Comets really can be described as giant snowballs whose movement is influenced by forces including the gravitational attraction of the sun. They are masses of ice, gas and dirt that measure a few kilometers in diameter at their core. The gases blasting out them also affect their orbits, which can be elliptical (egg-shaped) or parabolic (cone-shaped). A well-known example is Halley's Comet, named after the man who first calculated its orbit, Edmund Halley. He discovered that this comet passed the earth every 76 years. The next appearance of Halley's Comet to Earth dwellers will be in 2061.
The ice and gases forming the body of a comet consist of a variety of substances. These include ammonia, carbon dioxide, methane ice and water. There are three distinct parts of a comet. The nucleus, or head, is only a few kilometers in diameter. It is surrounded by a much larger area called the coma, which produces a bright light as the sun heats the dust and gas is released by the nucleus. This reaction also produces electromagnetic radiation. The coma can reach the size of a million kilometers in diameter.
The third and largest part of a comet is its tail. The size of a comet's tail can be as large as 150 million kilometers; in this solar system, a comet's tail is the largest object known to exist. There are two components in a comet's tail. The dust tail emanating from the nucleus leaves a trail that is pushed away from the sun by the pressure of its light; this gives the dust tail a slightly curved appearance. The gas tail, also called the ion tail, is produced by the sun's solar winds pushing ions (atoms or groups of atoms with a positive or negative charge) out of the comet and away from the sun. It is narrower and straighter than the dust tail.
Possibly the brightest comet ever to be seen from earth was the Hale Bopp Comet in 1997. Over a period of 18 months, Hale Bopp was visible to observers without the need for telescopes. It was named after the first two men who saw it: Alan Hale and Thomas Bopp. Hale Bopp was unique for its brightness and because scientists discovered it had a third tail, called a sodium tail. It was composed of neutral atoms, which have equal numbers of protons and electrons. Hale Bopp also was unique for containing argon, a gaseous element never before detected in a comet.