The nature and origin of ultra high-energy cosmic rays

The Earth is exposed to a permanent rain of cosmic particles from outer space [1]. Most of the particles are fully ionised atomic nuclei, moving with relativistic energies. The bulk of them with energies up to 1017 eV originate within our Milky Way. They are most likely accel- erated in supernova remnants. Some particles have a thousand times higher energies, i.e., around 1020 eV. This corresponds to the kinetic energy of a tennis ball with a mass of 50 g and a velocity of 90 km/h