The nature and origin of ultra high-energy cosmic rays

The Earth is exposed to a permanent rain of cosmic particles from outer space. Most of the particles are fully ionised atomic nuclei, moving with relativistic energies. The bulk of them with energies up to 10^17 eV originate within our Milky Way. They are most likely accelerated in supernova remnants. Some particles have a thousand times higher energies, i.e., around 10^20 eV. This corresponds to the kinetic energy of a tennis ball with a mass of 50 g and a velocity of 90 km/h.