Comets

Comets, like asteroids, are small celestial bodies that orbit the Sun. However, unlike asteroids, comets are composed primarily of frozen ammonia, methane or water, and contain only small amounts of rocky material. As a result of this composition comets have been given the nickname of "dirty snowballs."

## Overview

Some comets have highly elliptical orbits that bring them relatively close to the Sun. As these comets near the Sun the ices found within them melt and brilliant features are formed. Below is a list of these features.

* *Nucleus*: the nucleus of a comet is composed of ice and rocky material. The nucleus of most comets ranges from about 10 to 100 km in diameter, though they can be as large as 100 km in diameter.
* *Coma*: the cloud of gases that forms around the nucleus as the coma is heated is know as the coma. These gases are usually a mixture of water vapor, ammonia, carbon dioxide.
* *Dust Tail*: the dust tail of a comet is composed of gases and tiny dust particles blown away from the nucleus as the comet is heated. The dust tail is the most visible part of a comet.
* *Ion Tail*: the ion tail is a stream of ionized gases that are blown directly away from the Sun as a result of the comet’s contact with the solar wind.

Comets have been seen by humankind for millenia. However, scientists have determined that given the “shedding” of material each time a comet orbits closely to the Sun, a comet’s lifespan may only be thousands of years – relatively small number when compared to the lifespan of the Solar System. Thus scientists have concluded that if comets are still present in the Solar System today, then there must be a nursery of comets somewhere in the Solar System, otherwise all of the comets would have run the course of their lives long ago. Furthermore, since we have determined almost all of the inner Solar System objects the only place left for this reservoir of new comets to exist is in the outer Solar System.

Over the course of the second half of the twentieth century evidence provided by the orbits of comets suggested that there is not just one region of the outer Solar System producing comets, but two. The first of these regions is what is known as the Kuiper Belt, a band of comets similar in many ways to the Asteroid Belt found in the inner Solar System. Comets originating in this region have relatively short orbital periods and orbit the Sun in roughly the same plane as do the planets. The second region, called the **Oort Cloud**, is a region farther out than the Kuiper Belt and is essentially a spherical shell. Those comets coming from this area have orbital periods much longer than those of the Kuiper Belt. Additionally, the orbital plane of comets from the Oort Cloud may be drastically inclined to that of the orbital plane of the planets.

As for how a comet originating in one of these regions acquires an orbit that brings it into the inner Solar System, current theories propose that it may either be knocked into its new orbit through a collisions with another comet or its orbit may be altered by the gravitational force of an interstellar object passing nearby in the outer Solar System.

There are several famous comets, with some being known to ancient humans and some only recently appearing. Below are the most well known comets.

* + **Halley’s Comet**. In 1705, while studying the orbits of several known comets, Edmond Halley found that the comet observed in 1531, 1607 and 1682 was one in the same. As a result of Halley’s discovery, the comet was named after him. Halley’s Comet is visible every 75 to 76 years.
	+ **Comet Hale-Bopp**. Comet Hale-Bopp is best known to most people for the much publicized Californian cult who believed the comet to be a spacecraft. Comet Hale-Bopp was last visible in 1997 and will not be visible again for approximately 2,300 years. It is named after its co-discoverers, Alan Hale and Thomas Bopp.
	+ **Comet Shoemaker-Levy 9**. Shoemaker-Levy 9, also known as SL 9, was a string of comets that was captured by the gravity of Jupiter and subsequently began an orbit about the planet. The orbit of SL 9 assumed about Jupiter was, however, highly irregular and as a result of this irregularity SL 9 crashed into Jupiter in a brilliant display during the week of July 16, 1994. Shoemake-Levy 9 is named after its discoverers, Gene Shoemaker, Carolyon Shoemaker and David Levy.