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 fit any size astronaut, the new suit comes with modular components across the chest and waist that can be cinched or expanded.“We need to learn to live and work on the surface of another world for long periods of time, and in order to do that we need space suits,” Bridenstine told a roomful of NASA employees, students, and reporters at NASA headquarters in Washington.
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.