Comet

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A comet is a small, icy solar system body that is comprised of leftover debris from the formation of the solar system. Comets are composed of volatile ice, dust, gas and rock. These materials often outgas from the comet when it nears the Sun due to heat and solar radiation.

During this process a coma can form around the comet, causing a brilliant show of colors depending on the comet's composition.

This coma can also produce a following region of debris known as tails. Out of the two tails, one made of ionized plasma (ion tail) and one made of dust and debris (dust tail).

Comets are typically classified into two large groups and smaller, distinct groups: periodic, non-periodic: parabolic, hyperbolic, and interstellar. These classifications are based on their orbits and orbital period. Periodic comets, such as Encke, have orbits that are closer or shorter orbital periods, while non-periodic (long-period) comets have longer orbits that can take hundreds to hundreds of thousands of years to complete. Parabolic comets possess parabolic orbits, meaning they have enough speed to be ejected from the solar system. Interstellar comets originate from outside our solar system. To date, only one interstellar comet, Borisov, has been discovered.

According to the Solar and Heliospheric Observatory, there are currently 5,176 comets known (including fragments) as of June 5, 2025. This number is subject to change.

Physical characteristics

Comet ISON with the physical characteristics of a comet.

Comets possess numerous physical characteristics that distinguish them such their tails composed of decaying debris ejected from body or their coma, a sphere or halo surrounding the object that consists of particles originating from the main body.

Nucleus

103P/Hartley's nucleus as captured by a flyby.

The nucleus, or referred to as the body, is the main part of a comet. Its a solid structure that holds all of the materials. The nucleus is also a type of planetesimal, a structure that is the predecessors of planets. The nucleus originates from the Oort cloud and is ejected into the frontal solar system. It heats up rather quickly, not enough for the structure to keep itself stable, which causes the production of multiple different kinds of visible appendages, such as it's tail.

A nucleus may be composed of various materials, with rock being the most prominent. However, ice, dust, carbon dioxide, carbon monoxide, methane, and ammonia are also present on the surface. Organic compounds such as hydrogen cyanide exist both under the surface and on it. It has also been confirmed that DNA and RNA base molecular structures, (including phosphate groups, adenine, cytosine, etc), were found on comets. The structure of the nucleus is often referred to as a 'dirty snowball' or an 'amalgamation' of various materials.

Coma

The coma is the debris envelope surrounding the nucleus of a comet, formed when the comet approaches the Sun in its highly elliptical orbit. As the comet warms, parts of it sublimate, giving the comet a diffuse appearance when viewed through telescopes distinguishing it from stars. Comas are typically round when observed and are mainly very faint, being outshined by the comet nuclei and the tails.

The coma is mainly made of ice and comet dust. Water constitutes up to 85% of the volatiles that emanate from the nucleus when the comet is within 3–4.5 AU from the Sun. The coma typically forms the same way the tails do, due to a process known as outgassing, particles heated off of the surface may retain a temporary orbit around the nucleus, creating a fuzzy blur around it.

C/2011 L4 (PanSTARRS) shining its tail vibrantly in the morning sky.

Tails

The tail of a comet is the ejection of debris from the nucleus. As a comet approaches the Sun, the increasing heat causes the ices within the comet's nucleus to vaporize, releasing gas and dust. This process, known as outgassing, creates the coma. The pressure of solar radiation and the solar wind then push these materials away from the Sun, forming the comet's tail. Despite popular belief, the tail always points away from the Sun, regardless of the comet's direction of travel, due to the influence of these solar forces.

There are typically two types of tails: the ion tail and the dust tail. The ion tail, composed of charged particles like ions and electrons, is typically straight and points directly away from the Sun, fluorescing due to the influence of the solar wind. The dust tail, on the other hand, consists of larger, heavier particles that reflect sunlight. This tail is often curved, as the particles lag slightly behind the comet in its orbit. Together, these tails create the striking, glowing phenomenon visible from Earth, sometimes spanning millions of kilometers in length and providing a spectacular celestial display.

