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Scientists are grappling with our biggest limitation in spaceflight: our own bodies

We’ve sent people to space for decades — but we’re only beginning to learn what that means for human health.

Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

The human body has evolved, for hundreds of thousands of years, to thrive on the surface of the Earth. But what happens when you take such an earthbound body and put it in the weightlessness of space?

Things get weird.

Astronauts commonly report diminished eyesight upon their return home, possibly because the eyeball changes shape in space and tissues surrounding the optic nerves become swollen. Without the constant tug of gravity, bones become more brittle and muscles atrophy.

Now there’s momentum to send humans into space farther and longer than we’ve ever been before, subjecting our bodies to even more of this strange environment. The White House has tasked NASA with the (hasty) mission of returning to the moon by the year 2024, and establishing a more permanent human presence there. The plan involves a permanent “lunar gateway,” a space station to orbit the moon. Those efforts could lay the groundwork for an eventual crewed mission to Mars, which would place astronauts in space and on the red planet for years.

And there are even more far-fetched dreams incubating. Tech titans Jeff Bezos and Elon Musk have both stressed that humans ought to become an interplanetary species. “We are going to build a road to space,” Bezos said at a recent event unveiling a moon lander design for his rocket company, Blue Origin. “And then amazing things will happen.”

All these grand ideas, potential missions, and dreams of a long-term human presence in space depend on one thing: that our feeble human bodies can handle it. But the truth is, no one knows what happens to a body when it spends more than a year in space, or more than a year living on the surface of the moon.

What we do have is several very important, untested, and unresolved questions on what happens to the human body in space — and how we can protect the brave people who venture out there. Here are three of the biggest unknowns and the biggest risks.

1) How does the human body respond to radiation in space?

This week marks the 50th anniversary of the first moon landing. But since the Apollo program ended in the 1970s, human beings haven’t ventured very far out from our home planet.

The International Space Station is just 254 miles above the surface and is largely protected from the worst of cosmic radiation (streams of subatomic particles that spread through space like shotgun shot traveling at superfast speeds) by the Earth’s magnetism. The moon is nearly 240,000 miles away and offers no such protection. Neither does Mars.

“Radiation doses accumulated by astronauts in interplanetary space would be several hundred times larger than the doses accumulated by humans over the same time period on Earth, and several times larger than the doses of astronauts and cosmonauts working on the International Space Station,” physicists working with the European Space Agency reported in 2018.

When NASA sent the Curiosity Rover to Mars, it found that the one-way trip alone would expose unshielded astronauts to an extra 0.3 sieverts of ionizing radiation, equivalent to 24 CAT scans. That’s 15 times the annual radiation limit for workers at nuclear power plants, but not fatal. (For context, one sievert is associated with a 5.5 percent increase in cancer risk; eight sieverts can kill.)

The effects of this radiation — and how to mitigate them during spaceflight — aren’t entirely known. The only astronauts to have spent much time outside the protective bubble of Earth’s magnetism were the Apollo astronauts.

“There weren’t any genomics study done on astronauts in those days,” says Andy Feinberg, a Johns Hopkins epigenetics researcher who worked on the recent NASA “Twin Study,” which tracked astronaut Scott Kelly and his twin brother, Mark (who served as an on-the-ground control), for a year in space.

“It’s going to be very important to have an extended period outside of near-Earth orbit habitation by astronauts, for a long period of time,” he says, in order to study the effects of radiation on their genes.

NASA maintains a Human Research Roadmap that outlines the knowns and unknowns (the known ones) of risks to the human body in space. The list of gaps is currently very long. And many of them involve exposure to radiation — either in the deep reaches of space, or on the surface of the moon, which offers no protection from cosmic radiation.

For instance, on the road map, NASA reports it’s still working to determine the dose limits of radiation an astronaut can receive before getting seriously sick, and determining what, overall, this radiation does to an astronaut’s immune system. It also doesn’t know the probability that an astronaut will be sterilized (made unable to have children) in spaceflight. They don’t know how much radiation contributes to bone loss. Does radiation in space cause or worsen neurological diseases? That’s another gap.

2) Is there an upper limit for the amount of time a person can spend in space?

In 2015, NASA sought to increase their understanding of the risks of spaceflight by sending astronaut Scott Kelly up there for an entire year — double the length of the typical mission. Because of the mission, Kelly now holds the American record for number of consecutive days in space.

Aboard the space station, Kelly took part in 10 research projects in what NASA is calling the “Twin Study,” ranging from testing his cognitive abilities to assessing how changes to his genes are expressed.

The study is hard to draw conclusions from; after all, it had a subject pool of one. But some results raise new questions. When Scott Kelly returned to Earth after spending a year on the ISS, he wasn’t quite himself. For a year and a half afterward, he scored lower on tests of his cognitive abilities — tests that he actually improved on while in space. “It’s hard to concentrate when you’re not feeling well,” Kelly told the New York Times.

