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Comets are only weakly held together, like fluffy uncompacted snowballs. Small stresses, such as those due to heating during close passages to the Sun or from the tidal perturbation of a nearby planet, are often enough to break them apart. The most famous example of a shattered comet was Shoemaker-Levy 9 which split into twenty-odd fragments after a close approach to Jupiter in 1992. The fragments all inherit the speed of the parent comet and therefore move in unison like a flock of migrating geese.