Cosmic ray

Cosmic rays are energetic particles originating from space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, about 9% are helium nuclei (alpha particles) and about 1% are electrons. Note that the term "ray" is a misnomer, as cosmic ray particles arrive individually, not in the form of a ray or beam of particles. See wave-particle duality.

The kinetic energies of cosmic ray particles span over fourteen orders of magnitude, with the flux of cosmic rays on Earth's surface falling approximately as the inverse-cube of the energy. The wide variety of particle energies reflects the wide variety of sources. Cosmic rays originate from energetic processes on the Sun all the way to the farthest reaches of the visible universe. Cosmic rays can have energies of over 1020 eV, far higher than the 1012 to 1013 eV that man-made particle accelerators can produce. (The article on Ultra-high-energy cosmic rays describes the detection of a single particle with an energy of about 50 J, the same as a well-hit tennis ball at 42 m/s.) There has been interest in investigating cosmic rays of even greater energies.