Answer: Cosmic Radiation

The first astronauts were veritable guinea pigs, exposed to all sorts of conditions that no human had ever been exposed to before, including microgravity, temperature extremes, and full exposure to radiation outside of the protective envelope of Earth’s atmosphere. It was the last element, radiation, that produced a particularly curious set of circumstances for astronauts aboard the Apollo missions.

Both Neil Armstrong and Buzz Aldrin, Apollo 11 Commander and Lunar Module Pilot respectively, reported seeing bright flashes of light inside the cabin of the spacecraft. They saw these flashes even with their eyes closed. In subsequent Apollo missions, more astronauts reported the flashes, which were visible even when they were blindfolded. The flashes were frequent, occurring roughly 20 times an hour, and continued for as long as the astronauts were in space.

But what caused the persistent phantom lights in their vision? Cosmic radiation. We are well shielded from it here on Earth, but in space, both the spacecraft and the astronauts inside were constantly bombarded by it. Cosmic radiation is a high energy form of radiation that passes through most materials, but when a particle strikes some substances—such as the water-filled eyeball of an astronaut—some of that energy can be converted into light. The flashes experienced by astronauts, then, were tiny bits of cosmic background radiation putting on a tiny light show inside their very eyeballs.