The human body has evolved, for hundreds of thousands of years, to thrive on the surface of the Earth.
Things get weird
Astronauts commonly report diminished eyesight on their return home, possibly because the eyeball changes shape in the space and tissues surrounding the optic nerves become swollen. Without the constant tug of gravity, the bones become more brittle and muscular atrophy.
Now there's an impulse 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 commissioned 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," and a space station to orbit the moon. Those efforts could lay the groundwork for a possible crew 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 should become an interplanetary species. "We're 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 weak 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 is have several very important, untested, and unresolved questions about what happens to the human body in space ̵
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 have not ventured very far out of our home planet.
The International Space Station is just 254 miles above the surface and is largely protected by the Earth's magnetism from the worst of cosmic radiation, which is spread through space like a shotgun shot at superfast speeds. The moon is almost 240,000 miles away and offers no such protection. Neither does Mars.
"Radiation doses accumulated by astronauts in the interplanetary space would be several hundred times larger than those 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, "
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 sieves of ionizing radiation equivalent to 24 CAT scan. 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 – are not entirely known. The only astronauts that have spent much time outside the protective bubble of Earth's magnetism were the Apollo astronauts.
"There was no genomics study done on astronauts in those days," says Andy Feinberg, and 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 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 known and unknowns of the 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 example, on the road map, NASA reports it's still working to determine the dose limits of the radiation an astronaut can receive before getting seriously ill, and determining what, overall, this radiation does to an astronaut's immune system. It also does not know the probability that an astronaut will be sterilized (made unable to have children) in spaceflight. They do not know how much radiation contributes to bone loss. Does radiation in space cause or worsen neurological diseases? That's another gap.
In 2015, NASA sought to increase their understanding of the risks of spaceflight by sending astronaut Scott Kelly up there for an entire year
2) Is there an upper limit for the amount of time a person can spend in space? – double the length of the typical mission. Kelly has taken part in 10 research projects in what NASA calls the "Twin Study," ranging from testing his cognitive abilities to evaluate 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 was not 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 do not 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, and a psychiatrist from the University of Pennsylvania who led Kelly's cognitive testing. There's the higher radiation exposure, but just living in an isolated environment could play a role, he says. Plus, it could 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 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 do not know what it means for the future: On a trip to Mars, an astronaut will, after nearly a year-long voyage in space, descend to the surface of Mars. It will not 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 do not know. There are some clues, and concerns, that things just get worse for astronauts.
One of the most interesting findings in the twin study was that researchers noted in Kelly's genome and epigenome (markers on our genes that develop in response to environmental stressors) occurred during the last six months of the mission. What researchers do not know is whether these changes would continue to accelerate if the mission was extended beyond a year.
They also do not know exactly what those genome changes mean for health. Mostly, they appear to be a general indicator of stress. But would the researchers see even more – perhaps dangerous – changes if he was to stay longer? "We do not know what the maximum is," Lindsay Rizzardi, and 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 will have to send more astronauts for a year mission or longer. Including Kelly, only six people have spent more than 340 consecutive days in space
3) How does human mind cope with the isolation and loneliness of space travel?
This may be the largest, most potentially unsettling unknown. On the NASA Human Research Roadmap, one of the listed knowledge gaps is "identify [ing] the psychological and psychosocial factors, measures and combinations of these that can be used to compose highly efficient crews for autonomous, long duration and / or distance exploration missions. "
That is, how do we make sure crews will not kill each other on a long, cramped voyage?
The largest unknown, potentially, is the risk to psychiatric health. A trip to Mars could take place onboard and a smaller ship 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.
Basner has also studied what's going on in the brains of people who have stayed in the winter confined in Antarctica – perhaps similarly isolating experience. "You can actually see functional and structural changes in the brains of people overwintering," he says. "We have seen [brain] volume loss, basically widespread across the brain" in response to stress.
These changes are reversed after the winter ends. But it is unknown what brain changes might take in the isolation, 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 deal with them.
Perhaps the scariest risks are unknown
It could be that the human body and mind just can not 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 can 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 twin study Scott Kelly. They're going to have to spend long, lonely hours on the moon or in some place beyond Earth's low orbit, and do tests on their bodies, brains, and genetics themselves (they will not necessarily be able to ship back samples down to Earth for analysis.
There's a lot to discover yet. Another research gap: NASA scientists would like to know how toxic moon dust is to breathe in. As the Apollo astronauts found out, the moon dirt gets on everything, and they 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 would also like to know if drugs 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 'unknown unknowns.'"
How do we find them? We venture out farther than before.