How Can Astronauts See Stars With Their Eyes Shut?

Question posed by Jennie.

The first human beings to leave the Earth's protective shield were open to a plethora of new experiences, some foreseen, others a complete surprise. An example of unpredicted happenings was astronauts' reports of seeing frequent flashes of light, even when they had their eyes closed.

By Dinoj at en.wikipedia, from Wikimedia Commons

The first reported experience of this phenomenon was during the Apollo 11 mission, when the Commander (Neil Armstrong) and the Lunar Module Pilot (Buzz Aldrin) both reported these flashes after their eyes had become adapted to the low light in the cabin. Similar reports during the Apollo 12 and 13 missions led to subsequent Apollo missions including dedicated time to run experiments specifically looking at this strange occurrence. These experiments involved blindfolding crewmembers and recording their comments during designated observation sessions.

Three types of flash prevailed in the crewmembers' descriptions; the 'spot', the 'streak', and the 'cloud'; and all but one described the flashes as 'white' or 'colourless'. One crewmember, Apollo 15's Commander (David Scott), described one flash as "blue with a white cast, like a blue diamond." After crewmembers had become dark-adapted, they report a flash every 2.9 minutes on average. Only one Apollo crewmember involved in the experiments did not report seeing the phenomenon. This was Apollo 16's Command Module Pilot (Thomas Kenneth Mattingly Jr), who stated that he had poor night vision.

But where do the flashes come from, and how is it possible that they can be seen even when astronauts' eyes are closed?

'Cosmic rays' are high-energy charged subatomic* particles that originate from sources in outer space. They can travel for many thousands, millions or billions of years before reaching us, and rarely reach the Earth's surface, being stopped by the Earth's atmosphere** before they get here. The Apollo astronauts, being outside the Earth's magnetosphere, were not so protected and found themselves under a shower of cosmic rays.

These rays are very small and travelling very fast, so when they travel towards solid objects, it's a bit like firing a bullet at a chicken-wire fence: relatively speaking, there's a lot of empty space and they tend to pass straight through things like space ships without even noticing that they're there. They also pass through eyelids fairly well for the same reason.

But like firing bullets at chicken-wire, while most of them pass through the gaps, now and then one will hit the wire. When a cosmic ray hits a bit of a space craft or even someone's eyelid it we don't notice it.

When a cosmic ray hits part of an astronaut's eye, though, we do notice it. There are two possible explanations for this:

• When charged particles pass through certain types of matter, it causes the matter itself to produce light. The eye's vitreous humor is one such medium. The light produced is known as Cherenkov radiation. This effect is well-known and used in various situations including particle physics experiments in order to detect and identify particles which are otherwise elusive. It also has uses in astrophysics and biology.
• Some scientists think that the flashes are caused by cosmic rays hitting the retina directly.

And a third, less well supported idea deserves a mention:

• Some people have suggested the possibility that cosmic rays may interact directly with the visual centres within the brain itself. The Apollo cosmic ray visual phenomena experiments, however, suggested that some crewmembers were able to state which eye they saw the flashes in, which seems to work against this particular idea. Also, the fact that dark-adaptation is necessary reinforces the first two possibilities as retinal tissue houses the biochemical changes responsible for dark-adaptation.

* 'Subatomic' means 'smaller than atoms'.

** They do, however, produce an effect that can be measured at ground-level, when they hit particles in the upper atmosphere causing a 'domino effect' of particles being produced, rather prettily known as an 'air shower' (the image at the top of this post shows a simulation of one). This effect is what keeps some unstable isotopes being produced in the Earth's atmosphere. One example of such an isotope is carbon-14, which cosmic rays have kept roughly constant for the past hundred thousand years or so, allowing us to use carbon-dating in the field of archaeology.