Cosmic rays are extremely energetic subatomic particles that travel through outer space at nearly the speed of light. Scientists learn about deep space by studying galactic cosmic rays, which originate many light-years away (a light-year represents the distance light travels in one year). This photograph, taken in the late 1940s with a special photographic emulsion called the Kodak NT4, records a collision of a cosmic-ray particle with a particle in the film. A cosmic-ray particle produced the track that starts at the top left corner of the photograph; this particle collided with a nucleus in the center of the photograph to create a spray of subatomic particles.
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