The scientific community advanced several theories to explain the light flash phenomenon. The phenomenon was thought to have been attributed to penetration of the eye by cosmic ray particles with retinal stimulation occurring by one of two mechanisms. One explanation postulated that Cerenkov radiation was responsible for the light observed. A second explanation was that the bulk of the reported light flashes were caused by cosmic ray particles producing phosphenes due to direct interaction of ionizing radiation with nervous tissue in the retina. The experiments that followed on Apollo 14, 15, 16, and 17 were designed to determine which mechanism was more likely to be the primary cause of light flash sensations.
To determine if dark adaptation was necessary to observe light flashes, one subject attempted to destroy his dark adaptation by waving a flashlight in front of his eyes for 60 seconds after the experiment had been in progress for twenty-four minutes. After the additional light adaptation, the experiment continued, and all procedures were repeated.
Three one-hour observation sessions were scheduled on Apollo 15. The sessions occurred during translunar coast (TLC) travel, lunar orbit, and TEC. Using a protocol similar to that of Apollo 14, but without a precursory light adaptation phase, the astronauts recorded their observations on tape and reported light flash observations to the ground-based tracking station as they occurred. Light-tight eyeshades were worn for the first time during the Apollo 15 mission to provide a uniform and reproducible degree of darkness.
Two one-hour sessions were scheduled on the Apollo 16 and 17 missions. The sessions occurred during TLC and TEC. A system was developed for the Apollo 16 and 17 missions to obtain, for the first time, a direct physical record of incident cosmic ray particles that allowed correlation with crewmembers' reports of light flashes. The measurement system was known as the Apollo Light Flash Moving Emulsion Detector (ALFMED).
The protocol was identical on Apollo 16 and 17. During the TLC observation session, two crewmembers participated in the experiment. One subject wore the ALFMED, while the second subject wore eyeshades. The third crewmember transcribed the comments of the two subjects. During the TEC observation session, all three crewmembers wore eyeshades only; the ALFMED was not worn. The observation period began when the crewmembers reported by voice to the ground that they had donned the ALFMED and/or eyeshades. Thereafter, each crewmember verbally reported the occurrence and characteristics of each light flash. Personnel on the ground recorded times of occurrence and crew comments for each light flash event as reported. At the conclusion of the sessions, the crewmembers reported when the ALFMED and/or eyeshades were removed. Information reported by voice to the ground during the mission and obtained at post-mission crew briefings was tabulated from the ALFMED. The relationships between the occurrence of light flashes and the cosmic ray particles that passed through the ALFMED and the eyes of the crewmen were determined.
Light flashes were not reported during the Apollo 7-10 missions. On Apollo 11, crewmembers saw light flashes in the darkened command module during TEC while relaxing or wearing light-tight shields. All astronauts on Apollo 12 and 13 saw flashes with relative ease when the spacecraft was dark with their eyes open or shut.
Three basic types of flashes were reported on Apollo 14. The most prevalent was the "spot" or "starlike" flash sometimes referred to as a "super nova." Sixty-six percent of the flashes were of this variety. A type of flash described as a "streak" was the second most abundant occurring about twenty-five percent of the time. Some streaks were described as sharp lines, while others appeared to be diffuse. Still, others were reported as dashed lines with the most common version consisting of two principle segments with a gap in the middle. All streaks had a sense of movement appearing to be "going from left to right" or "coming straight at me." These streaks were hypothesized to be caused by particles with trajectories approximately tangent to the retina, and their apparent motion was due to either eye movement or the shape of the streak. The final type of flash was referred to as a "cloud" and occurred in eight percent of the cases. Clouds were flashes with no discernible shape and always appeared in the peripheral visual field. One subject on Apollo 14 described the clouds as resembling a lightning discharge when viewed from behind terrestrial clouds in the distance. Some of the cloud flashes were so large as to appear to fill the entire periphery while leaving the central visual field dark.
