Doctors are raising fresh concerns about the health of NASA's two stranded astronauts who were dealt a major blow last night.

The space agency once again delayed the return of Sunita Williams and Barry Wilmore until late March at the earliest, stretching their mission to over nine months.

Williams and Wilmore were only meant to spent eight days aboard the International Space Station when they launched on Boeing's faulty Starliner spacecraft on June 5.

Now, their names could be added to the short list of NASA astronauts who have spent more than 300 days in space. Only five have ever completed missions that long. 

Dr John Jaquish, a biomedical engineer, told DailyMail.com: 'By the time they get back to Earth, they will be at extreme risk of fragility fracture.

'They probably will not be able to walk under their own power. They will probably leave the space vehicle on a stretcher, and it will take them months to recondition their bodies.' 

This is not uncommon for astronauts after long mission in space, but doctors have expressed particular concern for Williams who they say is already showing signs of weight loss and malnourishment, though NASA has denied these 'rumors.' 

Williams and Wilmore also face risks to their psychological health, Dr. Carole Liberman, a psychiatrist based in California, told DailyMail.com. 

'Of course, astronauts are taught how to handle stressful circumstances,' she said, 'but this up and down, hope - no hope, scenario for 300 days is beyond what they would have expected.' 

'It plays havoc on their psyche, which then plays havoc on their body. They would have steeled themselves to hang in there for the previous dates they were given for their return, convincing themselves there’s only 10, 9, 8, 7, 6… days to go.

'But, each time it is delayed, a little voice inside themselves grows stronger, gnawing at them to wonder whether they will ever get home again.

'NASA needs to be transparent and tell them everything they're doing to fix the problem because this trust is all the astronauts have to go on,' she said. 

Helium leaks and thruster issues plagued Strainer before, during and after the spacecraft arrived at the ISS. NASA ultimately deemed it unfit to carry Williams and Wilmore home. 

The agency initially delayed the astronauts' return to February 2025, when they could hitch a ride back to Earth on the SpaceX Crew-9 Dragon spacecraft.

But now, NASA says the Starliner crew, along with NASA astronaut Nick Hague and Russian cosmonaut Aleksandr Gorbunov, will return to Earth after the four-member Crew-10 mission reaches the space station.

NASA and SpaceX are targeting no earlier than late March 2025 for the Crew-10 launch, which will carry NASA astronauts Anne McClain and Nichole Ayers, as well as Japanese astronaut Takuya Onishi and Russian cosmonaut Kirill Peskov to the ISS. 

Williams and Wilmore will still be coming home on Crew-9, but that mission will not depart from the ISS until after Crew-10 arrives in order to create what's known as a 'handover period,' according to NASA.

'It allows Crew-9 to share any lessons learned with the newly arrived crew and support a better transition for ongoing science and maintenance at the complex,' the agency stated Tuesday. 

But doctors told DailyMail.com that the longer Williams and Wilmore spend on the ISS, the more health risks they face.

Long-duration ISS missions typically last about six months. Studies have shown that there are significant health risks associated with spending that much time in space.

Those risks include bone and muscle deterioration, exposure to high levels of cancer-causing radiation, fluid shifts that can damage eyesight, kidney stones, cardiovascular issues and more.

By the time Williams and Wilmore come home, they will have been on the ISS for even longer than six months — potentially more than 300 days. 

While this isn't the longest period of time that an astronaut has spent in space (that record is 374 straight days on the ISS) it will take a serious toll on the Starliner crew, 

That's because 'when you remove gravitational pull you, the body doesn't see a reason to maintain bone density and muscle mass, so it starts to break down very quickly,' Dr Jaquish explained. 

Losing muscle and bone has greater consequences than just loss of mobility, such as the aforementioned risk of fragility fracture.

Severe muscle loss also increases the risk of organ failure, and can be detrimental to cardiovascular health, he added.

There are things ISS astronauts can do to mitigate some of these risks. To counteract the loss of bone, muscle and cardiovascular strength caused by low-gravity, they exercise for an average of two hours per day. 

But even though exercise can slow deterioration, spaceflight conditions still wreak havoc on the human body, Dr Jaquish said. 

'They would need to put very heavy loads to the body on a very regular basis to even just maintain any sort of strength or bone density,' he said, adding that the exercise equipment available on the ISS is not designed for that level of conditioning. 

What's more, NASA research has shown that daily exercise is not sufficient to counteract the effects of spaceflight on the heart, such as arterial stiffening and thickening of the artery walls, which can increase the risk of heart attack and stroke.

With all of that said, Jaquish believes that Williams and Wilmore have been well-prepared for this kind of scenario, as NASA 'does a very good job of contingency preparation.'

Dr. Vinay Gupta, a pulmonologist and Air Force veteran, agreed. 

'They're astronauts for a reason, because they've proven through their training and what they've been through that they're able to handle the stresses of being at the very highest of altitudes in a low-gravity situation, he told DailyMail.com.

'Having said that, there's a difference between training and real life,' he said, adding that the Starliner crew is at risk of weight loss, muscle atrophy, cardiovascular issues, sleep deprivation and hormonal changes, particularly to insulin and the stress hormone cortisol.

'I don't think anything that we're talking about here is life threatening,' he said.

'I think these will have impacts. They're going to need a medical assessment, I'm sure they're going to need recovery, but I don't think anything here will cause permanent issues.' 

Despite experts' concerns for the Starliner crew's physical and mental health, NASA has repeatedly stated that the astronauts are fine. 

The agency's chief health and medical officer Dr. James Polk previously said: 'All NASA astronauts aboard the International Space Station, including Butch Wilmore and Suni Williams, are in good health. It’s unfortunate that rumors persist otherwise.

'NASA and our partners have safely conducted long duration missions aboard the orbital laboratory for decades, studying the effects of space on the human body as we prepare for exploration farther into the solar system. 

'Crew health is regularly monitored by dedicated flight surgeons on Earth, and they have an individual diet and fitness regime to ensure they remain healthy throughout their expeditions.' 

NASA did not immediately respond to DailyMail.com's request for comment on the updated return date for Williams and Wilmore.