In general, spallation is a process in which fragments of material (spall) are ejected from a body due to impact or stress. In nuclear physics, it's the process in which a heavy nucleus emits a large number of nucleons as a result of being hit by a high-energy particle, thus greatly reducing its atomic weight. In the context of impact physics it describes ejection or vaporization of material from a target during impact by a projectile. In planetary physics, spallation describes meteoritic impact… (More on Spallation)