A comet is a celestial body whose name is derived from the Greek word “aster kmetes,” meaning long-haired or hairy. Sometimes called cosmic snowballs, most comets are approximately the size of a small town. However, it’s not uncommon for comets to be as large as a planet. Comets revolve around the Sun, as do planets and other types of celestial bodies; however, comet orbits have a distinction that has been studied for decades.
The general structure of a comet consists of a nucleus, coma, ion tail and dust tail. The nucleus serves as the “head” of the comet, and is primarily composed of gas and ice. Dust, rock and other materials within the nucleus help contribute to its solid, stable properties. While the entire body of a comet can be massive, the nucleus alone is small, many times measuring less than 10 miles in diameter. The coma is a dense cloud of gases and water that materializes from the nucleus and surrounds it, as the comet draws closer to the Sun. The coma is produced by the Sun’s heat reacting upon the ice and gases within the nucleus. Some of the coma’s gases include carbon dioxide, ammonia and carbon monoxide. The ion tail, also called the plasma tail or dust tail, of a comet is formed as solar wind and pressure from sunlight blows materials from the coma. This creates extensions, or tails, that are millions of miles long. A comet’s ion tail is composed of ionized gas, also known as plasma, and its dust tail consists of a variety of particles.
In 1577, Danish astronomer Tycho Brahe was the first to infer that comets did not orbit the Sun in a circular path, but rather, an elongated one. This was ultimately a conclusion more than a century later. Today, scientists analyze the orbit of celestial bodies by measuring their “eccentricity” – meaning their deviation from a circular path. While it’s been determined that almost all objects in our solar system have varying degrees of eccentricity, comets are known to be highly eccentric. A comet’s orbit is elongated or elliptical to such a degree that most comets travel well beyond Pluto’s orbit and are only seen once per millennium. Comets whose orbits are within Pluto’s range are called intermediate-period and short-period comets, ahd they are seen within every 200 years – as is the case with Halley’s Comet. Also, some comets are believed to have parabolic orbits. Unlike eccentric elliptical orbits that are closed-ended, a parabolic orbit has the appearance of the letter “V” with a curved endpoint rather than a pointed one.
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To date, the origin of comets is still unknown. While they were originally thought to have been derived from outside of our solar system; modern experts theorize that they were formed during the solar system’s development and are members of it. The Kuiper Belt theory is based on that premise.
The Kuiper Belt
In 1951, astronomer Gerard Kuiper theorized that a population of celestial bodies could be found beyond Neptune. He stated that this population was situated in the formation of a disc-like belt, and it included comets that orbited the sun. This theory was confirmed in 1992 by the discovery of the first of thousands of comets and others objects in what is now called Kuiper’s Belt.