Composition

The composition of a cometary nucleus may heavily vary depending on the structure, density, size, and weight. Most nuclei have a density of '0.6g/cm³' in which makes the density rather lightweight, which also concludes that the nucleus is no more then a loosely joined dust pile. However, there is some structural integrity underneath, and that has to do with the ice. Ice composes around 20-90%, with the mean average being 80%, of the nucleus on average and is like the glue of the comet, sticking materials together. As the nucleus barrels towards the inner solar system, the comet may have several holes or caverns that expose the ice and other raw materials, causing the outgas process. Carbon Monoxide on average takes about 10% of the cometary structure, being predominant in the mantle regions of the nucleus. Frozen Carbon Dioxide is also rather present below the crust. Ammonia, Methane, and Silicates take up the remainder of the pie chart. The average weight of a cometary body is about 3×10¹⁴ kg, which is about 300 trillion kg, which can be simplified using the metric system to 300 petagrams (pg). This average will change overtime as more comets are discovered, with the true mean likely being ~100 pg off.

Great Comets

A great comet is a comet that becomes exceptionally bright and noticeable when it passes close to the Sun, making it visible to the naked eye without the aid of telescopes. These comets often possess long tails and bright comas, earning them the title "great." Typically, their brilliance results from the sublimation of ices and dust in the comet's nucleus as it gets heated by the Sun, releasing vast amounts of gas and dust into space. This interaction with solar radiation and the solar wind causes their tails to stretch out dramatically, creating a spectacular visual effect that can be observed from Earth. Great comets are relatively rare, with only a few appearing each century, but when they do, they leave a lasting impression on observers and often become the subject of scientific study, historical records, and cultural references.

Throughout history, several great comets have captured the imagination of humanity to the point of where the great title was given. One of the earliest recorded great comets is Halley's Comet, which has been observed and recorded by various cultures for over two millennia. Named after the English astronomer Edmond Halley, who predicted its return, this comet appears approximately every 76 years. Its most notable appearances include its pass in 1066, which was famously depicted in the Bayeux Tapestry, and its 1910 return, which sparked widespread public interest and even some panic due to erroneous claims about its potential impact on Earth.

Hale-Bopp, visibly showing its 2 very prominent tails.

Another great comet is the Great Comet of 1680, also known as Kirch's Comet, after the astronomer Gottfried Kirch who discovered it. This comet was remarkable for its incredible brightness and long tail, which was visible even in daylight. Its appearance amassed a lot of popularity that it was mentioned in various contemporary accounts and influenced numerous scientific observations and theories about the nature of comets.

In more recent history, Comet Hale-Bopp, which was visible in 1997, stands out as one of the most widely observed comets of the 20th century. Discovered independently by amateur astronomers Alan Hale and Thomas Bopp, it was visible to the naked eye for a record 18 months. Its impressive brightness and the presence of both a blue ion tail and a white dust tail made it prominent among the great comets. Hale-Bopp's appearance also coincided with significant media coverage and public interest, cementing its place in popular culture.

The most recent great comet is C/2024 G3 (ATLAS) which shone at a magnitude of -3.3, making it one of the brightest comets known in the 21st century. However, ATLAS is quite dim compared to the likes of other comets, such as NEOWISE and Tsuchinshan-ATLAS.

List of Great Comets

The table below lists great comets that been deemed significant enough to earn the 'great' title by a majority of the public. This can vary from bright objects to various unique objects. The multiple apparitions of Halley's Comet are also included. There may be some inaccuracies as some sources don't always take in the benefit of the doubt. This table is subject to change.

Note: Several things here may be inaccurate due to numerous reasons. Use another source to properly educate and rely on if needed.

Periodic	Non-periodic (Non-parabolic)	Non-periodic (Near-parabolic)	Parabolic	Interstellar	Sungrazer	Extinct, Destroyed	Unknown
‡	♣	♠	♦	§	†	♥	0

