Apollo 17 Technical Crew Debriefing, 1973

This one-page excerpt comes from the technical debriefing NASA held with the Apollo 17 crew in the weeks after the mission. The speaker is Harrison Schmitt, the lunar module pilot and the only professional geologist to walk on the Moon. His remarks are about the light flashes many Apollo astronauts reported seeing with their eyes closed or in the dark.

What Schmitt told the debriefers

Schmitt says the flashes came “just about continuously” whenever the crew’s eyes were adapted to the dark. He singles out one he thought had been a flash on the lunar surface — apparently the same event he called out live during the flight, recorded in the air-to-ground transcript as a flash north of the crater Grimaldi.

He then adds a telling detail. During a session wearing blindfolds for the ALFMED experiment, there were no visible flashes at all, yet that night before sleep he was seeing them again.

What the ALFMED experiment was

ALFMED, the Apollo Light Flash Moving Emulsion Detector, was a helmet-like device lined with emulsion plates that record the tracks of passing charged particles. The idea was to compare the moments astronauts reported flashes with the particle tracks, testing the theory that cosmic rays crossing the eye cause them. Schmitt does not draw a conclusion here. Later NASA material in this collection, such as the Apollo 17 medical debriefing, describes the flashes as a now well-documented effect of cosmic rays on the retina.