Cosmic Rays with energies above ~1018 eV are referred to as "Ultra-High Energy Cosmic Rays" (UHECR). These are microscopic particles with a macroscopic amount of energy-about a joule (for comparison: an electron-volt is about 1.6x10-19 joules) or more. The existence of such energetic particles remains a mystery. The three main questions concerning UHECR's are:
To study the acceleration mechanism, one must make careful measurements of the energy spectrum of UHE cosmic rays to compare to the predictions from different acceleration models. To understand where the UHE cosmic rays come from, one needs to make a careful survey of the arrival directions, and search for both small- and large-scale anisotropies in their distribution.
Composition one of the most difficult measurements because UHE cosmic rays cannot be detected directly using conventional particle detectors (see Detection of Ultra-High Energy Cosmic Ray). Consequently, the composition must be inferred from auxiliary measurements. These are discussed in the other sections.