A comet is a small celestial body made of dust, rocks and ice with an elliptical orbit that passes near the Sun.
The word comet comes from the Greek word kometes and means long-haired. When seen from Earth, a comet is made up of two main parts: the head and the tail. The head is composed of a nucleus and coma, often called halo. The tail consists of a dust tail and an ion tail, also called a plasma tail.
- The head contains a nucleus of ice and rocks. When the comet approaches the Sun, the surface layer surrounding the nucleus is pulverized, releasing large amounts of gas and dust. A coma, or halo, surrounds the nucleus.
- The tail of the comet appears when it approaches the Sun and solar winds interfere with the coma. There are two types of tails. The ion tail, or plasma tail, is made of ionized gases (plasma), and is pushed away by solar winds, so it is oriented away from the Sun. A dust tail is made of dust and follows the comet’s orbit.
Orbit of a comet (C) going around the Sun (A) and crossing the orbit of Pluto (B)
The comet itself does not produce light. It is visible when the gas and dust reflect sunlight. There are currently over 3 000 known comets circulating in our solar system, but they are rarely seen. They are mostly located in distant areas at the outer limits of our solar system within the Oort cloud, and for most of these comets, it takes tens of thousands of years before they pass near the Sun. Each comet has its own cycle that varies based on its orbit.
Halley’s comet passes close to the Sun approximately every 76 years, and the last time it passed was in 1986.
The Hale-Bopp comet was visible in 1997 and passes close to the Sun every 2 533 years.
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The tails are estimated to be between 30 and 80 billion kilometres long.
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A comet’s mass is estimated to be between |10^{11}| and |10^{17}\ \text{kg}|, so they are truly large celestial bodies.
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The head’s halo is composed of gases, mostly water vapour |(\text{H}_{2}\text{O}),| but also carbon monoxide |(\text{CO}),| carbon dioxide |(\text{CO}_{2}),| methane |(\text{CH}_{4}),| ammonia |(\text{NH}_{3})| and hydrogen cyanide |(\text{HCN}),| as well as expelled nucleus fragments and dust.
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The coma’s diameter is estimated to be between 50 000 and 250 million kilometres long.
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The lifespan of a comet varies, but those that fly near the Sun tend to last less than 200 years. They are believed to originate from the Kuiper Belt, an area of the solar system beyond Neptune, between 30 and 50 astronomical units (au) from Earth. Other comets, with life spans longer than 200 years, originate in the Oort cloud, between 40 000 and 150 000 au from the Sun.