Introduction
The word comet is derived from cometa. The outer space is filled with a great number of surprises and mysteries. The comets are also one of the great mystery. Uncountable comets are rotating around the sun and are part of the solar system. The most successful comet discovery of all the times is of Jean-Louis Pons. He claims that he had discovered almost thirty-seven comets. The second most high-volume was of William Robert Brooks. The comets discovery dates back to 1680s by Gottfried Kirch. Comets were initially explained as solid rock particles floating the space, but after in-depth substance analysis, it is confirmed that they are small icy solar system part. When it passes close to the sun, it releases gases. The process of releasing the gases is known as outgassing. This gives the comet a tail, a bright coma shown behind a fast moving comet in the sky. Scientists have discussed a lot about the comets and have predicted their behavior. The life expectancy is also elaborated which resulted in the understanding of the origin of the earth as a planet. The structure of comets is completely different from the stars, asteroids and other planets in the outer space. There are many planets revolving around the sun and other stars. The comets are outnumbered, and they are also revolving around the heavy objects as well as sun and stars. The paper is overall discussing the comets and the human discoveries about them.
Discussion
The comet is an overall composition of solid rock and its long bright tail. The rock is said to be the nuclei of the comet, and the tail is the gas released by the comet which bright. The overall mass of the comet is composed of the rocky particles, ice, and the dust. The size of the rock (nucleus) is varied from a few meters to the tens of kilometers. The tail is found to be stretched to almost fifteen times more than that of the earth diameter. Since every particle, planet, moon, star, and galaxy have an orbit to follow. Similarly, the comets also have eccentric elliptical orbits. Their orbital periods range in years, respectively longer than the earth (Masoumzadeh et al.). The assumed one rotation of a comet ranges between millions of earth years (Shustov, 64). The orbits of the comets are classified into two categories. There are comets which follow the shorter orbit, and they are beyond the Neptune orbit in the scattered disk area. The long orbited comets rotate outside the Oort cloud and before the nearest star. Their orbits are usually calculated as millions and millions of years longer.
Some comets mostly passes outside the solar system and once in millions of years and are rotating around a big star which is thousands of light years away from us. These comets are usually named as the hyperbolic comets. Some comets also pass through the solar system, and they could be observed. Sometimes when a comet passes very closely from the sun, it loses its most of the ice and dust. It becomes smaller and looks like an asteroid. Comets are differentiated from the asteroids as both have completely different behavior and the lifespan. The comets have gravitationally unbound atmosphere. It surrounds the central nucleus. There is a tail which is known as the coma. It is composed of the dust and the ice blown out of the comet (Masoumzadeh et al.). The sunlight pressure and the solar wind plasma tends the tail to form and to light it up. There is a discovery of the central belt comets which resemble asteroids and the prominent difference among the asteroids and comets is heavily blurred.
At the end of 2014, there were almost 5253 known comets. This number is currently increasing because of the astronomers and the scientists. This five thousand observed number of comets is very less, and this could not be wrong to say that it is a fraction of the overall percentage of comets available in the outer universe. The estimated bodies which are available in the solar system is one trillion. It would be hard to say that if the scientists could be able to find all or a good number in this decade because according to scientists, only one or two comets are available to be viewed by naked eyes. Others are mostly unspectacular even with highly great telescopes. The bright examples which could be observed by the naked eyes are known as the great comets. Random researches are being done on the Comets. The European Space Agency was the first to land a robotic spacecraft over a comet. NASA has blasted a comet to study its interior. The fig (1) is obtain from the international sources. Fig: 1 (Lamy)
The comet is a solid rock, dust, frozen carbon dioxide and water ice. Carbon monoxide, methane, hydrogen cyanide, formaldehyde, ethanol, and ethane and also some of the highly sophisticated long-chain hydrocarbons and amino acids. This discovery of the composition of a comet resulted in the expectation of DNA, RNA or any other biological structure which would be responsible for the life transfer from one area of the space to another. The outer surface of the comet is normally covered with the tar or crude oil.
There are comets whose radius is greater than thirty kilometers are also observed and are classified in the high comets (Faggi et al.). Their exact size, weight and body shape was difficult to identify because of a higher speed and limited technology. In the past, very rare comets are identified in the outer space but now with proper telescopes and satellites floating above in the sky resulted in the discovery of as small as 100-300 meters comets which would be passing beyond the human eye could see.
The coma of the comet is the dust and the gas which is released and creates a thin and huge atmosphere around a comet. There is a pressure of sunrays and the sun radiations which causes the increase of the length of the tail from pointing in the opposite side of the sun. Water and gas are the most prominent substances. It is found that the comets release extreme quantities of X-rays. It was a surprise to the astronomers because the X-rays were associated with only higher temperature bodies, but the comet consisted of dry ice, water ice, and low-temperature areas as well. The discovery of X-ray in comets was in 1996, and a great discussion about the formation of the comets was again started officially in which the Russian, European and NASA scientists discussed these concerns collectively. After a long time, it was confirmed that because of the interaction of the comets with the solar wind, the cometary atmosphere gets charged by the ions present in the solar wind. Both the atmosphere and the solar wind ions collide, and the stealing of the electrons from an atom or the molecules process (charge exchange) results in the x-rays and the far ultraviolet photons.
