Two Apollo 17 crew members saw flashes of light during the mission. They noticed them both in orbit around the Moon and on its surface.
The flashes appeared at different points during the mission, but their source was not necessarily outside the spacecraft or on the Moon. During a post-mission medical debriefing, the crew and debriefers discussed the phenomenon: particles from space can make the eye see light. What looks like a flash in space may actually begin inside the eye.
Two Out of Three
The Apollo 17 crew had three members. Two reported seeing flashes at different points during the mission. The summary says the flashes appeared both in orbit around the Moon and on its surface.
The reports appear in an excerpt from a follow-up medical debriefing held after the mission. The crew and debriefers discussed the flashes of light. According to the summary, the phenomenon was new at the time and is now well documented.
How the Eye Sees Light Without an External Source
The explanation discussed during the debriefing involves high-energy particles from space. They can pass through the eye and strike the retina, the light-sensitive layer at the back of the eye.
The impact can make a person see streaks of light or flashes, even when no ordinary light source is shining on them. So a flash seen during spaceflight is not necessarily light coming from outside the spacecraft. According to the account, it is the eye reacting to particles passing through it.
What We Still Don’t Know
The summary does not identify the two crew members, say how many times they saw flashes, or specify exactly when they saw them. It describes the explanation discussed in the debriefing: particles from space struck the retina and produced the sensation of light.
How can a person see light when no visible light source is present?