NASA experts in a special study found, that nearly 300 astronauts have experienced the loss of visual acuity after space flight. It is reported, that the majority of astronauts suffer loss of visual acuity, were long on board the International Space Station. According to scientists, the loss of visual acuity of the astronauts is due to swelling of the optic nerve, which arises from the high-pressure cerebrospinal fluid in conditions of weightlessness. So long stay in weightlessness and leads human to deterioration of sharpness of vision.