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Glossary term: Comet

Description: A comet is a small object in the Solar System consisting of a nucleus made up of different types of ice with rocky, dusty material embedded in it – a dirty snowball. Comet nucleii can range in size form a few hundred metres to tens of kilometres across. Most comets have highly elliptical orbits. When the comet approaches the Sun, some of the surface ice evaporates and is blown back by the solar wind to form the distinctive coma and tail features. We see comets due to the sunlight reflecting off the coma or tail or (for comets far from the Sun) the nucleus. Comets are classified as either "periodic" or "short-period" if their passage has been observed more than once, or their period is known to be less than 200 years, and "non-periodic" otherwise.

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Term and definition status: This term and its definition is still awaiting approval

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

Related Media

Multiple image of a comet, its tail pointing away from the horizon, form an arc in the night sky over an urban area

Neowise's metamorphosis, by Tomáš Slovinský and Petr Horálek, Slovakia

Caption: First place in the 2021 IAU OAE Astrophotography Contest, category Comets. This image uses the chronophotography technique to capture the evolution of the comet C/2020 F3 (Neowise) over time, as it became visible in the northern skies in July 2020. Orbits of comets are extremely elliptical, which means that during part of their orbit they get close to the Sun. As a comet approaches the Sun, it gets heated, releases gas and dust creating an envelope or coma around the nucleus. The solar wind and photons (particles of electromagnetic radiation) interact with the coma producing the cometary tail, which can be seen clearly in this image. The tail of a comet always points away from the Sun, and extends as much as tens of millions of kilometres. This tail has two parts: the relatively straight bluish gas (ion or plasma) tail, which is made up of charged particles interacting with the magnetic fields of the solar wind; and the whitish dust tail compose of very small dust particles that are pushed by the radiation pressure from the Sun into a curve due to their slower speeds. Two regions in the Solar System are often associated with being “storehouses” of comets: the Kuiper Belt and the Oort Cloud. Comets with periods up to about two hundred years come from the Kuiper belt, a reservoir of cometary nuclei material with a disk-like shape located beyond Neptune. Longer period comets come from the Oort cloud, another huge reservoir of icy objects, with a spherical shape surrounding the Solar System. The outer limit of the Oort Cloud is not known as yet, but it could be as much as 10 thousand times the Sun-Earth distance, or even more. Due to gravitational disturbances, some of these cometary nuclei might be ejected towards the inner regions of the Solar System, sometimes approaching the Earth, offering some of the most spectacular views of a celestial body. The image also shows some prominent constellations and asterisms like the Big and Little Dippers, and also the North (Pole) Star – Polaris.
Credit: Tomáš Slovinský and Petr Horálek/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
A comet with two tails, one is yellowish and gradually spreading away from the nucleus, the other is blue and compact

Comet C/2020F3 (Neowise) with separate dust and ion gas tails and a green glowing coma, by Dietmar Gutermuth, Germany

Caption: Second place in the 2021 IAU OAE Astrophotography Contest, category Comets. Comets have a very interesting structure comprising of four main parts: the nucleus, composed of rock, dust and frozen gases, typically spanning a few kilometres, although bigger ones have been observed; a small atmosphere of gas surrounding the nucleus (only present when the comet approaches its closest point to the Sun), called coma; and the two distinctive cometary tails (there is at times third tail). The green colour of the coma is due to carbon and nitrogen present in the coma reacting with the Sun’s ultraviolet radiation. The tail that we are mostly used to observing – dust tail and is composed of micron sized dust particles, the second tail composed of charged particles – ion or gas tail. The tails are released only when the comet approaches the Sun at a distance where the heat and radiation emanating from our star is intense enough to vaporize the frozen gases. The dust tail is curved, while the gas tail is straight and always points away from the Sun as this is carried by the solar wind - flow of charged particles emitted by the Sun. As comets are formed by leftover material, they carry with them important information about the early stages of the Solar System’s formation. This beautiful image shows the comet C/2020 F3 (Neowise), as seen from Germany in July 2020, with three of the four structures clearly visible – coma, gas, and dust tail.
Credit: Dietmar Gutermuth/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
A woman in silhouette appears to greet a comet that appears behind bands of light cloud

Hello Comet, shall we dance?, by Robert Barsa, Slovakia

Caption: Third place in the 2021 IAU OAE Astrophotography Contest, category Comets. This beautiful and poetic image taken from Slovakia in July 2020 captures the comet C/2020 F3 (Neowise). The direction of the tails of the comet provides a clue as to the position of the Sun. In the past, the appearance of a comet in the skies could be accompanied by apprehension and even fear from those who did not know what these objects really are. Through careful observations and the applications of knowledge from physics, chemistry and geology, we now understand that comets are objects left over from the earliest days when the Solar System formed. The most distinctive features of a comet are the bluish ion (gas) tail, and whitish dust tail, which can extend for tens of millions of kilometres. These distinctive features, easily observable with the unaided eye together with an understanding of the science, are no longer cause for fear, rather they help us understand the history of our Solar System, and bring awe, joy and contemplation, as portrayed in this image.
Credit: Robert Barsa/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
Image of a typical comet with a wide white tail and a second blue tail tilted 30 degrees counter-clockwise to the white tail.

Comet Hale-Bopp

Caption: Image of comet C/1995 O1 (Hale-Bopp), taken on 4 April 1997, with an exposure time of 10 minutes. The field shown is about 6.5°x6.5°. Two tails extend from the bright coma: one white-yellowish dust tail and a bluish gas tail, always pointing away from the Sun.
Credit: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria credit link
License: CC-BY-SA-3.0 Creative Comments Attribution-ShareAlike 3.0 Unported icons
This comet nucleus looks like two large, bumpy lumps joined together. A small jet of material is being blown off the nucleus

Comet 67P/Churyumov-Gerasimenko

Caption: Comet 67P/Churyumov–Gerasimenko's nucleus is a "dirty snowball" made of a mixture of frozen materials and dust. It is shaped like two large lobes: one 4.1 km × 3.3 km × 1.8 km, the other 2.6 km × 2.3 km × 1.8 km. These lobes are connected by a small bridge. When a cometary nucleus such as this nears the Sun its frozen, icy material is heated, turning into gas. This, combined with the embedded dust, provide the material for the comet's characteristic coma and tail.
Credit: ESA/Rosetta/NAVCAM credit link
License: CC-BY-SA-3.0-IGO Creative Comments Attribution-ShareAlike 3.0 IGO icons