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8.2 Comets

Comets are the most visually spectacular of the small bodies in the Solar System. When they pass close to the Earth, they appear as bright objects visible from much of the world. A comet bright enough to see with the naked eye appears in our skies, on average, about once per decade. When a comet passes through the inner Solar System near the Earth, it can be seen drifting slowly from night to night among the stars. (Writers sometimes incorrectly describe comets as "flashing across the sky" like shooting stars. But in reality, they don’t move that quickly. Instead, they hang motionless and ghostly among the stars, and can be seen to change position only from hour to hour or from night to night.) Young Tycho Brahe used his observations of a comet to show that it passed among the planets. This insight shattered the ancient Greek idea of crystalline spheres and set the stage for the Copernican revolution.

Halley's comet in 1986. Click here for original source URL.

Close up of Comet Hartley 2. This image was captured by NASA's EPOXI mission between Nov. 3 and 4, 2010, during the spacecraft's flyby of comet Hartley 2. It was captured using the spacecraft's Medium-Resolution Instrument. Click here for original source URL.

Comets have several parts. The bright, diffuse part is the comet head, or the coma. The comet tail is a fainter glow extending out from the head, pointing away from the Sun. Although we see bright comets widely discussed in the media, many people are disappointed when they actually see one, because they observe it from a city. City lights wash out the delicate glow of a comet. A drive to a dark site in the country, away from the city glare, produces a completely different impression: a bright comet’s tail may extend across twenty degrees or more of the sky, like a delicate, arcing searchlight beam.

telescope reveals a brilliant, star-like point of light at the center of the comet head. The comet nucleus at the center of this light is the only substantial, solid part of the comet, but it’s too small to be distinguished by telescopes on Earth. A typical comet nucleus is a tiny world of dirty ice, only about 1 to 20 kilometers (or a few miles) across, which is tiny compared to most planets and satellites. The visible part of the comet’s head and tail is made up of diffuse gas and dust emitted from the nucleus. A comet’s glowing head is usually larger in volume than Jupiter, but it’s only a very thin cloud of gas and dust.

As a comet nucleus moves from the cold outer solar system to the inner Solar System, sunlight warms it and causes the dirty surface ice to sublime into gas. Expansion of the gas carries it away from the nucleus, together with dislodged dust grains. As this cloud of gas and dust spreads, it runs into the thin solar wind rushing outward from the Sun. Because the solar wind is moving away from the Sun, they always carry the comet’s gas and dust in a direction away from the Sun. Solar radiation ionizes gas molecules in the tail, so they’re affected by the Sun’s magnetic field. Larger dust grains are too massive to be strongly affected. (Similarly, if you release a handful of gravel and dirt, the fine particles are blown by the wind, but large stones are not.) Thus two separate tails form, one of ionized gas, and one of larger dust particles.

Comet 17P/Holmes?with the Ion Tail. Click here for original source URL.

Comet tails often stretch more than an astronomical unit, longer than the distance from the Earth to the Sun! Caught in the solar magnetic field and solar wind, the tails stream out behind the comet as the comet approaches the Sun, but lead as the comet recedes from the Sun. They might be likened to a woman’s long hair streaming out behind her as she walks into the wind, but in front of her if she reverses direction. In fact, the word comet comes from the Latin word for “hair.” The ion tail and the dust tail are usually slightly separated, due to the different strengths of the forces acting on them and the different masses of the constituent material.