Cosmic rays are energetic particles (mainly protons) that constantly bombard the Earth’s atmosphere from outer space. They were discovered more than a century ago, but their origin is still debated. This is because cosmic rays are strongly deflected by interstellar magnetic fields, which makes their particle distribution function almost perfectly isotropic. This implies that the observed arrival direction of a cosmic ray particle does not point to the site of its production, i.e. cosmic ray astronomy is not feasible.
