A type of nuclear fission that takes place when matter is bombarded by high-energy atomic particles or cosmic rays. Meteoroids, asteroids, and the Moon are all bombarded by solar particles and cosmic rays, chiefly very high-energy protons. These interact with elements in their surface rocks, producing atoms such as helium-3, neon-21, and argon-38. Spallation also changes the overall composition of high-energy galactic cosmic rays when they collide with stationary atoms of interstellar matter. New atoms of light elements such as lithium, boron, and beryllium are produced. Consequently, these elements are a million times more abundant in cosmic rays than in stars.