Comet Puzzles

Abstract

Martin Duncan can watch them for hours. Little dots wriggling around on the computer screen. In the middle is the Sun. The four larger dots circling it are the giant planets: Jupiter, Saturn, Uranus, and Neptune. All of the others, smaller ones beyond the orbit of Neptune, are comets. They move around the center in a slow procession, like Muslim pilgrims around the Kaaba during the Hajj. But every now and again, one of them will fall toward the center, its orbit disturbed for some reason or another. Once a comet comes adrift in this way, sooner or later it will cross the orbit of one of the giant planets and receive another jolt from its gravity. Eventually it will end up in a small, elliptical orbit, which will take it through the inner solar system on each circuit. It will then pass close to the Earth, where people will gaze in wonder at the impressive sight of a ‘star with a tail.’