This is simultaneously cool and horrifying. Cosmic rays are actually particles—tiny protons and neutrons, that shoot through space. They’re too small to see, but astronauts may still be seeing them.
One of the things they don’t tell you about being an astronaut is the fact that you will see little flashes of light. You will see them only once you’re out of Earth’s magnetosphere, which gives scientists a clue as to what may be causing the flashes. The magnetosphere is what protects us from many of the cosmic rays zooming through space.
“Cosmic rays” are badly named. They’re not rays, they’re tiny particles—usually single protons, but sometimes several protons stuck together (otherwise known as a small atomic nucleus).
Obviously, these are much to small to be directly seen. Researchers took a look at the question of what the astronauts were seeing up there. They came up with three ideas. One was that the particles were interacting with the optic nerve. The other was that they were directly hitting the vision center of the brain.
The third theory is somehow creepier than either other option. Cherenkov radiation is a glow usually emitted by material around radioactive objects. Material, whatever it is, slows down photons. Only in a vacuum does light travel at the “speed of light”. Light has a characteristic speed in different materials—glass, water, air, diamond—and within those materials, that speed can be exceeded. When the speed is exceeded, by particles emitted by the radioactive object, there is a kind of cascade, or sonic boom, of photons. The material emits light. It’s possible that what the astronauts are seeing is Cherenkov radiation caused by these particles passing through the vitreous humor of their own eyes. The light is coming from inside their eyeballs.