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Comets are considered to be among the most attractive celestial bodies due to their peculiar shape and the fact that sometimes they can be seen from the Earth with the naked eye. Just as meteorites and asteroids, comets belong to the small bodies of Solar system. Their history, physical and orbital characteristics have been studied for a long time, and although many comets are well-observed, some characteristics of these bodies remain unknown.

While most of space bodies did not move, or followed a trajectory, comets appeared unpredictably from nowhere which filled ancient people with horror. The word “comet” itself means “hairy star”. The earliest records of comets date from 240 BC, and the earliest comet ever noticed was called Comet Halley. In the period when scientific knowledge was poor, all the inexplicable phenomena were regarded as mysticism. Moreover, people often blamed comets for natural disasters, accidents, death, diseases etc. Finally, appearance of a comet could be interpreted as an omen of Armageddon. However, hundreds of years ago, there were people who believed that comets were purely natural phenomena which did not influence life on Earth in any of the ways mentioned above and who said that comets should have their own place in the classification of celestial bodies, because they had their own trajectory and did not die out. Up until 1577, people thought that comets existed in the atmosphere of the Earth, but Tycho Brahe concluded that they travelled beyond it. Later, there were many other scientists, each of whom was closer to understanding of what a comet was.

Nowadays, everybody knows that a comet is “an icy body that releases gas or dust”. These bodies consist of multiple elements, such as ammonia, ice, carbon dioxide and others and they are sometimes referred to as “dirty snowballs”. Just because they have water in their chemical composition, some scientists claim that this might have been a comet which started life on our planet. Comets’ orbits are situated around the Sun, but there is a hypothesis that some of them go far beyond Pluto. Comets can only be seen when they are close to the Sun, because they have complex orbits.

According to Arnet, active comets consist of such parts as nucleus, coma, hydrogen cloud, dust tail and ion tail. The nucleus consists of ice, dust and other organic materials. When a comet approaches the Sun, the ice in it starts melting, therefore, coma is formed. The most distinctive component of a comet’s structure — a tail — appears because of the radiation of the Sun, its light and wind, thus the tail points away from the star. The shape of the tail depends on the nature of the particles and how they are influenced by gravity. Some comets may have more than one tail.

As to the parameters of the comets, their nucleus measure up to 10 miles, while their comas “can reach nearly 1 million miles (1.6 million kilometers) wide, and some have tails reaching 100 million miles (160 million kilometers) long”. Despite the fact that, due to their small sizes, most comets cannot be seen from our planet without telescopes, some of them reflect the sunlight, so people can observe them with the naked eye. The extent to which a comet “shines” also depends on its age. This means that younger comets melt faster than the older ones which accumulate refractory particles on their surface. So, after a comet completes the orbit many times, volatile compounds cover its surface preventing the ice from evaporating. Such surface might have holes and cracks on it. Generally, a life cycle of a comet consists of such periods as departure, extinction, breakup and collision. So, having lost all its ice, a comet turns into a body similar to asteroids or break up into dust. It is also worth mentioning that some comets cause meteor showers, because they leave debris. An example of this could be the Perseid meteor shower. Moreover, some comets have the orbits which interfere with other planets or their satellites, and many craters resulted from this process.

Comets are also classified according to their orbital characteristics. Based on the duration of an orbit around the Sun, they are short-period, long-period and single-apparition comets. The first two types need less and more than 200 hundred years respectively to complete the orbit. Unlike planets and most asteroids which follow the same elliptical trajectories, thus enabling people to predict them, comets also revolve around the Sun, following oblong elliptical orbits. Still, they cannot follow precise conic sections, because there is the gravitation of the planets of the solar system which obviously influences the way a comet moves. An actual way of a comet is winding and it is possible to calculate the orbit only approximately.

Comets are usually named after those who discover them. The most famous comet is Halley’s Comet, which travels the solar system every 75-76 years. The end of the twentieth century was marked by the considerable interest in these celestial bodies. In 1995 all the astronomers concentrated on Shoemaker Levy Comet which broke into many pieces. They fell on Jupiter causing great disturbance in its atmosphere. In 1996, there was Hyakutake Comet, one of the brightest objects in the sky for a few weeks. A special feature of the comet was the inclination of its orbit towards plane of the ecliptic which enabled scientists to study it in detail. Other famous comets are Lexell’s Comet, the Eclipse Comet of 1948 which was discovered by accident, the great January Comet and others.

So, comets are celestial bodies consisting of ice, dust and many other components. They have their own structure, life cycle and quite a complex trajectory which have been studied for centuries. Many comets are well observed by modern astronomers, but there are plenty more which are yet to be studied.

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