Astronauts who've spent long periods in space have experienced blurred vision, a problem that could jeopardize long missions like a trip to Mars, NASA says.

In a NASA survey of about 300 astronauts, 30 percent who have flown on two-week space shuttle missions and 60 percent who've spent six months aboard the International Space Station reported a gradual blurring of eyesight, the Orlando (Fla.) Sentinel reported Monday.

NASA said it is conducting intensive research into the problem.

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

While the condition normally goes away once an astronaut returns to Earth, at least one astronaut reportedly has never regained normal vision.

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

Similar to an Earth-bound condition called papilledema, the blurred vision is thought to be caused by increased spinal-fluid pressure on the head and eyes in microgravity.

Multiyear missions such as a trip to Mars could see the blurring of vision become a serious problem, researchers said.

"No one has been in space long enough to know how bad this papilledema can get," said Dr. Bruce Ehni, a neurosurgeon who has worked with NASA on the issue.

"When they [NASA] start going [to] long-distance [destinations] like Mars, you can't end up having a bunch of blind astronauts."