1. Nuclear and subatomic particles moving through space at high speeds; radiated from the sun and other stars.
2. Streams of ionizing radiations from space, largely of protons, alpha particles, and other atomic nuclei.
3. Very high energy nuclei moving at velocities close to that of light which are probably produced by supernova explosions.

On striking the earth's atmosphere, they produce cascades of other particles (by collision with nuclei in the atmosphere) called air showers.