Astronauts aboard the International Space Station can see flashes of light even with their eyes closed — not from light entering the eye, but from high-energy space particles passing through their body and triggering the retina or visual pathway directly.

Float in a darkened module on the International Space Station, close your eyes, and wait. Before long, most astronauts report the same thing: an occasional flash, a spot or a short streak of light, against the dark behind their eyelids. No light is reaching them. The flashes are made by particles from space passing straight through the eye.

To the crews it is an old and familiar oddity. To everyone else it tends to sound invented. It is neither, and it has been documented and measured for more than half a century.

First noticed on the way to the Moon

The astronauts of Apollo were the first to describe it. Buzz Aldrin mentioned seeing the flashes during Apollo 11 in 1969, once his eyes had adjusted to the dark of the cabin, and other crews soon reported the same. When they paid attention, they counted them, roughly one every few minutes.

NASA took it seriously enough to fly an instrument for it. On later Apollo missions a device strapped to an astronaut’s head recorded the tracks of charged particles while the wearer noted each flash, so the two could be matched.

The phenomenon was real, and it had a culprit.

What is actually happening

The culprit is cosmic rays: fast-moving protons and heavier atomic nuclei, flung across the galaxy by violent events and arriving with enormous energy. Out beyond much of Earth’s protective shielding, they pass through almost everything aboard a spacecraft, including the people inside it.

When one passes through an eye, it can produce a flash in more than one way. It can ionise the retina directly, ploughing through the light-sensing cells and exciting them as if they had caught a photon. It can also generate a faint but genuine glow, through a process called Cherenkov radiation, when a particle moves through the fluid of the eye faster than light itself travels in that fluid; the retina then registers that glow as it would any other. Either way, the brain receives a signal it can only read as light.

Measuring something you cannot photograph

Pinning this down meant catching the particle and the sensation together. On the Russian station Mir, and later on the ISS, experiments called SilEye and then ALTEA placed silicon particle detectors around an astronaut’s head, logging each particle’s path while the astronaut marked every flash.

The records lined up. Heavy, high-energy nuclei and energetic protons were the ones setting off the flashes, by both the direct and the Cherenkov routes. The broad picture is settled. The finer detail, which particles matter most, and how much of the effect happens in the retina rather than deeper along the visual pathway, is still being worked out.

Why it is more than a curiosity

The flashes are the only part of the space radiation environment a person can actually perceive. Each one is, in effect, the track of a single cosmic-ray particle through living tissue, briefly made visible.

That is also why they are studied rather than simply noted. The same particles that produce a harmless flash are the ones that raise long-term questions for astronauts’ eyes and central nervous systems on extended missions. The flash itself passes in an instant. What it marks, a steady arrival of high-energy radiation, does not.

What to watch

The effect scales with exposure, and exposure rises sharply away from Earth. The Space Station still sits partly within the planet’s magnetic shelter, while a crew bound for Mars would have none of it, and would see the flashes more often as the dose climbed.

That makes the phenomenon a small, personal readout of one of the central problems of long-duration spaceflight. Working out how to shield a crew from that radiation, over a journey measured in years rather than months, is the part still waiting to be solved.