How does Jupiter "protect" Earth from comets and such? I've commonly heard that Jupiter protects Earth by flinging comets out of an orbit that would otherwise leave them heading for Earth. I understand that Jupiter can throw comets and other objects out of their current orbit when they pass by and that Jupiter can add energy with a gravitational slingshot sending the comets far from the center of the solar system. However, why doesn't Jupiter fling comets into an orbit that will hit Earth as often as it flings them out of orbit? Can't it also gravitationally slow down the comets as well? Why does interaction with Jupiter give a net decrease in comets headed towards Earth rather than just evening out? Thanks!