The encounter between Comet Shoemaker-Levy 9 and the planet Jupiter was the first collision between solar system bodies ever directly observed by humans. As the comet approached Jupiter in the spring of 1994, it broke up into a long chain of fragments. Astronomers observed with amazement in July 1994 as these fragments crashed, one by one, into the gas giant’s thick atmosphere. Some of the collisions were more powerful that all of Earth’s nuclear bombs detonated at once; had those fragments struck Earth, human civilization would probably have been destroyed.