Cosmic Rays of Extreme Energies

The origin of the highest energy cosmic rays is a challenging mystery defying decades of observations. The sources of these extremely energetic particles should be among the most powerful extragalactic sources. Extremely energetic cosmic rays (EECRs) reach interaction energies orders of magnitude beyond the LHC probing the frontiers of particle physics. Possible explanations for their origin have narrowed down with the confirmation of a GZK-like spectral feature. Hints of anisotropies in the distribution of arrival directions raise hopes for observing source images directly, however, composition measurements reported by Auger suggest a surprising interpretation. A clear resolution of this mystery calls for much larger statistics at extremely high energies beyond the reach of current observatories. A future space program can achieve several orders of magnitude in additional exposure to EECRs. The first step in such a program is the Extreme Universe Space Observatory at the Japanese Experiment Module (JEM-EUSO). JEM-EUSO is a large field of view ultraviolet telescope to be deployed at the International Space Station. JEM-EUSO can increase the annual exposure to EECRs by an order of magnitude.