Comets.

Comets. What are they? Have you ever through about is? Where do they come from? Have you ever asked yourself about it? Of course we all know that comets consist of a body and tail, but if I ask you for much more information, you probably won't for answer me. But I also didn't know much about comets before I had read some books about them. I have always been interested in astronomy, space, and especially comets. Have you ever thought why comets don't fall on our Earth? You might say that this is a regularity, that they don't fall on Earth. It's all has been provided by scientists in facts.

In this article I would like to reveal you all the secrets about comets that were closed for you. I would to give you a key that will open the door into the strange and unknown world of comets, for better understanding of this phenomenon.

Comets appear to be very large, and they are spectacularly beautiful. They are also mysterious, because nobody knows for sure where they come from. There have been several theories of their origin. One theory claimed that comets were remnants of volcano located on one of the larger planets in our solar system. Today most scientists think comets may have created from a huge cloud of particles that surrounds our entire solar system.

Comets caused great terror amongst people in ancient times. Many thought the brilliant and mysterious light of a comet to be the warning of coming disaster - a plague, a war, an earthquake, or the death of an important person. Others feared that comets would fall directly on the Earth, causing instant destruction. Even today, some people still think that comets will hit our planet, but that is not very likely, because their orbits are so far away from the Earth.

Throughout history, comets have been the subject of many stories.

Ancient Egyptians believed that comets were sky pictures of a woman with long, untidy hair. In ancient Greece, comets through to be orbiting planets with people on them. American Indians considered comets to be the spirits of stairs.

Comet Halley has its own share of interesting stories. The comet appeared in the year 1066, just before the Normans invaded England, and the English blamed it for the defeat of Kind Harold.

Today, we know that none of the ancient beliefs about comets is true, and that the appearance of comet during important historical events was mere coincidence.

So, in conclusion, I would like to stay to say that in this article I've given you, to my mind, only a little key to the first door of the comets world.

Comet in our universe.

Comet (Latin Stella cometa, "hairy star"), nebulous celestial body revolving around the Sun. A comet is characterized by a long, luminous tail, but only in the segment of the comet *s orbit when it passes closest to the Sun.

History

Appearances of large comets were regarded as atmospheric phenomena until 1577, when the Danish astronomer Tycho Brahe proved that they were celestial bodies. In the 17th century the British scientist Sir Isaac Newton demonstrated that the movements of comets are subject to the same laws that control the planets in their orbits.

Composition

A comet is generally considered to consist of a small, sharp nucleus embedded in a nebulous disk called the coma. American astronomer Fred L. Whipple proposed in 1949 that nucleus/ containing practically all the mass of the comet, is a "dirty snowball" conglomerate of ices and dust.

The head of a comet/ including the hazy coma, may exceed the planet Jupiter in size. The solid portion of most comets, however, is equivalent to only a few cubic kilometres. The dust-blackened nucleus of Halley's comet, for example, is about 15 by 4 km (about 9 by 2.5 miles) in size.

As a comet approaches the Sun, the solar heat evaporates, or sublimates, the ices so that the comet brightens enormously. It may develop a brilliant tail, sometimes extending many millions of kilometres into space. The tail is generally directed away from Sun, even as the comet recedes again. The great tails of comets are composed of simple ionised molecules, including carbon monoxide and dioxide.

As a comet recedes from the Sun, the loss of gas and accompanying dust decreases in quantity, and the tails disappear. Some of the comets with small orbits have tails so short that they are practically invisible. On the other hand, the tail of at least one comet has exceeded 320 million km in length. The variation in length of the tail, together with the closeness of approach to the Sun and the Earth, accounts for the variation in the visibility of comets. Of some 1400 comets on record, fewer than half the tails were visible to the naked eye, and fewer than 10 percent were conspicuous.

When several comets with different periods travel in nearly the same orbit, they are said to be members of a comet group. The most famous group includes the spectacular Sun-grazing comet, Ikeya-Seki, of 1965, and seven other having periods of nearly a thousand years.

Comets were once believed to come from interstellar space. Although no detailed theory of origin is generally accepted, many astronomers now believe that comets originated in the outer, colder part of the solar system from residual planetary matter in the early days of the solar system.

Comets have long been regarded by the superstitious as portents of calamity or important events. The appearance of a comet has also given rise to the fear of collision between the comet and the Earth. The Earth, in fact, has passed through the tails of occasional comets without measurable effect. The collision of the nucleus of a comet with a large city would probably destroy the city but the probability of such an event occurring is exceedingly small.

In 1992 Comet Shoemark-Levy 9 broke apart into 21 large fragments as it ventured into the strong gravitational field of the planet Jupiter. During a week-long bombardment in July 1994, the fragments crashed into Jupiter's dense atmosphere at speeds of about 210,000 km/hr. Upon impact, the tremendous kinetic energy of the comets was converted into heat through massive explosions, some resulting in fireballs larger than the Earth.