They knew they were in a risky business. But now, as American astronauts spend more and more time in space, they've noticed they're returning to Earth with a surprising malady: They cannot focus their eyes properly after they come home, and for some the problem seems permanent.
Astronauts with 20/20 vision found they needed glasses for the first time, says NASA. A few, their names withheld to protect their privacy, were told it would be unwise for them to fly in space again. At least a couple could no longer pilot private planes.
Now, a team from the University of Texas Health Science Center in Houston has done MRIs on 27 astronauts who spent more than month in space, and reports in the journal Radiology that 60 percent have what, on the ground, would be called intracranial hypertension -- high fluid pressure in the skull. The more time they spent in flight, the more likely they were to have the problem.
"We've known about vision changes on orbit but in some cases we've actually found that it can be permanent," said Peggy Whitson, who has flown twice on the space station herself and is now chief of the astronaut office. She spoke with ABC News last year when the pattern among veteran space flyers first became apparent.
A fifth of the astronauts tested showed a flattening of the rear of the eyeball, affecting their ability to focus their eyes. A third showed expansion of the space surrounding the optic nerve that's normally filled with cerebral spinal fluid.
Dr. Larry Kramer, who led the team at the University of Texas that did the MRIs, said the findings could someday be useful to non-astronauts, but at the moment he's most concerned about space flyers.
"What does this mean when we want to travel beyond Earth orbit, on longer missions to Mars and elsewhere?" he said in an interview with ABC News. "There's no way to predict it, so we ought to study it now."
Superficially, the cause would seem clear, though doctors say they worry that the mechanism may be more complicated. On Earth, the human body is attuned to gravity, with the heart pumping to keep fluids from pooling in the legs. In weightlessness, the body keeps forcing fluids upward -- even though there's no gravity to pull them down. So most astronauts report a feeling of fullness in the head. On short flights, Tabasco sauce and horseradish have proved popular on astronauts' menus; spicy food seems to clear out the sinuses.
But Kramer's study found more complicated consequences of long spaceflight. In addition to the trouble some astronauts had focusing their eyes, he said, there may be greater risk to the brain if any of them suffer head injuries.
"Some of it is reversible," said Whitson. "Some people get reverses and they come back to the same level that they were at pre-flight, and some were not reversible. We don't know enough yet to understand the mechanisms for how that happens."
While 60 percent of the astronauts showed signs of trouble if they spent more than a month of their lives in weightlessness, the other 40 percent seemed immune -- and Kramer said there was no guessing what would protect some but not others. "What makes them so special?" he said. "We don't know."
This is a looming issue for the future, but one that also matters now. NASA, since the end of the space shuttle program in 2011, has only been sending astronauts on multi-month missions, launched by Russian Soyuz spacecraft. Most of the space agency's long-term plans -- whether in Earth orbit, or going to Mars or to passing asteroids -- involve lengthy flights.
"At this point we're just raising the issue," Kramer said. "But once you're headed to Mars, there's no turning back."