Some astronauts who spend months in space are reporting blurred vision, and NASA is just now getting an idea of how serious the problem is, and what it could mean for long term space travel.
NASA research has found that astronaut vision erodes the longer they spend in space. And some doctors believe that in very long duration flights they could risk blindness.
Among the next crew heading to the International Space Station is veteran astronaut Don Pettit.
"Sometimes your eyes start to water just because you're looking at Earth and it's such a beautiful sight," he said. "Of course you're going to have some blurred vision after that."
Pettit wouldn't tell us if he's ever experienced a true change in his vision during or after a mission. But a new report indicates many have.
Chief NASA Astronaut Peggy Whitson said, "Right now, all of our crew members are potentially vulnerable. We don't know yet how to determine who will be vulnerable and who won't be vulnerable."
In the study of 300 astronauts, 30 percent of those who have flown two week shuttle missions, and 60 percent of those who have endured longer six month stints on the space station noticed things became blurry either during or after the missions.
"Some of it is reversible," Whitson said. "Some people, it reverses and they come back to the same level that they were at pre-flight. Others have not been reversible."
Researchers think the issue stems from long term exposure to microgravity. But astronauts insist the whole point of going to space is to study such effects on the body.
"Right now we don't have enough data on this to see what is going to happen," Pettit said.
If we plan to one day send astronauts to Mars, they say we need to know what will happen to those who carry out that mission. NASA is still researching how to protect astronauts' vision.