The study of planetary orbits doesn’t have the same glamour as other aspects of planetary sciences. It provides no pretty pictures of Jovian clouds or Martian sunsets. The only science-fiction movies that mention orbital dynamics (usually getting it wrong) are disaster flicks about impending killer asteroids.

But if asteroids can follow killer orbits, might the planets themselves ever run into each other? That question never arose in medieval cosmology, where Earth sat immobile at the bottom of the chain of Creation and the planets were points of quintessence moving in an unchanging perfect heaven. Then the Copernican idea of Earth as one planet among many orbiting the Sun led to a whole new set of puzzles to solve.