In July of 1994, Comet Shoemaker-Levy struck the planet Jupiter, depositing 7 *1022 joules of energy, and incidentally giving rise to a series of Hollywood movies in which our own planet is threatened by an impact by a comet or asteroid. There is evidence that such an impact caused the extinction of the dinosaurs. Left: Jupiter’s gravitational force on the near side of the comet was greater than on the far side, and this difference in force...