Observations of the Light Flash phenomenon in space

Light Flashes (LF) in space are abnormal visual perceptions thought to originate from the interaction of cosmic radiation with the human visual system. They are perceived by astronauts usually after a period of dark adaptation of the eye. These flashes are described to occur in a variety of shapes (streaks, star-like flashes, etc.) apparently differing from the diffuse glow reported by patients subjected to X-ray exposure. Usually these flashes are reported by astronauts after a period of dark adaptation of the eye, although there have been reports of flashes even under normal lighting conditions. Frequency, type and shape are greatly dependent on the observer; in addition it is usually necessary to brief the astronauts on what to expect in order for them not to discount LF without paying attention to them. After being instructed, most (but not all) astronauts were able to observe this phenomenon.

Space Borne experiments: 1969-1975

Light Flashes were originally predicted by Tobias in 1952 long before the first space flights and a detailed knowledge of cosmic ray radiation was available. The original intuition was found to be correct. Light Flashes were indeed reported for the first time on board Apollo 11 (1969). Subsequent observations on Apollo 12 and 13 missions, resulted in the first dedicated observation sessions on Apollo 14 and 15. On Apollo 16 and 17, LF observation sessions were carried out using the ALFMED (Apollo Light Flash Moving Emulsion Detector) experiment, consisting of an helmet with cosmic ray sensible emulsion plates to correlate passage of particles with the astronauts' visual system.
Average LF observation rate for the Apollo missions was 0.23 LF/min², compatible with the hypothesis of cosmic ray nuclei interacting with the human visual system. LF frequency during translunar coasting (going toward the Moon) was higher (0.39 LF/min) than that (0.34 LF/min) during transearth coasting (coming back to Earth). This phenomenon was particularly manifest during the Apollo 17 transearth phase, where neither of the three astronauts observed any LF. In addition to the lower LF frequency, dark adaptation time in the former case was lower (11 min) than in the latter (22.6 min). No correlation was found between the relative position of the spacecraft, the moon and the geomagnetic field, solar activity or other phenomena.

In 1974 LFs were investigated in low Earth orbit on board Skylab. Two observation sessions by the pilot of Skylab 4 mission were performed: the first (70 minutes on day 74 of the mission) passed through the edge of the South Atlantic Anomaly and reached higher geomagnetic latitudes; the second (55 minutes on day 81) passed through the center of the South Atlantic Anomaly. A large number (112) of flashes were observed during the passage in the SAA (which lasted 12 minutes), mostly during the second session where the anterior-posterior axis of the head of the astronaut was parallel to the local magnetic field, suggesting an important contribution by trapped protons. Also marginal evidence for a latitude effect was reported but not deemed conclusive at the time.

Light Flashes were also observed in 1975 in the framework of the Apollo-Soyuz Test Project (ASTP). Also in this case two sessions were planned and resulted in the observation of a total of 82 flashes. The frequency of flashes was strongly dependent on latitude, showing an increase at high latitude regions where particle flux is higher due to the lower geomagnetic shielding (which allows lower energy particles to reach low earth orbit). Contrary to the Skylab, no flash increase was observed in the SAA. This was explained in terms of the lower altitude (225 km on Apollo-Soyuz and 443 km on Skylab) and the higher shielding of the Apollo capsule respect to the Skylab: these two causes concurred to reduce trapped proton flux in the Anomaly region.

The Sileye experiments: 1995-2002

As in the case of ground tests, Light Flash research in space was resumed after 20 years on board Mir Space Station with the Sileye experiments, which placed two detectors (Sileye-1 in 1995 and Sileye-2 in 1998) and involved 6 astronauts in LF observations on MIR between 1995 and 1999. A third detector, Sileye-3/Alteino was used on the International Space Station (ISS) in 2002 during the Soyuz-34 mission. Overall these experiments have resulted in 35 observation sessions by 7 astronauts.

All detectors share the same philosophy of the previous devices. They consist in an helmet which contains a cosmic ray silicon tracking detector: the astronaut wears the helmet (with a light shielding mask), undergoes a period of dark adaptation and pushes a button when he observes a LF. In parallel the silicon detector measures cosmic rays above 40MeV/n from protons to Iron. Results of the observations are resumed in Table 1 and compared with previous observations: it is possbile to see how LF perception on board Mir is lower than that observed on previous experiments (0.13 LF/min). This is probably due to a lower orbit and and higher shielding of the station.