Comets

Comets are “celestial bodies that appear fuzzy with a head usually surrounding a bright nucleus, that has a usually highly eccentric orbit, that consists primarily of ice and dust, and that often develops one or more long tails when near the sun.” (Merriam-Webster). When we think of comets, we usually think of Halley’s Comet, but there are almost 1 trillion comets out in the deep reaches of space. Comets have been around since the creation of our solar system, and until us humans go extinct, we will see comets in the night sky every so often.

History of Comets

Comets have been around for ages, but the first visual evidence of one was in 1858 by astronomer William Usherwood. The comet that he painted was later named Comet Donati, and was later verified by fellow astronomer George Bond. Coincidentally, Bond had discovered the comet only a day after Usherwood. Edward Barnard, an American astronomer, was the first to make a photographic discovery of a comet in 1892. It was first named Comet Barnard, but later was changed to Comet Barnard/Boattini. The first spectrogram (a spectrum record on film) was of Comet Tebbutt (C/1881 K1), taken by English astronomer William Huggins on June 24, 1881.(Britannica.com). Halley’s Comet is arguably the most famous comet ever, due to its orbital period. It is visible from earth every 75-76 years, making it possible for a person to see it twice in a lifetime. Halley’s Comet was first spotted in 1758, but was not photographed until 1910. Edmond Halley, whom the comet is named after, discovered in 1705 that the sightings were not separate objects, but rather the same comet with a timed orbit. Using previous research, Halley also discovered distinct patterns that occurred within a certain time period. This helped him greatly in discovering that comets reappear every number of years on a pattern. Photos of tens of thousands of comets have been taken since the first photograph in 1858, and astronomers have been discovering new comets every year.

Types of Comets

There are 4 main types of comets, periodic comets, non-periodic comets, lost comets, and comets with no orbit.

Periodic Comets

Periodic comets, also known as short periodic comets, have orbital periods between 20-200 years. Comets like Halley’s Comet are a part of the periodic comet family.

Non-Periodic Comets

Non-periodic comets have a period of over a 1000 years and do not have enough velocity to escape our Solar System. Comets like the Great of 1680, 1807, and 1811 are a part of the non-periodic comet family.

Lost Comets

Comets with no orbit, known as hyperbolic comets, are comets from the Oort Cloud. Since the Oort Cloud is not gravitationally attached to the sun, comets can come in and out from any orientation. This means that the comet can only travel through the Solar System once. Comets like Caesar's Comet are a part of the hyperbolic comet family.

Comets With No Orbit

Lost comets are comets that were lost after discovery and have not been seen since the day they were discovered. Comets like Lexell’s Comet are a part of the lost comet family.

How are Comets Formed?

Comets are commonly suspected to be remnants of forming planets, similar to asteroids and other space debris. Unlike the rock and metal formations of asteroids, comets are primarily made up of frozen gases. Carbon, methane and ice are all major components of a comet. The large buildup of dust in the comet is also a major factor in the dirty appearance of the comet. The solar winds and pressure from the Sun creates the distinctive “tail” of the comet, melting away gases and dust to force a large trail to be left behind. That is why comets will always feature a tail that is pointing opposite to the Sun’s relative direction.

Where Do Comets Come From?

Comets come from two places, the Oort Cloud, and the Kuiper Belt. Comets from the Oort Cloud are non-periodic comets, and comets from the Kuiper Belt are short period comets.

Kuiper Belt

The Kuiper Belt, named after Gerard Kuiper, is located on the outer Solar System and is disc shaped. It extends from approximately the orbit of Neptune, to about 50 AU (Astronomical Units) from the Sun. It is very similar to the asteroid belt in terms of what it contains and how it is shaped, but is much larger. Most of what is contained in the belt is remnants from the formation of our Solar System.

Oort Cloud

The Oort Cloud, named after Jan Oort, is located about 200,00 AU from the Sun at any point in the cloud, and contains icy large objects shaped like discs. The Oort Cloud lays beyond the heliosphere and is considered to be located in interstellar space. The gravitational pull of the cloud is very easily affected because the cloud is not directly tied to our Solar System, that is why Comets often never stay within the cloud’s gravity field.

References

Comet. (n.d.). Retrieved February 03, 2021, from https://www.merriam-webster.com/dictionary/comet Halley's comet. (2021, January 22). Retrieved February 03, 2021, from https://en.wikipedia.org/wiki/Halley%27s_Comet History. (n.d.). Retrieved February 03, 2021, from https://www.britannica.com/science/comet-astronomy/History How many comets are there? (n.d.). Retrieved February 03, 2021, from https://www.esa.int/Science_Exploration/Space_Science/Rosetta/How_many_comets_are_there Temming, M. (2020, April 20). All about comets: What is a comet? Where do comets come from? Retrieved February 03, 2021, from https://skyandtelescope.org/astronomy-resources/what-is-a-comet/ Where do comets come from? (n.d.). Retrieved February 03, 2021, from https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question40.html#:~:text=Long-period%20comets%20%28those%20which%20take%20more%20than%20200,around%20the%20Sun%29%20originate%20from%20the%20Kuiper%20Belt.