Astronauts can see blinding light flashes with their eyes closed
Beginning with Apollo 11, astronauts traveling beyond Earth's thick magnetic bubble reported mysterious white streaks and flashes of light while trying to sleep. This phenomenon occurs when high-energy cosmic rays zip directly through the spacecraft hull and penetrate an astronaut's eyeball. As these relativistic subatomic particles strike the retina or trigger Cherenkov radiation in the eye's fluid, the optic nerve registers bright, phantom flashes in total darkness.
Flashes in the Dark
During the Apollo lunar missions, crew members resting in the command module noticed a startling visual effect. In the darkened cabin, with their eyes closed to sleep, astronauts observed unexpected bursts of light. These were not external reflections or cabin illumination leaking through their eyelids, but internal visual sensations occurring inside total darkness.
The light phenomena varied widely in appearance from one observation to the next. Crew members described seeing faint pinpoint flashes, elongated streaks resembling shooting stars, and diffuse luminous clouds. The apparitions appeared randomly, typically every few minutes, whether the astronauts rested with their eyes open in an unlit cabin or closed under sleep masks. Because the flashes persisted regardless of eyelid position, it became clear that the stimulus bypassed ordinary optical pathways.
Cosmic Rays and the Space Environment
The source of these phantom flashes lies in the hostile radiation environment of deep space. Beyond the protective shield of Earth's atmosphere and the bulk of its magnetosphere, space is permeated by galactic cosmic rays and solar energetic particles. Galactic cosmic rays consist of atomic nuclei stripped of their electrons, accelerated to relativistic velocities by energetic astrophysical events across the galaxy.
These high-energy particles possess immense kinetic energy, allowing them to penetrate the metallic hull of a spacecraft and the tissue of an astronaut's body with little deceleration. Standard spacecraft walls offer negligible shielding against such heavy, fast-moving charged ions. As a consequence, space travelers are constantly traversed by subatomic particles traveling near the speed of light.