Cosmic rays can loosely be defined as radiation consisting of energetic particles originating outside of the Earth. Cosmic rays are composed mainly of ionized nuclei, roughly 87% protons, 12% helium nuclei and most of the rest being made up of heavier nuclei. Electrons, gamma rays, and very high energy neutrinos also make up a much smaller fraction of the cosmic radiation.
The kinetic energies of cosmic ray particles span over fourteen orders of magnitude, with the flux of cosmic rays on the Earth's surface falling approximately as the inverse cube of the energy. The wide variety of particle energies is reflected in 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 up to 1020 eV. There has been interest in investigating cosmic rays of even greater energies.