WASHINGTON - If NASA ever wants to send astronauts to Mars, it first must solve a problem that has nothing to do with rockets or radiation exposure.

A newly discovered eye condition -- found to erode the vision of some astronauts who've spent months aboard the International Space Station -- has doctors worried that future explorers could go blind by the end of long missions, such as a multi-year trip to Mars.

While blindness is the worst-case scenario, the threat of blurred vision is enough that NASA has asked scores of researchers to study the issue and has put special eyeglasses on the space station to help those affected see what they're doing.

"We are certainly treating this with a great deal of respect," said Dr. Rich Williams, NASA's chief health and medical officer. "This is comparable to the other risks like bone demineralization and radiation that we have to consider. ... It does have the potential for causing mission impact."

According to one NASA survey of about 300 astronauts, nearly 30 percent of those who have flown on space shuttle missions -- which usually lasted two weeks -- and 60 percent who have completed six-month shifts aboard the station reported a gradual blurring of eyesight.

Williams put the figure lower -- at roughly 35 percent for station crew -- but did not dispute the severity of the problem or the mystery surrounding it.

The disorder, similar to an Earth-bound condition called papilledema, is believed to be caused by increased spinal-fluid pressure on the head and eyes because of microgravity, although the exact cause is uncertain.

Oftentimes, the problem goes away once an astronaut returns to Earth. But a recent study by the National Academies noted that there had been "some lingering substantial effects on vision" and that astronauts were "not always able to requalify for subsequent flights" -- at least not immediately.

Williams declined to discuss specific cases but acknowledged at least one astronaut never regained normal vision.

"We have seen visual acuity not return to baseline," he said.

Though it will be years before NASA has a rocket powerful enough to launch humans to Mars, the agency has long worried about the effects on astronauts of the nearly three-year-long round trip. But the chief worry has been exposure to cosmic radiation and, to a lesser extent, loss of bone mass due to microgravity.

For decades, though, NASA had also heard of anecdotal evidence of vision problems. But the agency began studying the issue in earnest only around 2005 when an unnamed astronaut came forward.

"You didn't hear about it at all until you had one fellow come back [from space] and had problems and was very open about it. His openness led to other people reporting the same," said Garrett Reisman, a former astronaut who spent three months aboard the station in 2008.

Much of the research so far has centered on seven unnamed astronauts who have shown symptoms, including one astronaut whose eyesight was so affected that by his third month on the space station he could "only see the Earth clearly while looking through the lower portion of his progressive reading glasses," according to a draft of a paper to appear in the medical journal Ophthalmology.

The seven cases were reviewed at a conference last February in Houston of about 75 scientists and doctors, from a wide range of fields.