Probably due to swelling of the optic nerve and changes in eye structure caused by the space environment, about 60 percent of International Space Station astronauts and 29 percent of space shuttle astronauts have reported a decline in vision clarity. After a few weeks aboard the ISS in 2007, for example, Canadian astronaut Bob Thirsk reported that his close-up vision had changed to the point that he had to ask for help in focusing cameras [source: NASA].
In some cases, the effects could cause permanent damage to the eyes, researchers worry. While space might not be able to make astronauts blind, it could reduce their vision enough to put them at risk of suffering a fatal accident. NASA medical researchers are looking for a solution, but in the meantime, they've developed special adjustable-focus glasses for astronauts to wear in space [source: NASA].