What a year in space and prolonged periods in orbit can do to the human body

Astronauts Suni Williams and Butch Wilmore recently returned to Earth having been stranded in space for nine months.

Their trip to the International Space Station in June 2024 was only supposed to last eight days, but technical problems meant their Boeing Starliner spacecraft had to return to Earth without them, with the pair “enjoying” an extending stay in orbit.

Upon their return, their physical appearances were remarked to be noticeably different, and it was revealed they would have to reacclimatize to life on Earth, their bodies having been changed by the difference in gravitational pull on our planet compared to in space.

How the human body changes in space

Studies have shown the human body is affected in all kinds of different ways by spending prolonged periods in orbit. These are some of the main changes.

Muscles and bones

With space being a low gravity environment, there is much less pull on limbs, which leads to muscle and bone mass shrinking. The back, neck, calves and quadriceps are the most affected parts of the body, as they don’t have to work as hard to maintain posture.

Research shows muscle mass can decrease by 20% in two weeks, and by 30% in three-to-six months.

Bones also lose strength, again due to not having to work as much. Astronauts lose 1-2% of bone mass a month, and up to 10% in six months.

Height

Similarly, lower gravity means the body isn’t pulled down as much as it is on Earth. That results in astronauts growing a little taller, which was found with Williams and Wilmore.

The spine elongates in space and then shrinks back on Earth, which can cause neck and back issues.

Eyesight

Less gravity means blood which normally circulates downwards can get stuck in the head, some of which ends up near the back of the eye and around the optic nerve. That can lead to oedema, which in turn causes decreased sharpness in vision and structural changes in the eye.

These changes can be temporary, resolving themselves after a year back on Earth, but previous experience shows they can sometimes be permanent.

Astronauts can also experience other eye problems such as flashes of lights in the eyes, which is caused by exposure to galactic cosmic rays and energetic solar particles, which the Earth’s atmosphere protects us from.

Cognitive performance

One of the most noticeable changes is said to occur immediately upon returning from space. Although cognitive performance isn’t affected while in orbit, gravitational change means movement, balance and orientation is off when back on Earth, usually for a period of around six months.

The longest ever trips to space

In history, four people been to space for a period of over a year:

In addition to the quartet, many of the changes mentioned above were observed in Scott Kelly, a NASA astronaut who was onboard the International Space Station for 340 days and took part in the most extensive study on the effects of long-term spaceflight, which his twin brother, who stayed on Earth, was also involved in.

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