Cosmic rays, or astroparticles, are a means through which astronomers can explore the Universe. These charged particles, which are mostly protons and the nuclei of atoms stripped of their electrons, travel through space at close to the speed of light. By tracing them back to their sources, scientists can learn more about the forces that have shaped the Solar System and the Milky Way galaxy at large. When cosmic rays reach Earth, most are deflected by Earth's magnetosphere, but some manage to penetrate our atmosphere and reach the surface.