For thousands of years, human beings have been amazed by the sight of bright comets, temporary visitors to Earth's night skies. Occassionally, a comet will be bright enough to be seen in daylight hours. These comets, as well as fainter ones visible at night without optical aid, astounded and frightened our ancestors. Today we have the advantage of centuries of study with telescopes and other astronomical tools not the least of with is mathematics. Even though we have discovered much about what comets are and what makes them tick, they are still regarded warily by some individuals.Physical Characteristics
No two comets are identical, but they all share similar characteristics. Each comet has a head and a tail. The head is surrounded by a atmosphere that appears as a roundish, diffuse nebula. The head is also called a coma which includes the atmosphere of the comet. The coma is usually as large as Jupiter, but it can be larger still. Inside the coma, comet's hide their nucleus. The nucleus is the "real" comet, consisting of water ice and other substances such as carbon dioxide, carbon monoxide, methane and ammonia which are in the form of ice, until the comet comes close to the sun, when it is evaporated and added to the atmosphere and the tail. The tail is really just an extension of the comet's atmosphere. It developes only when the comet approaches the sun (estimate distance) It can stretch from the orbit of the Earth to the orbit of Mars, making it (temporarily) the largest object in the solar system. But it is very tenuous and is composed practically nothing by Earth standards. There are two types of tails. Type I is the most prominent and is the plasma tail. It is usually straight and shines with a bluish color. It actually emits light because it is ionized. Type one tails are variable, they may change hour by hour and are certainly different for each comet. They will show knots of gas or irregular brighting. Type II tails are composed of dust. They are usually curved, shorter than the plasma tail and faintly yellow. They glow by reflecting light from the sun. The comet's tail will always stretch away from the sun, no matter where the comet is. This is because the material is not left behind by the comet, but blow away from the comet by the solar wind which exerts pressure on the dust particles and gases that depart from the nucleus.Cometary Orbits
Like the Earth, comets are in orbit around the sun. They are members of our solar system. Most of them are in parabolic orbits around the sun, although a few of them are hyperboloid. This is different from the Earth and other planets which are in an elliptical orbits around the sun. Comets approach the sun briefly - closest approach to the sun of all objects is called parahelion. When objects are at their farthest distance it is called aphelion. Thus we see comets when they are close to the sun.
Periodic comets are comets whose orbit is elliptic (?), it brings them back into the inner solar system again and again. The comet with the shortest known period is comet Encke, it returns to the sun every 3.3 years. The comet with the longest period we have established is comet Rigollet which orbits in 151 years. The most famous periodic comet is comet Halley, one of the few comets not known by the name of it's discoverer, but by the astronomer who first noted its periodosity and calculated its return. It returned in 1985 and will come again after another 76 years to grace our Earthly skies in 2061.
Beam me back, Astra!