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A human expedition into space offers many complexities, but what does spending more than a year in space do to the human body? The record length for a single trip into space currently stands at 437 days. As impressive as that might be, prolonged periods in orbit can alter an astronaut's body in some surprising ways, including changing their muscles, brains, and even their gut bacteria.

So click on to learn more about the risks of spending too much time in space.

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On June 5, 2024, astronauts Sunita Williams and Butch Wilmore set off into space on an eight-day mission.

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They were traveling on the Boeing Starliner when the spacecraft experienced problems on its way to the International Space Station (ISS), including leaks of helium, which pushes fuel into the propulsion system. Several thrusters also failed to work correctly.

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These circumstances have left the two astronauts stuck on the ISS for over nine months. But what are some potential risks to their bodies, orbiting around Earth that long?

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Without the constant tug of gravity on our limbs, muscle and bone mass quickly begin to diminish in space. The most affected areas are the muscles that help to maintain our posture in our back, neck, calves, and quadriceps. This is because in microgravity they no longer have to work nearly as hard.

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After two weeks muscle mass can fall by 20%, and on longer missions of three-to-six months, it can fall by 30%.

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For similar reasons, the bones of astronauts also start to lose strength. In fact, astronauts can lose 1-2% of their bone mass every month they spend in space, and up to 10% over a six-month period. As a comparison, on Earth, older men and women lose bone mass at a rate of 0.5%-1% every year.

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This can increase their risk of suffering fractures and increased healing time. It can take up to four years for bone mass to return to normal after returning to Earth

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To combat this, astronauts undertake 2.5 hours a day of exercise and intense training while in orbit on the ISS. This includes a series of squats, deadlifts, rows, and bench presses. They also take diet supplements to help keep their bones as healthy as possible.

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However, a recent study showed that even this exercise regime wasn't enough to prevent losses in muscle function and size. Higher loads in resistance exercises and high-intensity interval training were recommended after testing.

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Although weight means very little while in orbit, maintaining a healthy weight is a challenge while in space.

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Astronauts are provided with a diverse range of nutritious foods, including most recently a few salad leaves grown on board the space station. However, it can still affect an astronaut's body.

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NASA astronaut Scott Kelly (pictured) took part in the most extensive study of the effects of long-term spaceflight after staying onboard the ISS for 340 days. Compared to his twin brother who stayed back on Earth, Kelly lost 7% of his body mass while in orbit.

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On Earth, gravity forces the blood in our bodies downward while the heart pumps it up again. Although the body does adapt somewhat in space, this process is disrupted and blood can accumulate in the head more than normal.

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Some of the blood can end up at the back of the eye and around the optic nerve, leading to edema. This can lead to vision changes, such as decreased sharpness and structural changes in the eye itself.

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These changes can start to occur after just two weeks in space. However, the risk increases as time goes on.

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Some of the vision changes reverse within about a year of astronauts returning to Earth, but others can be permanent.

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After his time on the ISS, Kelly's cognitive performance was found to have remained relatively similar to his brother's on the ground.

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However, researchers noticed that the speed and accuracy of Kelly's cognitive performance decreased for around six months after he landed, possibly as his brain readjusted to the Earth's gravity and his lifestyle back home.

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A study on a Russian cosmonaut, who spent 169 days on the ISS in 2014, also found some changes to the brain. It found there were changes in the levels of neural connectivity in parts of the brain relating to motor function, and also in the vestibular cortex.

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One of the keys to good health is the diversity of microorganisms living in and on our bodies. The gut microbiome can influence how we digest food, affect the levels of inflammation in our bodies, and alter the way our brains work.

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Researchers examining Kelly after his trip found that his gut microbiome had changed profoundly. Exposure to radiation, the use of recycled water, and changes to his physical activity could all have played a role.

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Kelly's skin was found to have heightened sensitivity and he suffered a rash for around six days after he returned from the ISS.

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Researchers speculated that a lack of skin stimulation during the mission may have contributed to his skin issues.

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The ends of each DNA strand are called telomeres. They help to protect our genes from damage. As we age, they get shorter, but research on Kelly and other astronauts revealed that space travel seems to alter the length of these telomeres.

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One possible cause of the change in these telomeres could be exposure to the complex mix of radiation in space.

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There were also some changes in gene expression, the mechanism that reads the DNA to produce proteins in cells. Some of these are related to the body's response to DNA damage, bone formation, and the immune system's response to stress. However, most of these changes had returned to normal within six months of Kelly's return to Earth

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Kelly received a number of vaccines before, during, and after his trip into space and his immune system reacted normally.

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However, research has found that astronauts do suffer some decreases in white blood cell counts. This is largely due to the doses of radiation they receive while in orbit.

Sources: (BBC) (NASA) (Washington Post)