So far, about 200 explosions and at least 5 collisions in space have occured. Further explosions and collisions are very likely. The explosions are mainly caused by onboard energy sources, either due to pressure build-up in propellant tanks, battery explosions, or the ignition of hypergolic fuels. Each explosion creates thousands of small debris objects. The most prominent collision event and the first known one between two catalog objects was in 1996 between the Cerise Satellite and a fragment of an Ariane upper stage explosion.