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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