His doctors don’t really know why he had such a long time recovering his mental capabilities.

There are “so many things,” that could contribute to it, says Mathias Basner, a University of Pennsylvania psychiatrist who led Kelly’s cognitive testing. There’s the higher radiation exposure, but also just living in an isolated environment could play a role, he says. Plus, it might be mentally taxing going from a microgravity environment to a full-gravity environment on Earth.

“It takes some time for the brain to adapt to the [space] environment, and apparently it also takes some time to adapt back to the gravity environment,” he says. “There are 20 things going on at the same time” that could all result in changes in cognition.

Researchers also don’t know what it means for the future: On a trip to Mars, an astronaut will, after nearly a year-long voyage in space, have to descend to the surface of Mars. It won’t be ideal for that astronaut to set foot on Mars and have her thinking become clouded.

The overall lesson: There are many stressors in the space environment. They all impact the body and mind in hard-to-understand ways. And again, the twin study was just a year long. What happens to the human body in space on a two-year mission, a three-year mission? We don’t know. There are some clues, and concerns, that things just get worse for astronauts.

One intriguing finding in the twin study was that changes the researchers noted in Kelly’s genome and epigenome (markers on our genes that develop in response to environmental stressors) occurred in the last six months of the mission. What the researchers don’t know is whether those changes would continue to accelerate if the mission was extended beyond a year.

They also don’t know exactly what those genome changes mean for health. Mostly, they appear to be a general indicator of stress. But would researchers see even more — perhaps dangerous — changes if he were to stay longer? “We don’t know what the maximum is,” Lindsay Rizzardi, a Johns Hopkins biologist who studied Scott Kelly’s genome for the twin study, says.

There could be an upper limit for the amount of time a human body can spend in space. To find out, we’ll have to send up more astronauts for a year mission or longer. Including Kelly, only six humans have spent more than 340 consecutive days in space.

3) How does the human mind cope with the isolation and loneliness of space travel?

This may be the biggest, most potentially unsettling unknown. On the NASA Human Research Roadmap, one of the listed knowledge gaps is “identify[ing] psychological and psychosocial factors, measures, and combinations thereof that can be used to compose highly effective crews for autonomous, long duration and/or distance exploration missions.”

That is, how do we make sure crews won’t kill each other on a long, cramped voyage?

The biggest unknown, potentially, is the risk to psychiatric health. A trip to Mars could take place aboard a ship smaller than the International Space Station, potentially with fewer people on board.

What’s more, there would be delayed communications with Earth as the astronauts travel farther and farther away. It will be a long, lonely, cramped journey with bad food, poor sleep, and unnatural light. What happens to people’s minds in those conditions when they last for years?

Basner has also studied what happens to the brains of people who’ve had to stay the winter confined in Antarctica — a perhaps similarly isolating experience. “You can actually see functional and structural changes in the brains of the people overwintering,” he says. “We have seen [brain] volume loss, basically widespread across the brain” in reaction to the stress.

These changes are reversed after the winter ends. But it’s unknown what brain changes might take place in the isolating, stressful conditions of deep space. And for that matter, we’re not sure how to treat them. “Astronauts are going to experience psychiatric problems, because they’re human,” Feinberg says. And not only does NASA need to figure out all the ailments that may befall the human mind in space, but it also has to learn how to cope with them.

Perhaps the scariest risks are unknown

It could be possible that the human body and mind simply cannot withstand living in space indefinitely. There may be an upper limit for the amount of time we spend there.

Whatever the case, we know any long-term mission to the moon, or Mars or beyond is going to be dangerous. It may push the human body to a new limit. But the only way we’re going to find out how to mitigate those risks is for astronauts to continue to undergo rigorous evaluation like in the Scott Kelly twin study. They’re going to have to spend long, lonely hours on the moon or in some place beyond low Earth orbit, and do tests on their bodies, brains, and genetics themselves (they won’t necessarily be able to ship back samples down to Earth for analysis).

There’s a lot to yet discover. Another research gap: NASA scientists would like to know how toxic moon dust is to breathe in. As the Apollo astronauts found out, moon dirt gets on everything, and irritated their noses and lungs.

Scientists would also like to know if the negative effects of low gravity are mitigated on the surface of the moon or on the surface of Mars, both of which have less gravity than Earth. Heck, they’d also like to know if medicines to treat kidney stones work in space. There’s so much to learn.

“The greatest unknowns, and perhaps the most dangerous,” says J.D. Polk, NASA’s chief medical officer, “are those we have not considered or are unaware of, colloquially termed the ‘unknown unknowns.’”

How do we find them? We venture out farther than before.


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