Now a new study has looked into what might be happening to astronaut eyes at a molecular level. In a small study, scientists sent six mice to the International Space Station for 13 days. (The little guys rode aboard the last Discovery shuttle mission.) After the mice returned to Earth, the scientists, who came from several Houston research institutes, found the mice's optic nerves had physical damage. The researchers also saw increased signs of stress in the cells in the mice's eyes. These signs subsided within seven days after the mice returned to Earth, although the physical damage didn't heal. The study didn't extend past seven days after the mice's return to Earth.