A new study of 418 former astronauts and cosmonauts has found that the time they spent in space doesn’t appear to carry any increased risk of death from causes associated with radiation exposure.
The study did not, however, find that future deep-space ventures, such as a return to the Moon or a mission to Mars, are equally safe.
Studies of atom-bomb survivors and nuclear workers have shown radiation exposure to be associated with increased risk of death from cancer and cardiovascular disease (CVD), says Robert Reynolds, an epidemiologist and biostatistician with Mortality Research & Consulting, Inc., in City of Industry, California.
Radiation doses experienced by most astronauts are low enough, however, that standard methodologies have been unable to find any increased risk, Reynolds says. “NASA has done a good job of limiting people’s doses, from a mission planning perspective and an engineering perspective,” he says.
That, of course, is good news, both for space buffs and for astronauts hoping for long, healthy retirements.
But that doesn’t mean there’s no reason to dig deeper. “What we’re always looking to do is to say, ‘Is there any reason to doubt that?’,” he says.
In a study published in the journal Scientific Reports, a team led by Reynolds did just that, by taking advantage of the fact that radiation is known to cause both cancer and CVD.
The study began by tabulating publicly available data from every astronaut and cosmonaut who ever made it into space, looking to see if they are still alive and, if not, what they died of, and when.
The statistical methods were complex, but what the scientists were looking for was to see if there was a linkage between space travelers’ rates of cancer deaths and CVD deaths.
Such a linkage, Reynolds says, would suggest a common cause, such as radiation exposure. “But we don’t see that,” he says. “Therefore, radiation cannot be a common cause, because there’s no common cause.” {%recommended 7927%}
Other scientists’ reactions are mixed. “I think it’s an interesting paper, and I like the approach,” says Kira Bacal, a space medicine specialist formerly with NASA’s space medicine team, now at the medical school of the University of Auckland, New Zealand.
Her main concern is that other than the 27 astronauts who visited the Moon (or flew around it) during the Apollo program, none of the 391 other astronauts or cosmonauts had been outside the protection of the Earth’s magnetic field and its Van Allen radiation belts, which extend several thousand kilometres above the surface.
“In terms of radiation impacts on the human body, there is a substantial difference between the kind of exposures below the Van Allen Belts and above,” she says. “It’s apples and, like, baseball bats. Two completely separate things.”
One of the risks outside of the Earth’s protection, she says, is from solar flares, which can send huge blasts of radiation hurtling toward thinly shielded spacecraft.
“The Apollo astronauts were extremely fortunate that one solar flare was missed by their mission by literally days to weeks,” she says.
It is likely, she adds, that if the flare had caught the astronauts on the Moon or in transit between the Earth and the Moon, the radiation would have been intense to kill them, if not immediately, then shortly afterward, from cancer.
Also dangerous above low-Earth orbit (LEO), she says, are cosmic rays, which contain super-energetic particles as large as the nucleus of an iron atom. When something like that hits a cell, she says, “it’s going to come through like a fully loaded semi-truck”.
A study with mice, she adds, even suggests that this type of radiation might trigger brain changes akin to early-onset dementia – and might do it quickly enough to kick in during the course of a Mars mission.
Jeff Chancellor, an applied physicist at Louisiana State University, Baton Rouge, whose research focuses on mitigation of space radiation risks to astronauts, has additional concerns, including the fact that the study lumps together cosmonauts and astronauts.
“There are tremendous lifestyle differences – for example smoking, diet, exercise – between Russian and American cultures, which have the ability to impact lifespan,” he says.
Also, he adds, not all astronauts spend the same amount of time in space. “Most are short-duration fliers,” he says. “Short-duration versus long-duration mission exposures are two entirely different beasts and cannot simply be lumped together.”
Reynolds recognises that his study isn’t designed to address all such issues. To begin with, he says, astronauts going to the Moon or Mars are going to get a lot more radiation, and potentially of different types, than those in low-Earth orbit.
What his study is designed to do, he says, is to check that what we think we know about the relatively low dangers of radiation exposure in LEO isn’t wrong.
In other words, he says, don’t interpret the study as saying that there’s no reason to worry about radiation, but rather as reality check on whether the things we think we know about low-dose space-radiation exposure are wrong.
And so far, he says, the answer is “no”, they aren’t.
