Cosmic ray visual phenomena

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Cosmic ray visual phenomena, also referred to as phosphenes or "light flashes", are spontaneous flashes of light visually perceived by astronauts outside the magnetosphere of the Earth, such as during the Apollo program. Researchers believe that cosmic rays are responsible for these flashes of light, though the exact mechanism is unknown. Hypotheses include one or all of: Cherenkov radiation created as the cosmic ray particles pass through the vitreous humor of the astronauts' eyes, direct interaction with the optic nerve, or direct interaction with visual centres in the brain.[1]

Astronauts almost always reported that the flashes were white, with one exception in which the astronaut observed "blue with a white cast, like a blue diamond." There were a few different types of flashes: "spots" and "stars" were observed 66% of the time, "streaks" were observed 25% of the time, and "clouds" were observed 8% of the time. Once their eyes became adapted to the dark, Apollo astronauts reported seeing this phenomenon once every 2.9 minutes on average. They also reported that they observed the phenomenon more frequently during the transit to the Moon than during the return transit to Earth.

During the Apollo 16 and Apollo 17 transits, astronauts conducted the ALFMED Experiment[2] where an astronaut wore a helmet designed to capture the tracks of cosmic ray particles to determine if they coincided with the visual observation. Examination of the results showed that two of fifteen tracks coincided with observation of the flashes. These results in combination with considerations for geometry and Monte Carlo estimations led researchers to conclude that the visual phenomenon were indeed caused by cosmic rays.

More recently, the SilEye/Alteino and ALTEA projects have investigated the phenomenon aboard the International Space Station, using helmets similar in nature to those in the ALFMED experiment.

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