In July 1969, as Apollo 11 flew toward the Moon, astronauts noticed something unusual: brief flashes of light that appeared even with their eyes closed. At first, this was attributed to fatigue, but the flashes recurred in different crews. Gradually, it became clear: cosmic rays were to blame — tiny particles hurtling through space at tremendous speeds. They pass through the spacecraft, through the eyelids, and enter the eye. But how exactly a particle turns into a visible flash remained a mystery.
A new study built a computer model of the human eye and fired a stream of cosmic particles into it. It turned out that in ordinary space, between planets or in low orbit outside special zones, the flashes are created by a phenomenon similar to a light shock wave. A particle travels through the fluid inside the eye faster than light propagates in that fluid. This produces a faint glow, like a sonic boom but for light. The eye captures this light, and the brain sees a flash. The main "igniters" are iron nuclei, as well as oxygen and carbon. They are heavy and leave enough light in the eye to trigger retinal cells.
On Earth's surface, such flashes are not visible. The particles that reach us are mostly muons, light relatives of electrons. They can also glow in the eye, but their light is too weak to wake the retina. So on Earth, we sleep peacefully in the dark, while astronauts have to get used to a celestial "fireworks" behind their eyelids.
Astronauts see flashes on average once every few minutes, and they come in different shapes: dots, streaks, even whole clouds.
In Stanislaw Lem's novel "Fiasco," pilots see light trails from particles passing through the eyeball — science fiction that turned out to be reality.