Astronauts can sometimes perceive brief flashes in darkness because energetic cosmic particles pass through the eye or nearby visual tissue and stimulate the nervous system. Apollo crews reported the phenomenon, and NASA experiments investigated its connection to cosmic radiation. It is a visual sensation produced inside the observer, not a flash from a nearby object.
A light sensation without visible light
On Earth, the atmosphere and magnetic field shield us from much cosmic radiation. In space, high-energy particles can reach spacecraft crews more readily. An energetic particle passing through the eye may create a phosphene—a sensation of light without light entering from the outside. Some Apollo astronauts described flashes when their eyes were closed, supporting an internal trigger. [1]
What Apollo experiments found
NASA studied the reports on several lunar missions, including with detectors worn by crew members. Investigators connected the flashes with cosmic-ray nuclei interacting with the visual apparatus. The precise mechanism may involve direct stimulation or secondary particles created in tissue; a simple single explanation for every flash is not established. [2]
Why scientists still care
The phenomenon is both a clue about how radiation interacts with the body and a reminder that spaceflight exposes crews to a different radiation environment. Modern missions use shielding, dosimeters and mission planning to manage exposure. These flashes are not the same as the familiar visual specks people may see after pressing on their eyes. Read about why space looks black for another question about what astronauts see.
Does every astronaut see flashes?
No. Reports vary, and the flashes have been documented particularly in Apollo-era experiments; they are not a constant experience for every crew member.
Sources and evidence
- NASA — Seeing Cosmic Rays in Space ↗
NASA describes astronaut reports and the Apollo light-flash detector experiments.
- NASA Technical Reports Server — Light Flash Phenomenon ↗
Apollo-era investigation connecting observed flashes with cosmic nuclei and the visual apparatus.
Sources consulted: 11 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Initial publication; the explanation was checked against the linked institutional and research sources.