High-energy charged particles present in space with velocities near the speed of light. Cosmic rays consist mostly of protons (90%), the remainder being alpha particles (helium nuclei, 8%) plus a few electrons and heavier nuclei. Cosmic rays have energies ranging from 10⁸ to 10²⁰ eV and probably beyond. Higher-energy cosmic rays (above about 300 MeV) come from outside the Solar System and are thought to be accelerated in interstellar shock waves created by supernovae; those with the highest energies of all are accelerated many times. The entry of galactic cosmic rays into the inner Solar System is modulated by the solar wind and the interplanetary magnetic field. The number of cosmic rays hitting Earth is largest at solar minimum. Lower-energy cosmic rays are generated intermittently by shock waves near the Sun (solar energetic particles) and at the termination shock where the solar wind slows as it encounters the local interstellar medium.

A cosmic ray from space is known as a primary cosmic ray. When such particles collide with atoms in the atmosphere they are absorbed, causing both nuclear and chemical reactions. Higher-energy cosmic rays produce secondary particles (secondary cosmic rays), particularly neutrons, that can be detected on the ground. Cosmic rays of very high energy produce cosmic-ray showers. Cosmic rays were first detected during a balloon flight in 1912 by V. F. Hess, and the term was coined in 1925 by the American physicist Robert Andrews Millikan (1868–1953).