After the Apollo 17 mission, two of the three crew members said they had seen flashes of light. They discussed the experience during a post-mission medical debriefing.
The flashes appeared in two very different settings: in orbit around the Moon and while the crew was on its surface. The proposed explanation has nothing to do with a light source in the landscape or spacecraft. It starts with particles from space passing through the eye.
Light Without a Visible Source
During the debriefing, the crew discussed a phenomenon described as flashes of light. Two members said they had seen them at different points during the mission. According to the summary, the flashes appeared both while the crew orbited the Moon and while the astronauts were on its surface.
High-energy particles from space are called cosmic rays. They can pass through the eye and strike the retina, the light-sensitive tissue at the back of the eye. That interaction can make someone see a flash or streak of light, even when there is no visible source nearby.
Not Necessarily Eye Damage
The word “strike” might sound like it implies damage to the eye, but the summary describes an effect that creates the sensation of light. It does not say the astronauts suffered eye injuries. Nor does it describe a specific flash or the precise moment it appeared.
The phenomenon was considered new at the time of the mission; today, it is described as a recognized biological effect. In other words, particles from space can affect the eye and make someone see light. This explanation helps account for how a flash can appear without a visible light source.
What Remains Unknown
The summary does not explain why only two of the three crew members reported flashes. It also does not establish that this explanation was confirmed in their individual cases; it presents the phenomenon as a recognized biological effect.
In orbit around the Moon and on its surface, a flash can begin inside the eye.