Space radiation comes from two main sources: solar storms and galactic cosmic rays. Solar storms are intense, short-lived, and infrequent. Fortunately for Mark, there weren’t any during his mission. He dodged that bullet. However, he and his crewmates could not have avoided cosmic rays. These are high-energy particles that arise from supernovas, colliding neutron stars, and other violent events happening all the time in the Milky Way. They are ever-present, 24/7, and there is no way to avoid them. So far, NASA has developed no effective shield against these sub-atomic cannon balls from deep space. “Doubling a nominal spacecraft shielding thickness only reduces the GCR [galactic cosmic rays] exposure by a few percent,” notes Turner.
In the movie, Watney is actually safer than the crew of the Hermes. Turner explains: “The radiation exposure is significantly less on the surface of Mars. For one thing, the planet beneath your feet reduces your exposure by half. The atmosphere, while thin, further reduces the dose. The dose rate on Mars, while high, is only about 1/3rd of that on the Hermes.”
The biggest threat from cosmic radiation exposure is the possibility of dying from radiation-induced cancer sometime after a safe return to Earth. NASA’s radiation limits today are set to limit this life-shortening risk to less than three percent. Taking into account many factors, such as the phase of the solar cycle and the number of days the crew spent in deep space and on the surface of Mars, Turner has calculated the total dose of cosmic rays absorbed by Watney (41 cSv) and the crew (72 cSv). “cSV” is a centi-Seivert, a unit of radiation commonly used in discussion of human dose rates.
There is considerable uncertainty in how these doses translate into an increased risk of cancer. Turner estimates the added risk to Watney as somewhere between 0.25% and 3.25%. For members of the crew, the added risk ranges from 0.48% to 7.6%. The high end of these ranges are well outside NASA safety limits. The crew especially could be facing medical problems after their homecoming.
Post-flight cancer is not the only problem, however. “There is some additional concern that sustained radiation exposure could lead to other problems that manifest during the mission, instead of years afterward. Possible examples include heart disease, reduced immune system effectiveness, and neurological effects mimicking the symptoms of Alzheimer disease.”