Comets are small chunks of ice and dust, hence their nickname “dirty ice balls.” The nucleus consists of ice, frozen gases (such as methane, carbon monoxide and carbon dioxide) and solid particles (including rock, dust and dirt). The nucleus is small compared to other celestial bodies, ranging from only a mile or two to dozens of miles in diameter. The nucleus of Halley’s Comet measures just 10 x 5 miles. A thin, dark crust surrounds the nucleus.
Comets are thought to originate from two areas: the Oort Cloud, a large, spherical region beyond Pluto, which may contain as many as 6 trillion comets; and the Kuiper Belt, a region extending from Neptune’s orbit to beyond Pluto. Occasionally, the influence of a passing star may disturb one of the comets, sending them hurtling toward the inner solar system and the Sun.
Until it approaches the Sun, a comet has no head or tail, and is difficult to see, because the nucleus shines only when it reflects the Sun’s light. As the comet nears the Sun, the gases are unfrozen and evaporated, and gas and dust escape through the crust, creating a hazy cloud around the nucleus. This shining, luminous cloud, called the coma, can be tens of thousands of miles in diameter. Its glow results partly from sunlight reflected by the comet’s dust particles, and partly from gases within the coma that emit their own light. The coma and the nucleus are together known as the head of the comet.
The comet’s tails form when forces from the Sun pushes dust and gas away from the comet’s head. There are two tails: the dust tail and the gas tail (also called the ion tail or plasma tail). The dust tail is curved and pale yellow, and results from particles being pushed away by the pressure of sunlight. The gas tail is blue, and is straighter and longer than the dust tail. The gas tail forms when gases in the coma become electrically charged, or ionized, when hit by the Sun’s ultraviolet light. The solar wind pushes these gases in a direction opposite the Sun. The comet’s tails always point more or less away from the Sun; when the comet is traveling toward the Sun, the tails are behind the nucleus. As the comet moves away from the Sun, the tails precede the nucleus.
As the comet retreats toward the outer solar system, the coma and tails disappear, leaving only the bare, frozen nucleus when the comet reaches Jupiter’s orbit. Some comets, called periodic comets, reappear regularly, with orbits ranging from 3 1/2 years to 151 years. Other comets approach the inner solar system only once. Because comets lose material each time they pass the Sun, even periodic comets have a limited lifespan. However, the material they shed may continue to be seen on Earth, in the form of meteor showers.
Halley's Comet
Marien, Robert
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