a work of fiction, and it’s going to have inaccuracies. That’s just how it is,” Weir wrote. “Getting a thumbs up from those experts and hearing them say ‘It’s more accurate than anything else in the field’ is a good feeling.”