Origin of energetic space particles pinpointed

A source of ultra-high-energy cosmic rays has been identified for the first time. The extremely energetic particles, racing at almost the speed of light, have been traced back to a pair of colliding galaxy clusters.Cosmic rays are fast-moving particles that constantly bombard the Earth. Some come from the Sun, whereas higher-energy rays are accelerated around the remnants of supernovae.But ultra-high-energy cosmic rays are at least a thousand times more energetic still, and extremely rare. Only one particle is expected to hit each square kilometre of Earth every century. Physicists already know that these particles are almost certainly protons whose energies are measured in exaelectronvolts (1018 eV) - the amount of energy that an electron acquires when it is accelerated by a billion billion volts. Each proton has a kinetic energy similar to that of a flying golfball, and travels at just one part in 1022 slower than the speed of light.