Extremely energetic atomic nuclei which travel through the Universe at practically the speed of light and strike the Earth from all direction. Almost 90% of all the incoming primary cosmic rays are protons, about 9% are helium nuclei (alpha particles) and about 1% are electrons (beta minus particles). Some cosmic rays come from the Sun (mainly due to solar flares), most come from galactic supernovae, and a few with the highest energy are suspected to originate from outside the Milky Way. As for their flux, about 1 charged particle per second per cm^2 impacts the Earth. The typical kinetic energy of these particles is about 10 MeV/nucleon to several GeV/nucleon, although there are some at higher energies. In fact, the cosmic ray with the highest energy has been measured above x 10^20 eV. These ultra-high energy cosmic rays are suspected to be extragalactic, as there is no plausible mechanism of acceleration to these energies by a supernova, for example. Again, compare these energies to those of solar neutrinos that have only 0.26 MeV. Cosmic rays may be divided into primary cosmic rays and secondary cosmic rays. Their energy ranges from 10^9 to 10^20 electron-volts.