Cosmic rays are energetic particles originating from outer space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are simple protons, with nearly 10% being helium nuclei (alpha particles), and slightly under 1% are heavier elements, electrons (beta particles), or gamma ray photons.
The variety of particle energies reflects the wide variety of sources – the Sun, black holes, supernovae... Cosmic rays can have energies of over
several tens of joules, energies the equivalent of the kinetic energy of a tennis ball in flight, far higher than the
to
that particle accelerators can produce.
A cosmic ray enters the earth's atmosphere in the diagram below. It generates a cascade of exotic particles which may reach all the way to the ground.
