Comets were previously thought of as a “dirty snowball” but due to the fact that a comet’s has more dust than ice, they are now referred to as an “icy dirtball”. The nucleus is mainly made up of H2O (water), with some parts NH3 (ammonia), CH4 (methane), and CO2 (carbon dioxide). When the Big Bang happened over 4.6 billion years ago the hydrogen and helium accreted into the first generation of stars, which quickly became supernovae during which the heavier elements (collectively called metals) were produced and in turn condensed again with hydrogen and helium to form the second generation of stars. Much of the "metals" condensed at that time to form the terrestrial planets, but some of that material became small lumps of frozen gas and dust. These lumps are located in the outer region of the solar system, where temperatures are cold enough to produce ice thus keeping the lumps frozen. That region is called the Oort Cloud or the Kuiper Belt.  The Oort Cloud is estimated to contain 1011 (one hundred billion) comets.
As the comets orbit our sun, they may be disturbed by close encounters with other objects and some of those move out of the Oort Cloud and toward the sun. As as they approach, their frozen gases become vaporized and dust is ejected and they form streamers called tails. The tails tend to flow behind the nucleus of the comet as the gases and dust are easily influenced by the forces applied to them by the sun's photons of light bouncing off them and by its electromagnetic field that acts on the charged particles. Comets have two tails: the ion tail (blue, composed of gas), and the dust tail (yellow). Meteor showers, like the Perseid or Leonids, occur when the Earth travels through a comet's dust tail.
One of the most famous comets is Halley's Comet, which was discovered by Edmond Halley in 1705. The comet had last been sighted in 1682; Halley predicted it would return in 1758. Unfortunately, he died in 1742, so he was unable to see his prediction come true. The comet was subsequently named after him.
Halley's Comet has a period of about 76 years. It came by the Earth last in 1986, and will return in 2061.
Chemical Diversity in Comets
It has been postulated that the chemical diversity observed in the population of comets found in the Kuiper Belt (region extended after Neptune orbit) as well as Oort cloud (spherical cloud lie roughly 50,000 AU from the Sun) is primordial.
The chemical abundances were observed in both classes of comets: Oort cloud comets and the Jupiter-Family of comets whose population consisting of short-period comets formed in the Kuiper belt Differences were noted between the two groups. This can be attributed to several factors including differences in the chemical and physical environments in comet-forming regions, chemical evolution during their long storage in the Oort cloud and Kuiper belt, and thermal processing by the Sun when entering the inner Solar System. The chemical composition of comets is investigated by remote sensing using spectroscopy. This investigation is indirect, since only the gas and dust of the coma can be observed after the nucleus ices have sublimated from the nucleus.
Observations were made at millimeter/sub-millimeter wavelengths with the IRAM 30-m, JCMT, CSO and SEST radio telescopes. Six Jupiter-family, three Halley-family, and fifteen long-period comets were observed from 1986 to 2001. The eight molecular species were HCN, HNC, CH3CN, CH3OH, H2CO, CO, CS, and H2S. HCN were detected in all comets, while at least two types of molecules were detected in nineteen comets. It was inferred that the HCN abundance relative to water varies from 0.08% to 0.25% from the sub-sample of comets for which contemporary H2O production rates are available. HCN is the molecule which exhibits the lowest abundance variation from comet to comet with respect to other species and was found in all the 24 Comets. HCN was found only in the four Comets and low CO abundances was measured in Jupiter-family comets, while several Oort cloud comets exhibit high CO abundances . According to the observation 23% of CO molecule with respect to water was observed in 5 comets. While 0.035% of HNC relative to water was observed in 5 comets. Fifteen comets exhibited CH3CN up to maximum 6.2%. And H2CO, H2S and CS was found in 13,11,9 comets respectively. They were found at a concentration maximum up to 1.3%, 1.5% and 0.17% respectively relative to water.
Comet C/2001 Q4 (NEAT)
Comet C/2001 Q4 NEAT was discovered in August of 2001 in the course of the Near Earth Asteroid Tracking (NEAT) program. The comet was found on Charged Coupled Device (CCD) images and the astronomers were able to confirm the discovery on August 24. The comet was described as a round nebulosity measuring about 8 arc seconds across at the distance of 10.1 AU from the Sun. The total visual magnitude was 20.0. Its perihelion date was 2004 May 26 and perihelion distance was 1.00 AU. It has an unusual, almost perpendicular retrograde orbit which brings it into the inner solar system by a deeply southward path. It initially emerged from its remote home where it spent most of its time near the south celestial pole. The comet brightened slowly during the remainder of 2001 and throughout 2002. It appeared fainter than magnitude 15 at the beginning of the 2003, but it steadily brightened as the year progressed, to magnitude 12 around mid-September. In September amateur astronomers could easily mark the time of comet in the Southern Hemisphere began supplying regular visual observations of this comet, at which time the diameter of the coma was typically measured as 0.6 to 1.2 arc minutes. In CCD images amateurs began observing a short fan-shaped tail pointing northward in July as on its approached to the Sun. In late September, the tail extended about 0.8 arc minutes toward the northwest. As 2003 came to an end, the comet became slightly brighter than magnitude 10, with a coma diameter of about 2 arc minutes. A faint, fan-shaped tail extended about 4 arc minutes toward the east. Finally, Comet C/2001 Q4 (NEAT) entered the inner solar system in 2004, reaching perihelion of 1 AU. It's peak magnitude of 3.3 rendered it clearly visible to the naked eye. C/2001 Q4 is referred to as an Oort cloud comet as it originated from Oort cloud. It is composed of HCN, CO, CH3OH, H2S and other species.
We know how a comet is formed because of the recent discoveries made in 2009 by NASA’s Deep Impact which impacted Comet Tempel 1 in 2005 that revealed what exactly a comet’s nucleus is made of and its structure.
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