The comets present in the outer area of the solar system are mostly cold and frozen. They are impossible to detect because of their inactive nature. Moreover, their sizes are too small, and the statistics obtained from the Hubble telescope data concluded that the comets of Kuiper belt are mostly dormant and are more enormous sizes, but because of their extreme distances from the earth and light sources like sun and other stars, they are rarely seen (Shustov, 64). As a comet out of the Kuiper belt and enters the solar system, they are found to be in contact with the solar wind which results in their volatility and the water starts vaporizing and gets the stream out of the nuclei with an addition of the dust (Shanklin). The comet has two tails. The gas tail is separated from the dust tail. They are seen mostly to be completely distinct from one another. 
These tails have names as tail I and tail II. The tail I is composed of the gases, ions and is extremely opposite of the sun and the solar winds. On the other side because of the orbital movement, the dust tail, Tail II is a bit tilted. It is composed of the dust particles. This is demonstrated in the picture obtained from NASA website, Fig (2).
Fig: 2 (Lamy)
Sometimes the comets have to face the uneven heating. This heating results in creating the holes on the comet nucleus and the gases with the mixture of dust start getting out. This emission of the streams of gases results in the spin of the nucleus and sometimes even split apart. It is revealed that the dry ice which is another name for the frozen carbon dioxide is the only reason of power jets of the material which comes out in the form of dust from the nucleus of the comet. There are some infrared images which are showing such jets existing as well as carrying the dust out of the comet in the form of the coma.
Most of the comets are highly small inside the solar system. Their orbits are long and elliptical. The path they follow usually makes them closer to the sun for some time and then takes away to the distant areas of the solar system. The comets are classified concerning their orbital lengths. The longer, the orbital path would be, the elongated the ellipse would be observed.
There are short period comets which are defined as the ones who have orbital periods of less than 200 years. Their orbital behavior is mostly elliptical and in the same direction as the planets in the solar system. The comets whose existence is found close to some more significant planet, they are usually known as its family. There are some comets which are named as well upon the person who discovered them. For example, Encke’s comet has an orbit that doesn’t reach the orbit of the Jupiter, and its complete name is an Encke-type comet (Dones). The comets whose orbital period is less than 20 years and have very low inclinations which is up to 30 degrees are known as Jupiter-family comets. The comets whose orbital period is between 20 to 200 years. It is observed that their inclinations extend from zero to even more than 90 degrees. Most of them are known as Halley-type comets. There are in total 557 Jupiter family comets are discovered yet and only 89 Halley type comets discovered in 2017 statistics (Faggi et al.).
Within the asteroid belt, there are also a new group of comets being discovered of entirely different class. They orbit in a circular path and are entirely within the asteroid belt. It is predicted that some comets have to pass closer to the giant planets which result in their maintenance of always on the path. There are comets whose elliptical as well as circular paths are only developed because of the sun and the other neighboring planet’s gravitational pulls. Astronomer’s current statistics have concluded that the Jupiter family comets are among the most prominent family known.
The long period comets are the one having higher eccentric orbits and period varies between 200 years to several thousands of years. When the eccentricity of a comet is greater than 1, then it doesn’t mean that the comet will leave the solar system forever. There are many comets which were seen by the naked eye from the earth, and they will never be seen for the next thousand or several thousand years. There is a comet McNaught, who have eccentricity 1.000019. It was seen in January 2007, and it is observed that its orbit is leading it away for the next 46,300 years away from the sun.
The non-periodic or the single-apparition comets are same as the long period comets because of their hyperbolic as well as slight parabolic trajectories while they are near the perihelion in the inner solar system. The scientists have discussed and finalized that no comet with the eccentricity great than one is observed yet, but their existence is certain in many other galaxies and systems of the universe (Masoumzadeh et al.). The comet C/1980 E1 has an orbital period of round about 7.1 million years, and its eccentricity is observed as the highest ne which is 1.057. It was known as the hyperbolic comet. There are an outnumbered comets which were used to enter the solar system once, and there is no expectation that they would ever return to the solar system again. Their path is expected to be linear or an infinite diameter of trillions of lightyears.
Comets have a significant impact on life. In an early time when the earth was at its initial stages, scientists have discovered that the comets used to bombard the young earth and brought a great quantity of water that is now present and could be seen in the great oceans of the earth. Some scientists agreed on this point, but some are still doubtful. The Comets have the majority of the organic molecules which explains which explains that they could be considered as the one who brought the precursors of life or directly brought the life on the earth. Also, it is observed that since the comet has a rocky and icy surface, so they have potential to create the amino acids through which the proteins and made. In 2015, scientists found the majority of oxygen which was in the molecular form.
