(Image Credits : Comet Hale-Bopp spotted somewhere in Croatia -Wikipedia)
The comets are ice rocks that make their journeys through our planetary system. A comet on arriving close to the Sun gets heated up, and gasses are released, which results in a gas atmosphere and tail around the comet.
Comets having an elliptical orbit around the sun are periodic visitors. Their period can be short or long, consisting of a few decades to a few hundred years. Halley's comet, which visits us every 76 years, is one such example and is visible with naked eyes. The comets having parabolic and hyperbolic orbits around the sun are just one time visitors, these comets come inside our planetary system only once to never appear again. The trajectory of comet ISON which visited us in 2013 was hyperbolic in nature.
(Image:The possible paths in which comets can approach sun, Credits : windows2universe.org)
From where these comets originate ?
Cloud of debris (rocks of metal and frozen volatile) is present in and around our planetary system. These regions are Asteroid belt, Kuiper belt, and Oort cloud. Comets are frozen rocks that originate from the Kuiper belt and the Oort cloud. These ice rocks present in the cloud regions get diverted due to gravitational perturbations in an elliptical, parabolic, or hyperbolic trajectory towards the sun. Leaving there host clouds, they start their journeys towards our planetary system to make a full swing around the sun.
(Image : The inner & outer solar system. Kuiper belt and Oort cloud location can be seen. Credits: NASA)
Comet which visits every 2500 years
The comet Hale-Bopp was spotted in 1995 by astronomers during its approach towards the sun. This comet on its approaching path crossed Jupiter's orbit in the year 1996, reached closest to the sun on April 1, 1997 (its perihelion), and finally crossed Jupiter's orbit on its way back in 2001. The comet had an elliptical trajectory with an orbital period of about 2500 years, which means it will not come within our planetary system for this long again. It was visible for 18 months with naked eyes in the sky in different regions and was named as Great comet of 1997.
(Animation: The path of Hale-Bopp. Earth's orbit is shown in blue and Jupiter's orbit is shown in green. The time, distance, and velocity is indicated at the top and down. Credits: NASA)
Comet which got disintegrated
Comet ISON was discovered in 2012, when it was approaching in a hyperbolic trajectory, on its maiden voyage towards the sun. In December 2013, on approaching near to the sun, the comet got fully disintegrated due to the surplus heat and tidal force of the sun. The smaller pieces were not observable with naked-eyes. This comet came to an end while passing through our planetary system.
Comet ATLAS (C/2019 Y4) was discovered in 2019 and had an elliptical orbit with orbital period of about 6000 years. In March 2020 the comet split into fragments due to the surplus pressure of its emitted gas and is not expected to get brighten. This comet did not reach naked-eye visibility. This comet's fragments, if survived would have passed the nearest point to Earth on May 23, 2020, and would have come to the perihelion (closest to the Sun) on May 31, 2020. The survived fragments will continue its journey along the path of the comet.
(Image : Fragments of Comet ATLAS spotted by Hubble Space Telescope. Credits: NASA)
Comet NEOWISE (2020)
Comet NEOWISE was discovered in March 2020 by astronomers using the NEOWISE space telescope. At the time of discovery, the comet was about 250 million km away from Earth. This comet is in a highly elliptical orbit that is almost close to parabolic, and its orbital period is estimated to be about 6800 years. The dimension of the comet is about 5km. The comet is likely to avoid any severe disintegration when passing close to the sun. The comet will get brighter as it approaches Sun, will make a full-swing around the sun, and will eventually head out of our planetary system. This comet will be visible during the month of July in the night sky just after sunset in the North-West direction. Once it disappears, it won't be back for another 6,800 years.
(Image: NEOWISE in sky from July 14-23. Credits: Sky & Telescope)
NEOWISE may be spotted with naked eyes if the sky is clear and light pollution is less. It will be closest to Earth on July 22-23, 2020. It will rise upwards in the sky every day. For more details on how to spot NEOWISE click here - NASA guide.
Comet plunging on planet
Comet Shoemaker-Levy 9 was discovered in 1993 by astronomers. The comet had an orbit oriented around Jupiter, making it the first active comet observed orbiting a planet instead of the sun. It is believed that during the years 1960-1970, Jupiter had captured the comet. Tracing back the comet's orbital motion revealed that the comet passed extremely close to Jupiter in 1992, where it had undergone some fragmentation. Studies predicted the possibility of comet's collision with the planet during its next close approach in 1994, and it happened indeed. The fragments of the comet created in 1992 continued to move along the same path as comet and eventually collided with Jupiter in 1994. The following visualization shows the fragments of Shoemaker-Levy comet falling one by one on Jupiter.
(Visualization of Shoemaker-Levy 9 comet fragments colliding with Jupiter. Credits : NASA)
Comet which may have become a moon
There exist hypotheses that, Phoebe the moon of Saturn may be celestial object captured from the Kuiper belt. The irregular surface, low density, and unusually dark surface of Phoebe are said to be consistent with this hypothesis. The similarities between comets and moons can establish such findings further, and in future few other moons associated with the planet Saturn and Jupiter can come out to be the ones captured from the Kuiper belt or Oort cloud. This makes an interesting journey by some comets to become moon and remain forever as part of our planetary system.
(Image : Pheobe the moon of saturn. Image Credit: NASA/ESA)
Why research on comet is important ?
Comet's composition: Kuiper belt and Oort cloud contain materials from the time when the solar system was formed, and these have remained without much change over these millions of years. With the help of the comets coming from these clouds, astronomers & astrophysicists try to gain insights into the early stages of our planetary system. These comets on their pass by near the sun with their spectacular lighting reveal a wealth of information on composition and structure. The type of elements present in these comets and age of comets can shed light on the formation of our solar system. To take a step further in this quest the NASA's Deep Impact spacecraft dropped a 372 kg impactor on the comet Temple-1 in 2014 and collected the data.
Comet Collision & Capture: The impacts of comet collision on our planet may have played a role in the evolution of the Earth during its earliest years. Some believe that comets brought water and a variety of organic molecules to Earth. The comet's interaction with our planetary system brings out the processes going on in an active solar system and the role of these wandering rocks. Also keeping an eye on comets approaching earth is important, because any of these may get diverted towards earth for a head-on-collision, but chances are very less and almost zero for such events.
Comets which originate from Kuiper belt and Oort cloud, make an amazing journey, some of them are periodic visitors, some are one time visitors, some end their life within our planetary system, and some gets captured to remain forever in our planetary system.
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