Suni Williams and Butch Wilmore returned to Earth on Tuesday after spending nine months in space.
The two were stranded on the International Space Station after their Boeing spaceship malfunctioned and they had to wait for a SpaceX spaceship to become available to bring them home.
Nine months in space isn’t a record by any means, but it’s long enough that Williams and Wilmore likely saw some changes to their bodies during their time on the ISS.
As with any astronaut, “there’s a muscular and cardiovascular reconditioning that has to happen,” Steve Stich, tk, said in a briefing after the duo splashed down in the Gulf of Mexico.
Much of what scientists are learning about how space affects the human body comes from NASA’s research on astronauts staying on the ISS, like its Twins Study: a research program involving former NASA astronaut Scott Kelly, who lived in space for nearly a year, and his identical twin brother, Mark, who lived on the ground at the same time.
Indeed, Stich said, “every single crew member that we fly in orbit, we collect medical research data,” including drawing blood, measuring bone density, and testing vision multiple times throughout their space mission.
The lack of gravity, higher radiation exposure, space-compatible diet, and other facts of life in orbit affected Scott’s body in significant and surprising ways. While Wilmore and Williams weren’t in space for as long as Scott, they likely experienced similar changes though perhaps not as extreme.
Here are nine biological oddities that researchers have found might happen to your body if you live in space for a long time.
Your body fluids shift.
When you orbit Earth, you’re effectively in free-fall around the planet, and weightless. This means there’s nothing to force blood and other bodily fluids toward your feet. The fluid shifting from your legs to your head in a year could fill a 2 liter bottle.
Your face looks different.
With less gravity, a lot of liquids move toward and into your head — so your face looks puffy.
Your sight could change.
For the same reason that your face puffs out, your vision might get worse due to pressure changes in the brain. Fluids near the optic nerve can push on the back of the eyeball.
Deep-space radiation might also promote cataracts and impair eyesight. Even high-flying commercial-airline workers face that risk because of the thinner atmosphere.
Your bone density can change.
If you don’t exercise while in space, you could lose about 12% of your bone density in a year. Researchers are still trying to understand why this happens, though it seems to be related to how microgravity affects the tissues that makeup bones and bone cell behavior.
“The cells that build new bone slow down, while the cells that break down old or damaged bone tissue keep operating at their normal pace so that breakdown outpaces growth, producing weaker and more brittle bones,” NASA says.
You get taller — until you get back to Earth.
Since gravity isn’t pushing you down, fluid-filled discs between each of the bony vertebrae in your spine don’t get compressed, stretching your height by about 3%. After Scott Kelly’s time in space, he returned 2 inches taller than his twin brother. But returning to Earth-like gravity reverses that effect.
Your muscles can shrink.
You don’t need muscles when you’re weightless, so they atrophy and absorb the extra tissue. This is why physical exercise is a part of every astronaut’s schedule. But nothing seems to maintain muscle mass better than the strain of living in the gravity found at Earth’s surface.
You’ll be sleepy.
You’d probably be sleep-deprived. Most astronauts only get 6 hours a night because sleeping in space feels weird.
Your cancer risk increases.
Radiation bombarding your body outside of Earth’s protective magnetic field can increase your risk of getting cancer.
NASA currently limits astronauts’ lifetime radiation exposure to 3,250 millisieverts for males, which is equivalent to about 400 CT scans of the abdomen.
Female astronauts typically have more tissue that’s susceptible to radiation, so their lifetime limit is 2,500 mSv.
Animal research suggests this threat could be worse in deep space than previously thought, though studies involving humans are needed to confirm that’s also true for astronauts.
Your genetic code behaves differently.
DNA is life’s basic blueprint, and genes — much like words in a cookbook — spell out the specific recipes to keep us alive. However, it’s equally important when and how much those genes are expressed, or turned on and off. A lot of that has to do with a person’s environment.
The Twins Study found that about 7% of Scott Kelly’s genes expressed a bit differently after a year in space than they did on the ground, and didn’t return to normal (or at least not quickly). The real-world ramifications of this are still being explored.
What if you die in space?
No one has ever died on the ISS. The farther humans travel from Earth, however, and the longer time they spend in space, the greater the risk is that someone could die from a medical event, a vehicle emergency like a fire, depressurization, electrical shock, or simply a lack of food and water.
According to NASA, if someone were to die in space one of the most immediate concerns would be how to ensure the safety of the rest of the crew.
“In the closed atmosphere of a space vehicle, the natural byproducts of decomposition and/or potential pathogens released during the decomposition process could contaminate the enclosed vehicle environment,” NASA explains in a technical brief from 2024.
If the crew is close to Earth, like on the ISS, there are a few options, NASA says: return the body to Earth, place it in a safe orbit around Earth, or allow it to burn up in Earth’s atmosphere.
Ultimately, the final decision would have to take into account multiple factors including minimizing risk to surviving crewmembers, potential forensics collection, biohazard containment, and legal jurisdiction.
If the crew are far from Earth, like on a mission to Mars, the option is to either try to preserve the body for return to Earth — which relies on the crew being able to handle the remains properly — or jettison the remains into space.
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