Spallation is a process whereby material is released from a body because of impact or stress. In impact mechanics, it is the release of material from a target during impact by a projectile. In planetary physics, it is the impact of a meteoroid on the surface of a planet and the effects of solar winds and cosmic rays on the atmosphere and surface of planets. In anthropology, spallation is a process used to make stone tools by knapping. In nuclear physics, spallation is the process in which a heavy nucleus releases many nucleons because it has been hit by a high-energy particle, thereby reducing its atomic weight.