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A comet is an object made up of ice and dust that resembles a fuzzy star, and travels along a definite path through the solar system. Some of the brightest comets develop a long shining tail when they come near the sun. The tail of a comet may stream across space for as far as 160 million kilometres. A comet has a distinct centre called a nucleus, which measures less than 16 kilometres in diameter. A hazy cloud called a coma surrounds the nucleus . Its diameter may be as large as 1.6 million kilometres. The coma and the nucleus make up the comets head. Most comets travel around the sun in elliptical paths. The time it takes a comet to make a complete orbit is called its period. Some comets have short periods of less than seven years. Others travel in huge orbits that they pass near the sun only once in thousands or even millions of years. These long - term comets travel many millions of kilometres beyond the outermost planets before eventually returning to pass by the sun. All comets seen by astronomers are considered part of the solar system.
When the word "comet" is mentioned people think of bright objects with long tails. Twenty or so comets are discovered or recovered each year and it is quite rare for one to become visible to the naked eye, and even rarer for one to be easily seen. Comet Halley, was quite bright during its 1985-86 appearance, and in March 1986 had a tail which more than filled the fields of binoculars. To the naked eye, however, it was just a fuzzy star. This comet has been around since 240 BC and is expected to return in 2061-2062.
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