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Chapter 7


Comets are, indeed, the remnants of material left over from the formation of the Solar System 4.54 billion years ago. They range in size from hundreds of meters to 100 km and are aptly described as dirty snowballs, being composed of water and other ices with embedded rock-like debris and dust. It is estimated that more than a trillion of them reside in orbits many billions of miles from the Sun, far beyond the outermost planets. Occasionally, passing stars or the interaction among several comets will fling one in toward the Sun. Their orbits are thus highly elliptical -- they swing very close to the Sun and then are flung back toward the outer reaches of the Solar System. These we see only once, as their orbital periods are measured in thousands to millions of years. But in the trip through the planets on the way to the Sun, the gravitational attraction of a nearby planet can perturb the comet's orbit such that it gets trapped in the inner solar system, orbiting once every few to few hundred years. Halley's comet is perhaps the most famous example, returning as it does every 76 years (it last appeared in 1986, so you should have a chance to see it after you retire).

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Halley's Comet Halley's Comet