A cosmic ray is a charged particle, usually a proton or atomic nucleus, travelling through space at close to the speed of light. Most come from supernova remnants within our own galaxy, while the very highest-energy ones almost certainly originate outside it, from sources still not identified. The most energetic ever recorded carried the kinetic energy of a well-struck tennis ball packed into a single particle. Because magnetic fields bend their paths, they arrive from scrambled directions, which makes tracing them back to their origin exceptionally hard.