Cosmic rays are fast-moving particles from space that constantly bombard the earth from all directions. Most of the particles are either the nuclei of atoms or electrons. Of the nuclei, most are single protons – the nuclei of hydrogen atoms – but a few are much heavier, ranging up to the nuclei of lead atoms. Cosmic ray particles travel at nearly the speed of light, which means they have very high energy. Some of them, in fact, are the most energetic of any particles ever observed in nature. The highest-energy cosmic rays have a hundred million times more energy than the particles produced in the world’s most powerful particle accelerator.