Electrons and the nuclei of atoms—largely hydrogen—that impinge upon Earth from all directions of space with nearly the speed of light. Before they enter the atmosphere they are typically referred to as primary cosmic rays, to distinguish them from the particles generated by their interaction with the terrestrial atmosphere. Secondary cosmic rays, comprising a large variety of species of charged and neutral particles, cascade down through the atmosphere all the way to the ground and below. Study of cosmic rays at high energy now is often referred to as particle astrophysics.