Imagine making the nine-month trek to Mars just to have such poor vision upon arrival that it's impossible to land.
That type of scenario would be a nightmare for NASA astronauts headed to the red planet — but it's a very real possibility if scientists don't develop a way to counteract a phenomena that leaves men visually impaired after long-term exposure to zero gravity.
"This is a big concern for astronauts, who are mostly pilots," said David Zawieja, regents professor at Texas A&M University's College of Medicine, who is studying the problem for NASA. "You have to have them be able to land and appropriately be able to see. They have to get there, land, do what they need to do and then return."
Researchers still don't know why it happens or why only men have been shown being affected, not women, Zawieja said. The leading hypothesis is that increased pressure in the heads of male astronauts is to blame.
RELATED: NASA isolation study may provide answers for mission to Mars
Scientists at Texas A&M and Florida State University are studying how fluid pressure changes in a person's head — such as the cerebrospinal fluid that cushions the brain from shock — might impact vision. They're also studying coronary artery function.
Space Flight-Associated Neuro-ocular Syndrome appears to be more prevalent during long-duration missions.
"Few data exist from which to determine the extent or cause" of the syndrome, according to a November 2017 risk report produced by Houston's Johnson Space Center.
Researchers expect to have results later this year from a study that launched 20 male mice to the International Space Station in August, with a goal of putting scientists one step closer to discovering why the vision problems happen to men.
Some scientists think zero gravity increases the cerebrospinal fluid pressure in the skull, increasing pressure in the back of the eyeballs and the optic nerve, Zawieja said.
"When there's increased pressure on the nerve, it doesn't act properly," he said.
NEW MISSION: New NASA mission to study little known boundary between Earth and outer space
Others think the syndrome might be caused by increased vascular fluids, such as blood and lymph, in the head. About 68 ounces of this fluid — the same as a two-liter bottle of soda — shift from an astronaut's legs toward their head in space, according to NASA.
Scott Kelly, the first American to spend a continuous year in space, wrote in his book, "Endurance," about the vision changes he experienced on many of his spaceflights. His vision would get blurry about 10 to 12 feet in front of him, he wrote, but eventually returned to normal back on Earth.
After his first long-duration flight, which lasted 159 days, Kelly wrote that doctors found swelling of his optic nerve as well as choroidal folds, similar to stretch marks, on his eye.
NASA researchers have identified these and other eye problems, such as the flattening of eye shape, in many male astronauts on long-duration space missions.
About 60 percent of 300 International Space Station Astronauts who were surveyed by NASA said their vision had degraded, according to the Johnson report. It did not specify if the respondents were male and female.
Researchers are attempting creative ways to counteract this problem, but it's difficult to do without knowing the exact cause. For example, as part of an experiment during his year on the space station, Kelly would don vacuum pants that literally sucked his body in an attempt to relieve the intracranial pressure experienced during spaceflight.
"The human body is roughly 70 percent fluids, which includes blood, lymph, and water contained within and around cells. On Earth, our cardiovascular system keeps those fluids distributed throughout our body despite the pull of gravity," according to a June 2015 NASA blog post from the space station. "During spaceflight, body fluids accumulate in the upper body, causing a noticeable puffiness in astronauts' faces."
The redistribution of fluids may contribute to vision problems in space, the post said.
"Reducing the pressure on our lower bodies also reduces the amount of fluid in our heads," Kelly wrote. "By studying the effects of [the Chibis pants] on our bodies, we hope to understand more about this problem."
SPACE: Earth's ozone is recovering thanks to decades-old chemical ban, scientists say
The main problem is that measuring this type of pressure requires an invasive procedure, such as a spinal tap or drilling a hole in one's skull, which cannot safely be done in space.
That's why Zawieja's mice are a great asset for researchers.
After the mice returned from their one month on the space station, scientists extracted tissues, arteries and veins from the neck and head, for example. Those samples were tested and will be compared to mice that stayed on the ground at Kennedy Space Center in Florida. Zawieja and other scientists are planning a trip to Florida in April to continue their research efforts.
Rodents, obviously, are different than humans, but Zawieja believes researchers can extrapolate from the findings. And he hopes that their findings eventually help NASA determine a way to combat the problem, perhaps through medication.
"That's the goal: to figure out how to prevent this so when we go to Mars that by time we return, we can see," he said.
Alex Stuckey covers NASA and the environment for the Houston Chronicle. You can reach her at firstname.lastname@example.org or Twitter.com/alexdstuckey.