NASA officials often talk about the “unknown unknowns” — the unforeseen problems that catch them by surprise. The eye issue caught them by surprise, and they are happy it did not happen in the middle of a mission to Mars.

In 2009, during his six-month stay on the International Space Station, Dr. Michael R. Barratt, a NASA astronaut who is also a physician, noticed he was having some trouble seeing things close up, as did another member of the six-member crew, Dr. Robert B. Thirsk, a Canadian astronaut who is also a doctor. So the two performed eye exams on each other, confirming the vision shift toward farsightedness.

They also saw hints of swelling in their optic nerves and blemishes on their retinas. On the next cargo ship, NASA sent up a high-resolution camera so that they could take clearer images of their eyes, which confirmed the suspicions. Ultrasound images showed that their eyes had become somewhat squeezed.

NASA is now checking astronauts’ eyesight before, during and after trips to the space station.

The issue turns out not to be new. Many space shuttle astronauts had complained of changes in eyesight, but no one had studied the matter.

“It is now a recognized occupational hazard of spaceflight,” Dr. Barratt said. “We uncovered something that has been right under our noses forever.”

Dr. Barratt said the vision shift had no effect on his ability to work in space. The concern, however, is that the farsightedness may be just a symptom of more serious changes in the astronauts’ health. “What are the long-term implications?” he said. “That’s the $64 million question.”