The frequency of the light flashes averaged about one per two minutes for each crewmember. The visual phenomena were observed with the eyes open and closed, and the crew was more aware of the phenomena immediately upon awakening than upon retiring. During the TEC session, one subject determined that it was not necessary for the eyes to be dark adapted to see the flashes if the level of illumination in the spacecraft was low. Researchers considered this by far the most significant experiment result. This evidence concluded that some, and probably most, of the flashes were produced by mechanisms other than Cerenkov radiation.
All of the Apollo 14 astronauts were positive that it was possible to tell which way a "streak" was moving. Considering simulation studies, researchers concluded that streaks may have been produced by low energy particles and not Cerenkov radiation, but the observations were not definite enough to allow firm conclusions on that point.
Several light flashes were apparently seen by two of the crewmen simultaneously. Coincidence of light flashes for two crewmembers, if a true coincidence, would substantiate that the flashes originated from an external radiation source and would indicate that they were generated by extremely high energy, highly ionizing particles, e.g., Cerenkov radiation. Low energy highly ionizing particles would not have the range through tissue to have reached both crewmembers.
Thus, results from Apollo 14 did not answer conclusively whether the light flashes were caused by low-energy particles or high-energy, high atomic number High Charge Energy (HZE) particles.
The tape containing detailed descriptions of events observed during TLC were lost during playback to the ground. During lunar orbit, a total of twelve events were reported with the first event occurring ten minutes after dark adaptation. During TEC, a total of twenty-three events were reported by the three crewmembers with the time of the first event occurring between seventeen to thirty minutes after dark adaptation.
Apollo 16 and 17
From the ALFMED data, researchers calculated the mean time between events after dark adaptation for each observer and the average value for all observers for each session. The average time between events was longer during TEC observation periods than during TLC sessions. TEC dark adaptation times (time to witness the first flash) were also considerably longer than those found during TLC sessions. In addition, most crewmembers commented that the flashes seemed not only less frequent during the TEC sessions but also much less brilliant. The most dramatic example of this difference occurred on Apollo 17 when all crewmembers reported that no events were seen during the entire one-hour TEC session. During a similar TLC session, the two observing crewmembers reported a total of twenty-eight events. The only other inability to observe the phenomenon occurred on the Apollo 16 mission when one subject reported he was unable to observe even a single event at any time during the entire mission. A number of possible mechanisms were examined in an attempt to explain the decrease in flash events during TEC observing sessions. None of these mechanisms offered an adequate explanation. The anomaly remained as an apparently real effect for which no unique explanation had been demonstrated.
Apart from the intrinsic interest of this phenomenon, it was important to find the cause in order to determine whether a hazard could be present. Using long-duration radiation measurement standards, the whole-body dose due to cosmic radiation was quite low during most space missions unless a major solar flare was encountered. However, the biological interactions of the HZE particles common in space were not well understood. Further investigation would be required to determine realistic human tolerance levels for extended missions and to evaluate the need to provide special spacecraft shielding.
The controversy over which mechanism, Cerenkov radiation or direct neural stimulation, was responsible for the observed effects was a matter of great interest to scientists and stimulated research by physicists, physiologists, and physicians. The major concern, of course, was the assessment of the potential hazards to astronauts on long-term missions. No changes in visual acuity of the astronauts were detected. Retinal photography performed before and after the mission revealed no evidence of changes attributable to radiation.
At the end of the Apollo program, it appeared that the direct neural stimulation hypothesis provided the more likely explanation of the light flash phenomenon. However, the alternative mechanism, Cerenkov radiation, was certainly plausible and could not be excluded as a possibility for at least some of the light flash events. Estimates of the effect of Cerenkov radiation proposed that a two hundred day mission would not result in any serious injury. The expected rate and intensity of Cerenkov light pulses within the eyes of Apollo astronauts exposed to radiation away from Earth appeared too low to account for the bulk of light flashes. The observed rate was about one per two minutes after some initial dark adaptation. Results of laboratory exposure of human subject to several types of particulate radiation and to energetic x-rays suggested that the bulk of reported light flashes were radiation-induced phosphenes, that is, sensations of light due to direct interaction of ionizing radiation with nervous tissue in the retina.