What happens to your body if you're stuck in space for nine months?

This is exactly what happens to your body when you're stuck in space.

In the six decades since the Soviets first launched Yuri Gagarin into space in 1961, there have been a handful of space-related disasters.

Launching humans into space is an inherently risky practice.

To prepare astronauts for the microgravity environment of space, they take part in rigorous training and simulations to get their bodies used to zero gravity and weightlessness.

Time spent in space can impact blood pressure, spatial orientation, balance, hand-eye coordination and even bone density.

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Two American astronauts, who were launched on a test mission to the International Space Station back on June 5 last year, have tasted fresh air again after they left their shuttle back to Earth this morning.

A livestream from NASA showed the SpaceX shuttle splashing down off the coast of Tallahassee, Florida, US, just before 9pm.

The SpaceX crew monitoring the splashdown erupted in cheers as the shuttle landed.

"On behalf of SpaceX, welcome home," the ground crew told the four astronauts aboard.

Shuttle captain Nick Hague said there were "four big smiles" on the faces of the crew.

That crew included Butch Wilmore and Suni Williams, who found themselves stranded on the International Space Station after Boeing rocket faults stretched their week-long space voyage into nine months.

The journey home from space took about 17 hours, with the astronauts now waiting about 30 minutes for recovery ships to reach them.

Over the past nine months, Wilmore and Williams have faced the tough reality that their bodies will slowly but surely feel the impacts of extended time spent in space.

Though they would have been well-prepared, some side effects are impossible to avoid.

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What are the physical effects of prolonged exposure to zero gravity?

According to NASA, extended periods without the earth's gravity can have an impact on human bones. Zero-gravity means the bones in the lower portion of the body (legs, hips, spine) are doing significantly less work.

Weight-bearing bones lose an average of 1 to 1.5 per cent of mineral density (bone mass) per month during space flight.

Low bone mineral density leads to an increased risk of broken bones. If too much is lost, there is a risk of developing osteoporosis.

Astronauts in space also face decreases in blood volume and can develop increased arrhythmias.

What happens to your body when you go to space?

There are other significant effects of extended periods in space on human health. The Centre for Space Medicine at the Baylor College of Medicine notes it can have a neurological impact on astronauts.

Without gravity, the messages the brain sends to muscles and internal organs can change.

An example of this is the inner ear receiving "conflicting" inputs from the brain than what is normally expected on earth. This can result in disorientation, space motion sickness, and a general loss of sense of direction.

Astronauts have reported this makes even basic tasks difficult while in space.

For their safety, returning astronauts are usually placed in chairs to recover immediately upon arriving back on Earth.

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There are also optical side effects of being in space. Astronauts can experience vision changes, including Associate Neuro-ocular Syndrome (SANS), which causes swelling behind the eye.

Astronauts may also notice a decrease in muscle mass, so those in space perform resistive exercises to mimic lifting weights to keep strength in their arms and legs.

NASA notes astronauts have reported back pain during space flight.

What happens to your organs in space?

In the absence of gravity, the gastrointestinal system can suffer. Gravity helps move food through the GI tract and without it, the system can experience a decrease in motility.

It also makes going to the toilet quite a difficult and unpleasant task in space.

What happens to astronauts' teeth in space?

Teeth are also bones. So astronauts will experience a loss of bone mass in their teeth, too.

NASA requires oral screening procedures for potential astronauts to avoid issues including dental fillings being weakened or the loss of crowns and bridges.

Does radiation affect astronauts in space?

Humans are protected from harmful space radiation while on Earth. Astronauts, however, are at risk of being exposed to this radiation beyond Low Earth Orbit (LEO).

Prolonged exposure to radiation can result in sickness such as degenerative disease or cancer.

The shell of a spacecraft works to protect astronauts from radiation.

"Currently, the main operational countermeasure against the adverse effects of radiation is simply limiting astronaut exposure, which means limiting the amount of time astronauts are allowed to be in space," NASA notes in its report Space Faring: The Radiation Challenge.

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Is it possible to reverse the effects of prolonged space travel on the body?

Some of the effects of space travel begin to reverse after returning to Earth, including vision changes and spatial orientation, but some effects are permanent.

Cognitive performance takes some time to readjust when getting used to the Earth's gravity again.

A study in Scientific found bone density can take a long time, if ever, to return to normal.

"We've seen astronauts who had trouble walking due to weakness and lack of balance after returning from spaceflight, to others who cheerfully rode their bike on Johnson Space Center campus to meet us for a study visit," said co-author Dr Steven Boyd, a professor in the Cumming School of Medicine at the University of Calgary (UCalgary), Canada. 

"There is quite a variety of responses among astronauts when they return to Earth."

"Bone loss happens in humans—as we age, get injured, or any scenario where we can't move the body, we lose bone," explained lead author Dr Leigh Gabel, an assistant professor in the Faculty of Kinesiology, also at UCalgary.

What psychological challenges do astronauts face during space missions?

Space exploration and extended periods in space can have an adverse effect on the mental health of astronauts.

The isolation, disruption of sleep and cognitive dysfunction can cause space travellers to feel irritable and emotional while in space.

NASA notes there can be a negative psychological impact on its astronauts and is looking into how to mitigate this risk.

This includes having astronauts learn a language, examining the idea of a 'space garden' for them to tend to and providing virtual environments for exercise equipment.