NASA's astronauts face 'life-altering' changes to their bodies due to being stranded in space by Boeing's faulty Starliner

The NASA astronauts stranded in space face 'life-altering' risks if their stay on the International Space Station (ISS) is extended until 2025, experts warn.

Boeing's faulty Starliner took Barry 'Butch' Wilmore and Sunita Williams to the ISS June for what they thought would be a matter of days, but the crew received news this week that they may not come home until next year.

Many studies have found that long stints in space can cause bone density loss, muscle atrophy, intense radiation exposure, vision problems and other significant health issues.

Astronauts also receive different levels of training depending on the length of the mission, including health tests, which suggest Wilmore and Williams are not prepared for the trails ahead. 

As one expert put it to DailyMail.com: 'The longer you're [in space], the bigger the change.'

Technical issues with Boeing's Starliner have left the astronauts stranded on the ISS for more than two months, when they were scheduled to return on June 14. 

Thruster failures and helium leaks on Starliner prompted NASA and Boeing to keep the two astronauts in orbit for longer over fears a trip back on the spacecraft could end in disaster. 

But now the crew faces another risk - their bodies could start deteriorating.

'Generally, almost all of the changes that we see with human spaceflight in terms of physiology seem to be dose dependent,' Rachael Seidler, a professor of Applied Physiology and Kinesiology at the University of Florida, told DailyMail.com.

'The longer you're there, the bigger the change - at least up to a point,' she added. 

The ISS only has about 90 percent of the gravity we experience here on Earth. 

Living in reduced gravity means that the body's bones and muscles aren't working as hard as they're meant to, and this results in significant bone density loss and muscle atrophy.

'Astronauts experience accelerated bone loss at an alarming rate - about 12 times faster than severe osteoporosis on Earth,' Kyle Zagrodzky, founder and CEO of bone health clinic, told Newsweek.

'Osteoporosis increases fracture risk, often leading to life-altering injuries. 

'Hip fractures pose a grave concern, as they can strip away independence and increase mortality risk.' 

Additionally, astronauts on long duration space missions can lose up to 50 percent of their muscle mass, according to the European Space Agency.

This can compromise their ability to do physically demanding activities both during and after the mission, and cause pain and discomfort.

Low gravity even changes the shape and function of astronauts' hearts. In space, the heart changes from an oval (like a full water balloon) to a round ball (like an air-filled balloon), according to NASA.

Muscles that work to constrict the blood vessels also atrophy during long space flights, which means they can't control blood flow as well.

During a long space mission, astronauts can lose up to 50 percent of their cardiovascular endurance, one study found. 

Plus, as bodily fluids shift upwards during spaceflight, that can cause problems with vision, cognition, balance, and coordination, Seidler told DailyMail.com. 

Some astronauts report experiencing 'space fog' while on a long mission. 

Similar to brain fog, this little known phenomenon can make astronauts feel off and disoriented. In extreme cases, it can cause nausea, vomiting and mood swings.

Low gravity can also cause Spaceflight Associated Neuro-Ocular Syndrome, which reduces astronauts' near-distance vision. 

Nearly half of astronauts report changes to their near-vision after returning from long ISS missions, according to the American Academy of Ophthalmology. 

The longer astronauts spend in space, the longer it might take them to recover from these changes once they return to Earth, Seidler said. 

For example, it might be several weeks before they can safely drive a car, she said. 

And if that's not bad enough, low gravity also puts astronauts at much greater risk of developing kidney stones.

There are some things astronauts can do to combat the effects of low gravity. 

For example, the ISS is equipped with special exercise equipment that astronauts use for two hours per day to reduce muscle and bone loss. 

A study conducted on the Skylab crew, who stayed on the first manned research laboratory in the early 1970s, found the astronauts had a negative nitrogen balance, which indicated skeletal muscle loss.

The group spent 171 days in space, which would be less that Williams and Wilmore should they stay on the ISS until next year. 

But there's one danger they can't do anything to avoid - radiation.

The ISS has shielding to reduce the amount of space radiation that astronauts are exposed to. But they still endure about 365 times more radiation than we do here on Earth.

In other words, for every day that an astronaut spends on the ISS, they're exposed to a year's worth of the radiation they would experience on Earth.

That may increase astronauts' risk of developing cancer later in life, according to NASA. 

All of this means that the sooner NASA can get Wilmore and Williams back to Earth, the better. Each additional day they spend on the ISS does more damage to their bodies. 

Starliner launched on June 5 with the intention of docking at the ISS for about a week before returning Wilmore and Williams safely to Earth.

But the mission was already off to a rocky start. In the weeks before, the launch was delayed several times due to technical issues with the spacecraft.

Even on the day of liftoff, Starliner experienced minor helium leaks that engineers determined were not severe enough to delay the launch again. 

It was all downhill from there. By the time Starliner reached the ISS, it had sprung more helium leaks and five of its 18 thrusters failed. 

Wilmore and Williams were able to safely board the ISS, but now they're left without a safe way home. 

If Starliner is deemed unfit to carry the astronauts home, they could potentially return to Earth aboard SpaceX's Dragon Capsule after the Crew-9 mission launches in September, NASA officials said in a press conference Wednesday.

But that would mean that Wilmore and Williams wouldn't come home until February 2025. By then, they will have spent nine months in space.

NASA has not made any set decisions regarding Starliner's return, the agency told DailyMail.com in an emailed statement. 

But Boeing is hopeful that their spacecraft could still return to Earth with Wilmore and Williams on board.

'We still believe in Starliner's capability and its flight rationale. If NASA decides to change the mission, we will take the actions necessary to configure Starliner for an uncrewed return,' A Boeing spokesperson told DailyMail.com in an emailed statement.