(NewsNation) — Butch Wilmore and Suni Williams, as well as two other NASA astronauts, have left the International Space Station after being stuck for nine months after a Boeing test flight.
The return trip will take around 17 hours, and the crew plans on landing in the splashdown zone in Florida if weather allows.
Wilmore and Williams have been in space since June 2024, so returning to Earth will take a toll on the astronauts’ bodies.
What happens to a human’s muscles and bones in space?
An astronaut’s muscle and bone mass can diminish due to the lack of gravity in space. According to the BBC, the muscles that help maintain posture are affected the most because the muscles don’t have to work as hard in space. The Canadian Medical Association Journal reported that muscle mass can fall by nearly 20% in two weeks and by 30% on three-to-six-month missions.
Astronauts’ skeletons can also be affected without Earth’s gravity. Bones can start to demineralize and lose their strength, according to the BBC. For every month an astronaut is in space, they can lose 1% to 2% of their bone mass. This can make it more likely for astronauts to have bone fractures and it can take longer for injuries to heal.
This image made from video shows astronauts, including Suni Williams, center, greeting each other after a SpaceX capsule docked with the International Space Station on March 16. (NASA via AP) 
NASA astronauts Suni Williams, left, and Butch Wilmore stand together for a photo enroute to the launch pad at Space Launch Complex 41 on June 5, 2024, in Cape Canaveral, Fla., for their liftoff on the Boeing Starliner capsule to the international space station. (AP Photo/Chris O’Meara, File) 
In this photo provided by NASA, astronaut Suni Williams tries on and evaluates her spacesuit aboard the International Space Station on Jan. 9, 2025. (NASA via AP) 
Boeing Crew Flight Test astronauts Butch Wilmore, left, and Suni Williams pose for a portrait inside the vestibule between the forward port on the International Space Station’s Harmony module and Boeing’s Starliner spacecraft on June 13, 2024. (NASA via AP, File) 
FILE – This image made from a NASA live stream shows NASA astronauts Suni Williams and Butch Wilmore during a press conference from the International Space Station on Friday, Sept. 13, 2024. (NASA via AP, File)
The BBC reported that it can take up to four years for the bone mass to return to normal after an astronaut returns to Earth.
To help with this, astronauts will do 2.5 hours of exercise per day and training while on the ISS. That training and exercise can include squats, deadlifts, and row and bench presses. According to the BBC, astronauts also take diet supplements to keep their bones healthy. However, one study showed that this training and exercise wasn’t enough to prevent all losses in muscle function and size.
Do humans lose weight in space?
It is actually quite the opposite. Since human bodies float around in space, keeping a healthy weight can be challenging. According to NASA, to help combat this, the agency ensures astronauts have access to many nutritious foods.
Scott Kelly, a NASA astronaut who was onboard the ISS for 340 days, lost 7% of his body mass while in space, according to the American Association for the Advancement of Science.
How is a human’s eyesight affected by space travel?
According to NASA, in space, blood can accumulate in the head more than normal. This can cause some fluids to pool at the back of the eye and near the optic nerve. This can cause vision changes, like decreased sharpness and structural changes.
This can happen after only two weeks in space, and the risk increases as time goes on. Some of these changes will reverse, but others will be permanent, according to NASA.
Galactic cosmic rays and energetic solar particles could also cause problems with an astronaut’s eyes. On Earth, we are protected, but in space, cosmic rays and solar particles can hit an astronaut’s retina and optical nerves.
How does a human’s skin react to being in space?
An astronaut’s skin can also be affected by prolonged space travel. According to Kelly, his skin was more sensitive and he developed a rash around six days after returning from space. The BBC reported that researchers have speculated this could be due to the lack of skin stimulation during space missions.
Once the Starliner astronauts return home, they will board a NASA jet to the Johnson Space Center in Houston where their families will be waiting. Medical teams will also be available to help the crew, as astronauts often experience muscle weakness in their first days back to Earth after extended periods in space.