NASA's robot spaceprobe Voyager 1, took this closeup image of the surface of Jupiter's crater scarred moon Callisto in 1979. A mysterious chain of craters is seen to extend diagonally across the image (upper left to lower right). What could cause the craters to line up in such a regular fashion? Scientists were at a loss to explain this crater chain along with several other chain like features observed on Callisto's surface. Fifteen years later, with the discovery of Comet Shoemaker-Levy 9, also known as the "string of pearls" comet, the mystery was solved. Comets whose orbits stray too close to Jupiter are torn apart by the strong gravity. When the individual pieces, strung out along the orbital path of the comet hit an object like Callisto, the sequence of impacts produces a crater chain.