Astronauts have seen some truly amazing sights: the Northern lights from above, the curve of the Earth and the dark side of the Moon. On top of this, astronauts aboard Skylab, the Shuttle, Mir, and the International Space Station have reported seeing strange flashes of light. These are caused by cosmic radiation zipping through their eyes like teeny tiny bullets. When one of these particles strikes the nerves in the eye it triggers a false signal that the brain interprets as a flash of light.