It’s not really why he signed up to be an astronaut, but like it or not, Mike Barratt and his eyes have become a science project.

The eye charts he reads, the red drops that turn his eyes yellow and the ultrasounds being performed on him could determine whether he or any other astronaut ever journeys into deep space or sets foot on other worlds.

NASA’s new priority is how to protect astronauts from going blind on the years-long trip to get wherever they are going.

“I absolutely agree that this is our number one priority,” Barratt said.

Why?

Because when Barratt blasted off to the international space station, he needed eyeglasses for distance. When he returned to Earth, his distance vision was fine, but he needed reading glasses. That was more than two years ago. And he’s not getting better.

“We really need to understand this. This is a critical point for understanding how humans adapt to spaceflight,” he said.

In the past few years, about half of the astronauts aboard the international space station have developed an increasing pressure inside their heads, an intracranial pressure that reshapes their optic nerve, causing a significant shift in the eyesight of male astronauts. Doctors call it papilledema.

Female space travelers have not been affected.

Some of the astronauts slowly recover. Others have not.

Space station astronauts typically spend about six months in orbit.

Barratt is one of 10 male astronauts, all older than 45, who have not recovered. Barratt returned from a six-month stint aboard the station in October 2009 and has experienced a profound change in his sight.

He used to be nearsighted. But now, the space veteran says he’s eagle-eyed at long distance but needs glasses for reading. There is no treatment and no answers as to why female space flyers are not affected.

CNN spent part of a day with Barratt, watching as doctors monitored his progress with high-resolution testing as they try to understand how the weightless environment of space is causing half of all space station astronauts to have this vision change. Today, space station astronauts fly with specially designed variable focus glasses to help combat the vision shift.

“The big benefit of these is that they allow us to adjust for significant prescription changes,” said Dr. Robert Gibson, a senior vision consultant, who was brought in to help study the problem.

Doctors have found that Barratt’s retinas have microscopic folds or wrinkles on them, and the back of his eye, the optic nerve, is no longer round but has flattened.

“I think this is showing that there are physiologic aspects of adaption to spaceflight we weren’t seeing before,” said Barratt.

This raises a red flag for all of NASA’s plans for long-duration human space flight. The space station is supposed to be the test bed for how humans would learn to live in space, but it opens profound questions on whether humans will ever venture to Mars or to an asteroid if they are unable to figure out how the outer-space environment is affecting the eyes.

“This has all of our attention,” said Terry Taddeo, the acting chief of space medicine at Johnson Space Center in Houston.

“It is a serious problem and one we are going to have to understand more about before we would be able to send somebody into a long-duration mission away from Earth, where they would be away for years,” he said.

Right now, the only data that doctors have are from six-month tours of duty on the space station.

NASA has begun doing extensive preflight and postflight eye exams, including high-resolution MRIs of the eyes. There have been anecdotes from some space shuttle astronauts who also complained about vision change, but it does not appear they had long-lasting effects from the much shorter space flights that typically lasted up to about three weeks.

“What we’re seeing appears to occur within the first couple of months of flight and appears to level off, plateau after about four to five months,” Gibson said.

“If it’s just a matter of giving them a stronger prescription, we can live with that,” he said. “But if there is an elevated intracranial pressure as the cause of this, we have to be concerned about other neurologic effects."

That means there could be other effects on the body that haven’t become apparent.

This is why a three-year mission to Mars is in question.

It would be humans' next great leap, and NASA is spending almost $18 billion over the next five years to develop a heavy lift rocket that would take astronauts to the Red Planet or even to an asteroid. They would travel in a new spacecraft, Orion.

But right now, a trip to Mars is still more science fiction than fact. No one is calling this vision problem a showstopper, yet the program’s price tag begs for a solution to be found fast so NASA won’t be building the world’s largest, fastest rocket to nowhere.

Dr. Bruce Ehni, a neurosurgeon at the VA Medical Center at the Baylor College of Medicine in Houston, has consulted with NASA and is the only neurosurgeon on their panel.

“If they can’t predict who is at risk ... they put his health in jeopardy. They put, possibly, the mission in jeopardy if he can’t see or do his job effectively,” he said.

But Barratt thinks that any deep space venture to Mars is still 20 years away. He’s hoping that spacecraft will be a whole lot faster than anything the space agency can fly now.

“You fly fast, and you don’t worry,” he said, with a grin.

“I’m still hopeful that in 20 years, we’ll have advanced propulsion capabilities that can get us there in a matter of weeks to a few months. Then, a lot of these problems go away,” he said.