Name	Year	Diameter	Orbital Period (days)	Magnitude	Notes	Type Label
Aristotle's Comet	373-372 BC	Unknown	146000-182500~	Unknown	The comet is sometimes referred to as occurring during 373-372 BC instead of 372-371 BC. Seneca wrote later that the sightings of the comet coincided with the destruction of Buris and Helice, suggesting a date of 373-372 BC.	‡ ♥
Halley's Comet	87 BC	11 km	27265.5	2		‡
Caeser's Comet	44 BC	Unknown	(Parabolic)	1.5	In the 1800s a possible match was speculated which would give it a period of about 575 years. This has not been confirmed because the later observations are similarly insufficiently accurate.	♦
Halley's Comet	12 BC	11 km	27265.5	1	May have related to the biblical 'Star of Bethlehem.'	‡
Halley's Comet	66	11 km	27265.5	1		‡
Halley's Comet	141	11 km	27265.5	-1		‡
Great Comet of 178	178	Unknown	Unknown	Unknown	Only recorded by the Chinese.	0
Great Comet of 191	191	Unknown	Unknown	Unknown	Only recorded by the Chinese.	0
Halley's Comet	218	11 km	27265.5	0		‡
C/240 V1	240	Unknown	Unknown	0-1	Likely parabolic, however this is not confirmed.	0
Halley's Comet	295	11 km	27265.5	0		‡
Halley's Comet	374	11 km	27265.5	-1		‡
C/390 Q1	390	Unknown	Unknown	-1	Only recorded by the Chinese.	0
C/400 F1	400	Unknown	Unknown	0	Orbital period may be non-periodic. Unconfirmed.	0
C/442 V1	442	Unknown	Unknown	1-2	Orbital period may be non-periodic. Unconfirmed.	0
Halley's Comet	451	11 km	27265.5	0		‡
C/565 O1	565	Unknown	(Near parabolic)	0-1		♠
C/568 O1	568	Unknown	(Near parabolic)	0		♠
Halley's Comet	607	11 km	27265.5	-2	May have been brighter then such.	‡
Halley's Comet	684	11 km	27265.5	1-2		‡
Halley's Comet	760	11 km	27265.5	0		‡
C/770 K1	770	Unknown	(Near parabolic)	1-2		♠
Halley's Comet	837	11 km	27265.5	-3	Likely the brightest comet of the first millennia.	‡
Great Comet of 838	838	Unknown	Unknown	Unknown		0
Great Comet of 891	891	Unknown	Unknown	Unknown	Only recorded by the Chinese.	0
Great Comet of 905	905	Unknown	Unknown	0	Only recorded by the Chinese.	0
Halley's Comet	989	11 km	27265.5	1-2		‡
Halley's Comet	1066	11 km	27265.5	-3.5	May have been directly involved with the Battle of Hastings during the Norman conquest of England. It was also depicted in the Bayeux Tapestry.	‡
Great Comet of 1106	1106	25 km<	Unknown	1	This comet passed very close to the sun and is perhaps the progenitor of the sungrazing comets of 1882 and 1965 or that of 1843.	0 ♥†
C/1132 T1	1132	Unknown	450000<	-1		♣
Halley's Comet	1145	11 km	27265.5	0		‡
Halley's Comet	1222	11 km	27265.5	1-2	Korean observers reported the comet was visible during the daylight hours on September 9th. This has not been confirmed.	‡
C/1240 B1	1240	Unknown	(Near parabolic)	0		♠
C/1264 N1	1264	Unknown	(Near parabolic)	0	One of the longest tailed comets known.	♠
Halley's Comet	1301	11 km	27265.5	1-2		‡
Halley's Comet	1378	11 km	27265.5	1		‡
C/1402 D1	1402	Unknown	(Parabolic)	-3		♦
Halley's Comet	1456	11 km	27265.5	0		‡
C/1468 S1	1468	Unknown	(Near parabolic)	1		♠
C/1472 Y1	1472	Unknown	(Near parabolic)	-3		♠
Halley's Comet	1531	11 km	27265.5	1		‡
C/1532 Y1	1532	Unknown	(Near parabolic)	-1		♠
Comet Apian	1533	1 km	(Parabolic)	-1	One of the most observed comets known.	♦
C/1556 D1	1556	Unknown	(Near parabolic)	-2		♠
C/1577 V1	1577	Unknown	(Near parabolic)	-3.5		♠
C/1618 W1	1618	Unknown	(Parabolic)	0-1		♦
C/1664 W1	1664	Unknown	(Near-parabolic)	-1		♠
C/1665 F1	1665	Unknown	(Near-parabolic)	-1.2		♠
C/1668 W1	1668	Unknown	(Near-parabolic)	1-2		♠
Kirch's Comet	1680	Unknown	3652425	1-2		♣
Halley's Comet	1682	11 km	27265.5	0-1	This was around the same time that it was found out that this comet was also many other comets	‡
C/1686 R1	1686	Unknown	(Near-parabolic)	1-2		♠
C/1743 X1	1743	4 km	(Near-parabolic)	-3		♠
Messier (P1)	1769	Unknown	(Near-parabolic)	0		♠
C/1807 R1	1807	Unknown	625610	1-2	First ever comet found to have an elliptical orbit after Halley.	♣
