A mysterious syndrome has been impairing astronauts’ vision on the International Space Station, causing untreatable nearsightedness that lingers for months even after they’ve returned to Earth.

The problem is so bad that two-thirds of astronauts report having deteriorated eyesight after spending time in orbit. Now scientists say they finally have some answers - and it’s not looking good for our prospects of getting to Mars.

"Nobody’s gone two years with exposure to this, and the concern is that we’d have loss of vision," Dorit Donoviel from the US National Space Biomedical Research Institute told The Guardian. "That is catastrophic for an astronaut."

Earlier this year, NASA reported that something in space has been messing with its astronauts’ perfect eyesight, causing long-term impairment to their quality of vision.

Astronaut Scott Kelly, whose exceptional vision was part of the reason he was selected to be America’s first astronaut to spend a full year in space, says he's been forced to wear reading glasses since coming home.

John Phillips, who spent time on the International Space Station (ISS) in 2005, brought his sudden bouts of blurry vision home with him, and during his post-flight physical, NASA confirmed that his vision had gone from 20/20 to 20/100 in just six months.

NASA suspected that the condition - called visual impairment inter cranial pressure syndrome, or VIIP - was caused by the lack of gravity in space.

The hypothesis was that the microgravity of the ISS was building up pressure in astronauts’ heads, causing roughly 2 litres of vascular fluid to shift towards their brains.

They say that pressure was responsible for the flattening of eyeballs and inflaming of optic nerves observed in returned astronauts.

"On Earth, gravity pulls bodily fluids down toward the feet. That doesn’t happen in space, and it is thought that extra fluid in the skull increases pressure on the brain and the back of the eye," Shayla Love reported for The Washington Post.

Now, a team from the University of Miami has conducted the first study to actually test this idea, and found that something else has been causing vision problems in astronauts.

The researchers compared before and after brain scans from seven astronauts who had spent many months in the ISS, and compared them to nine astronauts who had just made short trips to and from the US space shuttle, which was decommissioned in 2011.

The one big difference between the two was that the long-duration astronauts had significantly more cerebrospinal fluid (CSF) in their brains than the short-trip astronauts, and the researchers say this - not vascular fluid - is the cause of the vision loss.

Under normal circumstances, CSF is important for cushioning the brain and spinal cord, while also distributing nutrients around the body and helping to remove waste.

It can easily adjust to changes in pressure that our bodies experience when transitioning from lying down to sitting or standing, but in the constant microgravity of space, it starts to falter.

"On earth, the CSF system is built to accommodate these pressure changes, but in space the system is confused by the lack of the posture-related pressure changes," says one of the team, Noam Alperin.

Based on the high-resolution orbit and brain MRI scans taken of their 16 astronauts, the team found that the long-duration astronauts had far higher orbital CSF volume - CSF pooling around the optic nerves in the part of the skull that holds the eye.

They also had significantly higher ventricular CSF volume, which means they had more CSF accumulating in the cavities of the brain where the fluid is produced.

"The research provides, for the first time, quantitative evidence obtained from short- and long-duration astronauts pointing to the primary and direct role of the CSF in the globe deformations seen in astronauts with visual impairment syndrome," says Alperin.

The results were presented at the annual meeting of the Radiological Society of North America in Chicago this week (here's the abstract), and have yet to be peer-reviewed, so we have to wait for the results to be replicated by an independent team before we can know for sure that this is the answer.

But even if this, or NASA's original hypothesis turns out to be more accurate, we still have a big problem on our hands.

Very few astronauts have spent more than a full year in space, and astronauts are already facing at least 18 months in space to get to and from Mars.

And that's if they flew home immediately after arriving - if we want to think about colonisation or extended stays on Mars, we're going to have to consider blindness as a potential complication.

Right now, there are no solutions for how to treat or prevent fluid build-up in space, and with the brain damage that's also expected to come from long-term spaceflight, Elon Musk's warning that the first Mars colonists should be "prepared to die" has never been more salient.