Many hyperbolic comets were being discovered in the last ten years whose numbers are shown in the Table (1) underneath from 2007 to 2016.
| Year | Number |
| 2007 | 12 |
| 2008 | 7 |
| 2009 | 8 |
| 2010 | 4 |
| 2011 | 13 |
| 2012 | 10 |
| 2013 | 16 |
| 2014 | 10 |
| 2015 | 15 |
| 2016 | 4 |
Table (1)
From the time of 1200 A.D. t0 1650s, the comets were considered as the act of God, and there was a great fear existed in the society. There were many scholars including Gotthard Arthusius have published some discussions about the comets and the natural disasters (Mozhenkov and Vaisberg). There was an incident in 1680 when a great comet hit the earth, and the great floods started which pour water on the earth. Poisonous gas was detected in the tail of the comet in 1910 which resulted in the people’s buying gas masks and anti-comet pill and anti-comet umbrellas.
Comets orbits are not as much sophisticated and properly managed. Due to higher in number the collisions and the complete end because of the breakups. There was a comet which was found close to Jupiter, and after six days of its discovery, it broke into pieces, and those pieces feel into the Jupiter atmosphere. Due to thermal stress, many comets split into two parts, and there are 42p/Neujmin and 53)/Van Biesbroeck were parts of a parent comet.
Similarly, because of lower pressures and movement issues, many comets fall into the giant planets like Neptune and Jupiter. Also, some even fall into the sun and other planets and eventually their end happens. Sometimes a giant comet while traveling opposite to the other comets and asteroids, collides on a regular basis and ends up because of breaking down into pieces much time (Dones). The collisions of the comets according to the scientists is a spectacular view, and the last one was seen in 1994 when a comet broke into pieces and fell into the Jupiter atmosphere. All of the collisions and falling into the sun dates back to thousands of years.
There are some unusual comets which exhibit entirely different and unusual properties. These comets don’t exist in the asteroid belt but in some other regions like between Saturn and Jupiter, Saturn and Uranus as well as close to mercury. Their orbits are unstable and continuously changes. They were initially predicted as the asteroids, but after fainted tails and movements are observed, they were finally declared as comets.
There were an outnumbered comets discovered in the early part of the century, but then they were not found again. They changed their behavior and the predicted orbits were either wrong, or because of some unnecessary changes, they left their orbit. Currently anew comet is discovered, and according to the astronomers, it is concluded that it was one of the lost comets and is found back (Shanklin). There are eighteen comets which were rediscovered and fist in the category of rediscovery and the lost comets.
The people associated the comets, their appearances and hiding as the human’s birth and death. There is a truth and is observed by the number of people that when a bright comet appeared in the sky, it caused a huge amount of people drowned in the problem of hysteria and inspired panic. This caused these comets to be bad omens. In 1997, when freshly the poisonous gas existence in the comet was discussed, a newspaper published that a comet seen right now could save poisonous millions. It triggered a massive number of people to the suicide of the heaven Gate cult. That’s why the comets are discussed by the philosophers as the causes of threats and problems to the human population.
Conclusion
In a nutshell, it can be explained that the comet are beautiful artistic pieces of the universe and are bight as stars when viewed from earth but are close to planets and asteroids. They are very rarely seen in the sky and almost one or two per year. The comets are present inside the solar system, but their orbit decides if they will last inside or will leave sooner. Comets composition is different, mostly made up of gases and dust atmosphere with a solid nucleus. There are comets which have orbits greater than even thousands of years. There were many comets which were seen in the solar system, and it is expected that they will never be seen again. They have long tails multiple time bigger than their nucleus. Sun rays and solar winds effects their tail and their overall behavior and movement are controlled by the sun’s gravity and the neighboring plants. They are considered as the pieces which initially collided the earth and brought, water, organic substances, and other life-related substances. The humans were affected by the comets in the past as well as in the present. The appearance if comets were terrible for some people.
Works Cited
Dones, Henry C. Luke, et al. “Ways of Changing the Number and Size Distribution of Ecliptic Comets.” AAS/Division of Dynamical Astronomy Meeting. Vol. 48. 2017.
Shustov, Boris, et al. “Comets in UV.” Astrophysics and Space Science 363.4 (2018): 64.
Faggi, Sara, et al. “The Volatile Composition of newly-discovered C/2017 E4 (Lovejoy) before its dissolutionas revealed by iSHELL at NASA/IRTF.” AAS/Division for Planetary Sciences Meeting Abstracts. Vol. 49. 2017.
Masoumzadeh, Nafiseh, et al. “Opposition effect on comet 67P/Churyumov-Gerasimenko using Rosetta-OSIRIS images.” Astronomy & Astrophysics 599 (2017): A11.
Mozhenkov, E. R., and O. L. Vaisberg. “On the classification of comet plasma tails.” Solar System Research 51.4 (2017): 258-270.
Shanklin, Jonathan. “The brighter comets of 2011.” Journal of the British Astronomical Association 127.6 (2017).
Lamy, P. L., et al. “The nucleus and inner coma of Comet 46P/Wirtanen.” Astronomy and Astrophysics 335 (1998): L25-L29.
