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what are comets?

Comets are small planets, ranging in size from baseball-sized meteors to 1/3 the size of the moon. They are the rocky and icy bodies left over from the formation of the solar system. Their average diameters usually range from 750 meters (2,460 feet) or less to about 20 kilometers (12 miles). 

A comet can best be described as a "giant dirty snowball." It is a large mass of ice, dirt and rock that orbits the Sun. As it gets nearer to the Sun, parts of it begin to melt and make a vapor trail, that makes the "tail" of the comet.

The word "comet" came to the English language through Latin cometes from the Greek word komē, meaning "hair of the head"; Aristotle first used the derivation komētēs to depict comets as "stars with hair." The astronomical symbol for comets (☄) accordingly consists of a disc with a hairlike tail.

In Simple words, Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They usually follow highly elongated paths around the Sun. Most become visible, even in telescopes, only when they get near enough to the Sun for the Sun's radiation to start subliming the volatile gases, which in turn blow away small bits of the solid material.

In other words, A chunk of frozen gasses, ice, and rocky debris that orbits the Sun. A comet nucleus is about the size of a mountain on Earth. When a comet nears the Sun, heat vaporizes the icy material producing a cloud of gaseous material surrounding the nucleus, called a coma. As the nucleus begins to disintegrate, it also produces a trail of dust or dust tail in its orbital path and a gas or ion tail pointing away from the Sun. Comet comas can extend up to a million miles from the nucleus and comet tails can be millions of miles long. There are thought to be literally trillions of comets in our solar system out past Neptune and Pluto, but only once a decade or so does one become near and bright enough to see easily without binoculars or a telescope.

In other words, Comets are small Solar System bodies that orbit the Sun and, when close enough to the Sun, exhibit a visible coma (atmosphere) or a tail — both primarily from the effects of solar radiation upon the comet's nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, measuring a few kilometres or tens of kilometres across.

Comets have a variety of different orbital periods, ranging from a few years, to hundreds of thousands of years, while some are believed to pass through the inner Solar System only once before being thrown out into interstellar space. 

Short-period comets are thought to originate in the Kuiper Belt, or associated scattered disc, which lie beyond the orbit of Neptune.

 Long-period comets are believed to originate at a very much greater distance from the Sun, in a cloud (the Oort cloud) consisting of debris left over from the condensation of the solar nebula.


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