NASA's stranded astronauts may have endured nine uncertain months in space.

But the real challenge begins now that they have returned to Earth. 

Sunita Williams and Butch Wilmore may have lost up to half of their muscle mass in low gravity, and almost a fifth of their bone density.  

Former astronauts have found that it can take up to 1.5 times the length of the mission to recover. That means the stranded pair returning today may need more than a year before they feel entirely themselves again. 

Williams and Wilmore were initially scheduled to spend just eight days aboard the ISS when Boeing's Starliner launched on June 5. But a series of technical issues drove NASA to send Starliner back to Earth without them in September. 

Williams, 59, and Wilmore, 62, were immediately helped onto stretchers and taken for medical checks after splashing down off the coast of Tallahassee, Florida at 5:57pm ET Tuesday evening.

Their long road to recovery begins now, with their rehab split into three phases. 

Dr Vinay Gupta, a pulmonologist and Air Force veteran, said the astronauts are likely to need up to six weeks of rehab to get back into basic shape after living in low gravity for so long. This will include a multi-phase exercise program and guided nutrition plan.

But bone and muscle loss aren't the only health challenges they will have to face. 

Living in the harsh environment on the International Space Station (ISS) may have also increased their risk of eyesight impairment, skin issues and blood clots, according to British astronaut Tim Peake. 

Doctors raised concerns about Williams hours after splashdown when NASA shared an image of the astronaut looking noticeably frail.

The medical experts particularly pointed to Williams' 'visibly thin' wrists, which they told DailyMail.com could be a sign of rapid weight loss, muscular wasting in her arms, and bone density loss.

The experts also noted the IV sticking out of Williams' wrist is 'most likely' to help restore hydration and electrolytes, as microgravity tricks the body into eliminating necessary fluids, causing dehydration.

Striking before and after pictures of Williams show her with noticeably grayer hair, deeper wrinkles, and a more gaunt face.

But experts suggest the fact that the duo are already walking less than 24 hours after returning is promising - some feared they may not regain that ability for days.

Dr John Jaquish, a biomedical engineer in at Jaquish Biomedical, told DailyMail.com: 'That amount of time in space is crushing.' 

Williams and Wilmore are also feared to face 'chicken legs, 'baby feet' and increased risk of cancer due to radiation exposure.

Beyond looking unusual, this buildup of fluids in the head can also lead to serious medical complications. 

As pressure builds up in the head it presses on the eyes and on the optical nerves causing something called Spaceflight Associated Neuro-Ocular Syndrome (SANS).

Over time, microgravity even causes changes in the shape of the eye including swelling of the optic nerve, flattening of the back of the eye, and the development of folds in the retina.

SANS causes blurry or fuzzy vision in about 70 percent of all astronauts who go into space.

NASA astronauts who return from long ISS missions complete a 45 day rehabilitation program that requires them to exercise for two hours per day, seven days per week.

The program is tailored to the specific needs of each astronaut, according to NASA.

Phase one 

Phase one of a NASA astronaut's post-mission rehabilitation plan focuses on regaining strength, flexibility and the ability to walk. 

This may include gait training exercises, range of motion exercises and obstacle training.

Gait training exercises are movements designed to improve strength, balance and coordination during walking. Examples include squats, straight leg raises, standing on one leg and seated marching. 

To improve their range of motion, the astronauts may perform ankle pumps, which involve sitting or lying down while flexing the feet. They may also do stretches to loosen the calves, quadriceps and hamstrings. 

Astronauts may have to navigate an obstacle course or step over and around objects to improve their coordination. 

Phase two 

After making some improvement during phase one the astronauts move on to phase two, which adds proprioceptive exercises and cardio reconditioning.

Proprioceptive exercises strengthen the body and improve the mind's perception of its movement and position.

Examples include reverse lunges, banded toe taps and sumo squats with leg raises.

Some of these exercises are more complex. Astronauts may be asked to pick an object up off the floor while standing on one leg, which requires them to hinge at the waist and maintain their balance as they bend down. 

As for cardio training, the astronauts may use a treadmill, elliptical or stationary bike to get their endurance back to pre-flight status. 

Phase three 

Phase three, the longest phase, focuses on returning the astronaut to their optimal level of physical performance through functional development training. 

This training helps astronauts regain the skills and abilities they need in order to do their jobs and fully participate in their daily lives with ease and efficiency.

It may include more high-intensity exercises, such as jump squats and jump lunges, mountain climbers, planks and dead lifts. 

Most astronauts return to their re-mission fitness level after the 45 days, according to NASA. 

But it can take months or even years for some to recover, and research has shown that many astronauts never fully restore their bone density. 

