But in 2005, one unnamed astronaut came forward to reveal his affliction, prompting a survey of the corps. NASA discovered 35 percent of its former space station crew members, who typically spend about six months in orbit, experience visual acuity issues, agency spokesman Mike Curie told Discovery News.
"It wasn't until a couple of years ago when we started seeing this on station that we injected true acuity scans and MRIs (magnetic resonance imaging) to produce actual medical data. That type of exact data does not exist from the shuttle astronauts," who typically spent two weeks in space, Curie added.
The condition isn't serious enough to cause blindness in the short term, but it raises concern about what would happen to astronauts during a three-year mission to Mars. The vision loss seems to be due to a swelling of the optic nerve, a condition similar to a disease on Earth called pseudotumor cerebri, which mostly afflicts heavy women.
"Nobody knows why pseudotumor cerebri occurs, but it's a condition which, if left untreated in some people who are really afflicted badly with it, can lead to complete blindness," Ehni said.