Spallation is a nuclear reaction in which several particles result from a collision. In astronomy, spallation typically involves the collision of high-energy cosmic rays with atoms, either in interstellar space or on the surface of objects such as meteorites, asteroids, or the Moon, that are unprotected by an atmosphere. Spallation reactions between cosmic rays and interstellar matter results in the production of fast-moving nuclei of lithium, beryllium, and boron, so that these elements are found to be a million times more common in cosmic rays than in stars.