Dr John Jaquish, a biomedical engineer, told the Daily Mail they could still recover their pre-flight bone density if they use osteogenic loading, but it won't be easy. 

This involves exercises that strengthen bones by putting stress on them, such as squats, lunges or jumping.

But in order to stimulate bone growth, the astronauts' bones will have to bear a load 4.2 times their body weight, Dr Jaquish said. 

For reference, 'the world record squat is only four times body weight, so the minimum [weight] you need is more load than the world record holders,' he said.  

Wasted muscles  

To combat the effects of living in low gravity, astronauts exercise for at least two hours per day on the ISS. 

But this still isn't enough to stop muscle and bone loss, doctors told the Daily Mail.

Astronauts who spend long periods of time in low gravity 'lose musculature, they lose bone density,' Dr Jaquish said.

'The human body needs the Earth's gravitational pull, and in an absence of that, a lot of things are not functioning correctly.'

Research has shown that a 30 to 50-year-old astronaut who spends six months in space loses about half their strength. 

This not only leaves them physically debilitated when they return to Earth, but puts them at greater risk of bone fracture and premature osteoporosis, a disease that weakens bones as people age. 

To reduce these impacts, astronauts work to make sure they are in peak physical condition before they embark on an ISS mission and continue to workout on the space station to maintain their fitness.

But 'the reality is, they're effectively getting a fraction of the sort of exercise that we all take for granted just by walking in [Earth's] gravity,' Dr Gupta said. 

'Invariably, despite them doing all the right things (which I know they are) you're going to see a decrease in muscle mass and strength — no question.'

Heart health 

It's not just their muscles and bones taking a hit. Low gravity impacts their cardiovascular health too.

This is because blood and other bodily fluids shift upwards towards the head, which means the cardiovascular system doesn't have to work as hard to maintain blood flow to the brain.

This can result in reduced blood volume and reduced function of the heart and blood vessels, according to NASA.  

Extreme radiation 

Williams and Wilmore will also have been exposed to extreme levels of space radiation during their extended ISS mission. 

In just one week on the ISS, astronauts are exposed to the equivalent of one year's exposure on Earth. This may increase their risk of developing cancer, central nervous system damage, bone loss and some cardiovascular diseases, according to NASA.

'If I was their physician, I would think about a more proactive strategy for cancer screening,' Dr Gupta said. 

'We want to take a different approach here, given that they had such a unique exposure history,' he said. 

In addition to bone and muscle loss and cancer risk, Williams and Wilmore could be facing other debilitating health issues too.  

'We're still finding out about the long-term effects of spaceflight,' European Space Agency astronaut Tim Peake previously told LadBible. 'But we also know that our skin ages, our eyesight changes and [that] we're more prone to blood clots.'

'These are relatively new things that we've found out. I guess the longest effect is the radiation dose, as that's hard to quantify what the effects will be,' he said. 

In December 2015, Peake became the first British astronaut to visit the ISS. He completed his six-month mission aboard the floating laboratory in June 2016. 

Skin problems

Studies have shown that spending six months on the ISS wreaks havoc on the skin. 

One team of researchers found that astronauts' epidermis gets thinner by nearly 20 percent in space, perhaps due to low gravity, which may dysregulate the skin's ability to grow and repair itself. 

Another study found that skin rashes are the most frequently reported clinical symptom during six-month ISS missions, occurring 25 percent more frequently than they do for the general US population on Earth. 

These rashes may result from irritants or allergens found inside the space station, and the weakening effect that low gravity has on the immune system. Skin lesions also take longer to heal in space, according to NASA.

Fears over eyesight 

Low gravity also damages astronauts' eyesight during long-term ISS missions, sometimes leading to Space-Associated Neuro-Ocular Syndrome (SANS).

According to NASA, SANS results from bodily fluids shifting toward the head due the lack of gravity, and symptoms include swelling in the optic disc (where the optic nerve enters the retina) and flattening of the eye shape.

Astronauts' eyes typically return to normal once they return to Earth, studies have shown. 

The upward fluid shift their bodies experience on the ISS also increases astronauts' risk of blood clots, specifically through the development of a condition known as Spaceflight Venous Thrombosis (SVT). 

Some astronauts who develop SVT completely recovered after returning to Earth, but others need additional treatment, according to NASA.  

Williams and Wilmore spent more than nine months on the ISS, much longer than a typical long-term mission. 

This could mean that the health impacts they have experienced, and now must recover from, will be more severe than the average astronaut's. 

At this time, there is no evidence to suggest the Starliner crew is suffering from vision loss, skin lesions or SVT. But this is not out of the question.