50 years ago, Apollo astronauts may have witnessed lights due to cosmic rays.

Light flashes no danger for short space flights – Science News, May 12, 1973

The light flashes and streaks seen by [Apollo] astronauts have long been attributed to high-energy, heavy cosmic particles (HZE) passing through the eyes…. A new report … concludes that the particles are not a serious hazard for short trips to the moon or Earth-orbital missions such as Skylab.

Update

The mechanism behind the flashes described by Apollo astronauts remains a mystery. Perhaps the particles, which are components of c­osmic rays, emit radiation as they pass through part of the eye. Or perhaps they trick nerve cells to create the illusion of light. However the flashes happen, they are still an issue for astronauts. In 2006, about 80 percent of NASA and European Space Agency astronauts reported experiencing the flashes. How the phenomenon impacts astronaut health after months or years in deep space is unclear. As NASA plans crewed missions to Mars and the moon, scientists are devising new ways to protect astronauts from radiation (SN: 7/4/20 & 7/18/20, p. 18), such as portable magnetic shields that can deflect cosmic rays.