NASA Wants to Set a New Radiation Limit for Astronauts
February 11, 2021
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Now that the Biden administration has signaled its support for NASA’s Artemis mission to the moon, maybe we should think about the risks astronauts will face when they get there, and what might happen during a longer trip to Mars.

Of all the things to worry about while traveling in space—equipment malfunctions, the weird effects of weightlessness, collisions with space debris, and just being far away—one the most difficult to deal with is the health effects of radiation from the sun or cosmic events. This radiation consists of atoms that have lost their electrons as they accelerate in interstellar space, approaching the speed of light—something that happens right after a star explodes, for example. It comes in three forms: particles trapped in the Earth’s magnetic field; particles shot into space during solar flares; and galactic cosmic rays, which are high-energy protons and heavy ions from outside our solar system.

It’s also one of the “red risks” identified by a NASA study published last year on the highest-priority health problems faced by astronauts. Radiation damages DNA and can lead to mutations that can trigger cancers. It can also cause cardiovascular health problems such as heart damage, the narrowing of arteries and blood vessels, and neurological problems that can lead to cognitive impairment, according to a NASA website.

On Earth, humans are exposed to 3 to 4 millisieverts (mSv) of radiation a year, mostly from natural sources like some kinds of rocks and the few cosmic rays that get through the atmosphere. On the International Space Station, astronauts get about 300 mSv per year. Until now, a 55-year-old male NASA astronaut was limited to an effective dose of 400 mSv over his career, while a 35-year-old female astronaut could only be exposed to 120 mSv.

Now that NASA is planning to send people on much longer missions, the agency is considering raising that threshold to 600 mSv for astronauts of any gender or age. Under the existing standard, some veteran astronauts might have been excluded from longer-term space missions because they are bumping up against lifetime radiation limits. Younger astronauts have less flying time in space and hence less exposure, but the success of a big mission might require experience over youth.

NASA’s proposed new limit would still be lower than those for other space agencies; European, Russian and Canadian astronauts can be exposed to up to 1,000 mSv before they get grounded by their space officials. But NASA officials don’t apologize for their more conservative stance. “It’s a different risk posture in what we feel is acceptable risk,” says David Francisco, technical fellow for human spaceflight standards at NASA’s Office of the Chief Medical Officer. “We picked 600 because we feel it’s more acceptable to our culture. It’s something we constantly work on and go back and forth on. We debated on going to 1,000, and that’s one of the questions: Are we still being conservative with 600?”

To resolve that question, the space agency has asked an expert panel from the National Academy of Sciences to determine what’s the best number to use. The panel began meeting last month and is expected to complete its work by this summer. The experts will look at how NASA has calculated its new exposure limits, and how those match up with existing clinical data and animal studies.

To understand the links between radiation and cancers, medical researchers have long been following survivors of the atomic bomb blasts in Japan during World War II (as well as the health of their children). There have also been studies of medical workers who are exposed to x-rays, and nuclear plant workers, who receive low doses of radiation over the courses of their careers. But NASA doesn’t have much data on the health effects of radiation from space on its astronauts.

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