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Wednesday, November 29, 2023

NASA Wants to Set a New Radiation Limit for Astronauts

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.

Partly, that’s because although the International Space Station has been hosting astronauts for 20 years, and has been home to many studies on weightlessness, it’s not really a good place to study the effects of radiation on the human body—the station sits in the protective magnetic field of low-Earth orbit. Once a spacecraft and its human occupants travel beyond that protective bubble, the radiation risk jumps, says Amy Berrington de González, a senior investigator and cancer epidemiologist at the National Cancer Institute, and a member of the panel exploring radiation risk for NASA. “What we know is the cancer risk is likely to be higher, but exactly how much higher and whether different tissue will be affected in certain ways isn’t clear,” she says. “A dose of protons to the brain might be different than a dose of protons to the stomach. There’s a lot more uncertainty of the carcinogenic effects.”

Simple physics can tell what happens when a spacecraft collides with a piece of an old satellite or fast-moving astro-pebble, but it’s a lot more challenging to predict how the body will handle a burst of invisible solar particles or cosmic rays. Health issues like cancer can be triggered by many things, and radiation is only one one of them. An individual astronaut’s risk of developing cancer is also dependent on their age, gender, family history, lifestyle factors like their diet and if they’ve ever smoked, as well as the amount of damaging radiation they might get on any trip to outer space.

And it’s hard to predict how the radiation exposure on a given mission might affect an individual astronaut. Using sensors on recent rover missions to the Red Planet, NASA has measured how much space radiation lies between here and Mars. The real challenge is figuring out what that radiation will do to a human, says Berrington de González. “It’s even difficult to project risk for a CT scan,” she says. “When you take it to space radiation, there are all different types of space radiation exposure. It's not x-rays and gamma rays, it's also protons and other particles that we have very little data for.”

NASA hasn’t updated its cancer risk calculus for astronauts in more than a decade, and the agency wants to take advantage of more recent data from animal models and those long-term studies of medical and nuclear plant workers and bomb survivors, says J. D. Polk, NASA’s chief medical officer. Once NASA finalizes a new maximum radiation standard, engineers will use it to guide their blueprints in designing ways to protect astronauts during long-duration missions. For example, Francisco points out, a three-year round trip to Mars and back might expose an astronaut to a total of 1,000 mSv, so that means NASA needs to figure out how to reduce that exposure with shielding. Some ideas include a vest called AstroRad that is being tested on the space station that might protect astronauts from solar particles, or a small shelter inside the spacecraft to protect the entire crew against a blast of high-energy galactic rays.

And it’s not just cancer that astronauts have to worry about. Radiation can cause myocardial remodeling, in which the structure of the heart begins to change, and tough, fibrous tissue grows to replace healthy muscle, potentially leading to heart failure. Other effects include atherosclerosis in blood vessels, which can cause stroke or heart attack, or inflammation, cell death and DNA damage, according to Jane Grande-Allen, professor of bioengineering at Rice University. Grande-Allen’s lab has been funded by NASA to develop an early stage in vitro cell model to study the effects of space radiation on cardiovascular disease.

“Even if there is a whole lot of shielding, if there is a small amount of radiation over a long period of time, it’s going to be leading to cardiovascular disease,” Grande-Allen says. “There’s a lot we don’t know because we haven’t done this before.”

One big difference between people on Earth and people in space is that astronauts are selected for their physical prowess and histories of good health; plus, they are constantly exercising (albeit on treadmills). All of these lower their risk of both cardiovascular disease and cancer. But while it’s good that astronauts are healthy, so far most of the research on how bodies fare in space have been done on a very nonrepresentative group of people. NASA’s astronaut corps has overwhelmingly male and white since John Glenn became the first American in space back in 1962.

The agency had to extrapolate the cancer risk for female astronauts based on other kinds of radiation research. Studies of Japanese atomic bomb survivors have shown that women have a higher risk of radiation-induced lung, breast and ovarian cancers, which is why NASA has had a stricter standard for female astronauts. Over the years, that has led to complaints from some former female astronauts that they were being discriminated against and unable to advance their careers by spending more time in space. NASA’s new proposed radiation standard would be the same for all astronauts.

Polk says he believes it’s important to allow the best and most qualified astronauts to fly on long-duration missions beyond low-Earth orbit, and to ensure that experienced astronauts won’t be disqualified because of a health standard that is still a work in progress. “There’s always a risk balance,” says Polk. “You don’t want to remove someone based on an arbitrary number unless you have really good data to back it up.”

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