How fast do cosmic ray particles travel?

Cosmic ray particles travel at extremely high velocities, approaching the speed of light (c). Because these particles possess significant mass, they cannot reach or exceed the speed of light, which serves as a universal cosmic speed limit according to special relativity.123

The specific velocity of a cosmic ray depends on its energy level. For instance, some cosmic rays reach energies as high as ${10}^{20}$ electron volts (eV). At this extreme energy:1
* An electron travels at a velocity that differs from the speed of light by only about 1 part in ${10}^{28}$.1
* A proton travels at a velocity that differs from the speed of light by approximately 1 part in ${10}^{22}$.1

Because these particles move at such high fractions of c, their motion must be analyzed using relativistic mechanics.1