Astronauts who engage in long space missions may see the world differently once they get back to Earth. Literally.

Seven out the 50 subjects of a study reported having blurry vision even long after landing. The 50 subjects, who were at around age 50, were sent to the International Space Station, where they stayed for six months.

The astronauts said that their vision starts to become blurry during the sixth week of the mission. The severity of the sight problem, however, differed with each subject.

According to the study, some of the subjects claim that the blurry vision persisted up to a month after landing on earth.

"In astronauts over age 40, like non-astronauts of the same age, the eye's lens may have lost some of its ability to change focus," Space.com quoted Thomas Mader, an ophthalmologists of the Alaska Native Medical Center in Anchorage, as saying.

The study results could indicate how long space missions, like sending a man to Mars, could affect the astronaut's physical condition.

The vision problem only occurred on astronauts who spent more than six months on microgravity, which could suggest that stress caused by spacecraft launch and re-entry had nothing to do with the condition.

The research show that its is likely that intracranial pressure, an occurrence when pressure builds up inside the skull, could have tissue, fluid, and nerve abnormalities found in the subjects eye after the mission, which would have caused the blurry vision.

However, other symptoms of which usually goes with ICP, such as chronic headaches, double vision or ringing in the ears, were not present.

Other likely causes the researchers are looking at includes abnormal flow of spinal fluid around the eye's optic nerve, changes in blood flow in the tissues behind the retina, or changes related to the low pressure felt within the eye in microgravity.

According to the researchers, micro gravity could have caused some fluid to be displaced to the head, causing problems just mentioned.

The study was published in the October issue of Ophthalmology, the journal of the American Academy of Ophthalmology.