Cosmic Rays (ionized atomic nuclei) are the only matter from beyond our solar system or even from extragalactic space, that we can directly investigate. Up to energies of 10^17 eV they most likely originate in our Galaxy. The highest-energy cosmic rays (> 10^18eV) cannot be magnetically bound any more to the Galaxy and are most likely of extragalactic origin.
The pure existence of these particles raises the question about their origin – how and where are they accelerated? How do they propagate through the universe and interact? How can we directly probe extragalactic matter, locate its origin, and understand its physics processes?
An important key to understand the origin of cosmic rays is to measure the particle species (atomic mass). A precise mass measurement will allow discriminating astrophysical models and will clarify the reason for the observed suppression of the cosmic-ray flux at the highest energies, namely the maximum energy of the accelerators and/or energy losses during propagation, and/or even unthought causes.
