Astronauts see bright flashes of light in space caused by cosmic rays passing through their eyes.

The phenomenon of light flashes in space was first reported by Apollo 11 astronauts Buzz Aldrin and Neil Armstrong in 1969. These flashes are primarily caused by cosmic rays, which are high-energy protons and atomic nuclei traveling through space at nearly the speed of light. When these particles enter the vitreous humor of the eye, they travel faster than the phase velocity of light in that specific medium.This disparity in speed triggers Cherenkov radiation, which is an electromagnetic shockwave similar to a sonic boom but for light. Astronauts describe the visual sensations as 'streaks', 'stars', or 'clouds' that appear even when their eyes are tightly shut. A study by the ALTEA (Anomalous Long Term Effects in Astronauts) program on the International Space Station confirmed that these events are directly linked to heavy ion passage through the visual cortex or the retina.Beyond being a visual curiosity, these particles pose a significant health risk to long-duration space travelers. NASA researchers have found that cosmic radiation exposure significantly increases the incidence of radiation-induced cataracts among the astronaut corps. For example, a 2001 study published in 'Investigative Ophthalmology & Visual Science' showed that astronauts with higher radiation exposure developed cataracts earlier than those with lower exposure. Protecting crews from these invisible, high-speed particles remains one of the biggest challenges for future missions to Mars.