In space I see things that are not there. Flashes in my eyes, like luminous dancing fairies, give a subtle display of light that is easy to overlook when I’m consumed by normal tasks. But in the dark confines of my sleep station, with the droopy eyelids of pending sleep, I see the flashing fairies. As I drift off, I wonder how many can dance on the head of an orbital pin.

The retina is an amazing structure. It’s more impressive than film or a CCD camera chip, and it reacts to more than just light. It also reacts to cosmic rays, which are plentiful in space.

Cosmic rays are fragments of atoms—some the pieces of faraway exploded stars, some leftover debris from when the universe formed. These atomic fragments move at high speeds, and like X-rays, penetrate deep into material where they are eventually absorbed. Fortunately, our atmosphere absorbs most of them, so they do not pose significant problems for Earth dwellers (except for the many unfortunate effects to our bodies that we have collectively named “the aging process”).

Space is different. Free from the protection offered by the atmosphere, cosmic rays bombard us within Space Station, penetrating the hull almost as if it was not there. They zap everything inside, causing such mischief as locking up our laptop computers and knocking pixels out of whack in our cameras. The computers recover with a reboot; the cameras suffer permanent damage. After about a year, the images they produce look like they are covered with electronic snow. Cosmic rays contribute most of the radiation dose received by Space Station crews. We have defined lifetime limits, after which you fly a desk for the rest of your career. No one has reached that dose level yet.

When a cosmic ray happens to pass through the retina it causes the rods and cones to fire, and you perceive a flash of light that is really not there. The triggered cells are localized around the spot where the cosmic ray passes, so the flash has some structure. A perpendicular ray appears as a fuzzy dot. A ray at an angle appears as a segmented line. Sometimes the tracks have side branches, giving the impression of an electric spark. The retina functions as a miniature Wilson cloud chamber where the recording of a cosmic ray is displayed by a trail left in its wake.

The rate or frequency at which these flashes are seen varies with orbital position. There is a radiation hot spot in orbit, a place where the flux of cosmic rays is 10 to 100 times greater than the rest of the orbital path. Situated southeast of Argentina, this region (called the South Atlantic Anomaly) extends about halfway across the Atlantic Ocean. As we pass through this region, eye flashes will increase from one or two every 10 minutes to several per minute.

Our brain interprets its sensory input and creates a map of reality. Philosophers have for centuries contemplated this question. As Plato wrote, we see only the shadows of a larger and richer reality. On Space Station, I drift off to sleep, thinking of the nature of the “real” universe while observing my personal reality of dancing fairies.

Don's blog also appears at airspacemag.com.