C/1811 F1	1811	Unknown	2742-2974	0	Longest comet visible to the naked eye until Hale-Bopp (260 days)	♣
C/1843 D1	1843	Unknown	219000-292000	-3		♣ †
Comet Donati	1858	Unknown	634735	0-1		♣
Comet Tebbutt	1861	Unknown	148190	0		♣
Great Southern Comet of 1865	1865	Unknown	(Parabolic)	1		♦
Coggia's Comet	1874	Unknown	(Near parabolic)	0-1		♠
Great September Comet	1882	Unknown	255500	-3		♣
Comet Viscara	1901	Unknown	(Parabolic)	1		♦
Daylight Comet	1910	Unknown	3358000	1-2		♠
Halley's Comet	1910	11 km	27265.5	0-1	This return sparked widespread public interest and major panic because of claims about its potential impact on Earth. 'Anti-comet' pills and protection were sold by people attempting to take advantage of the situation.	‡
Comet Skjellerup-Maristany	1927	Unknown	(Parabolic)	1		♦
Southern Comet of 1947	1947	Unknown	(Parabolic)	-3		♦
Eclipse Comet	1948	Unknown	22810310	1-2		♠
Comet Ikeya-Seki	1965	Unknown	290175	-6.2	Probably the brightest great comet, Ikeya-Seki is the brightest of the sungrazers.	♣ †
Comet Bennett (Y1)	1970	Unknown	637655	0	Comet Bennett was the subject of multiple different experiments to prove that its outgassing depended on what materials and how much of those materials were on the surface. This was proven during multiple exposures.	♣
Comet West	1976	Unknown	(Near parabolic)	-2.4	Despite its brightness, Comet West went largely unreported in the popular media. This was partly due to the relatively disappointing display of Comet Kohoutek in 1973, which had been widely predicted to become extremely prominent	♠
Halley's Comet	1986	11 km	27265.5	-0.5		‡
Comet Hyakutake	1996	Unknown	18980000	0	Coincidentally, the Ulysses spacecraft approached within approximately 500 million km (3.3 AU; 310 million mi) of the nucleus, confirming that Hyakutake possessed the longest tail of any comet.	♠
Comet Hale–Bopp	1997	60 km	876000	-5.7	Hale-Bopp was the most widely observed comet of the 20th century, and the most observed comet of the 2nd millennia. Hale-Bopp used to be the largest comet, until Bernardinelli-Bernstein. Hale-Bopp also has the title of the longest period visible to the naked eye.	♣
Comet McNaught	2007	Unknown	2.1 billion	-5.4	The Ulysses spacecraft made an unexpected pass through the tail of the comet on 3 February 2007. The Solar Wind Ion Composition Spectrometer (SWICS) aboard Ulysses measured Comet McNaught's tail composition and detected unexpected ions. It was the first time that O3+ oxygen ions were detected near a comet.	♠
Comet Lovejoy (W3)	2011	0.1 km<	227030	-3.7	Well known for being a true sungrazer, Lovejoy passed through the Sun's corona on 16 December 2011 and miraculously surviving. It unfortunately fragmented into several pieces during its exit of the solar system, but its main fragment may have a chance of returning and spreading the show again in 27th century.	♣ †
Comet Borisov'	2019	0.5 km>	(Parabolic)	8	Although not as bright as the other comets, being at a magnitude where it would be invisible to a small telescope, Borisov was the first ever interstellar comet to be discovered, therefore, a lot of people still deem it to be great. Borisov had a tail which was 14 times the size of the Earth. Borisov was also nicknamed 'the Christmas comet.' Unfortunately, Borisov ruptured while it was exiting the solar system. The remnant main fragment, although its trajectory was slightly changed, still continues its legacy as it solemnly rushes out of the solar system.	§
Comet NEOWISE	2020	5 km~	2 million~	-5.5	Brightest comet in the northern hemisphere since Hale-Bopp.	♣
Comet Tsuchinshan-ATLAS (A3)	2024	3.2 km~	(Parabolic)	-4.9	Second brightest comet in the northern hemisphere since Hale-Bopp. Was one of three comets expected to become great, however such was the only one.	♦
Comet ATLAS (G3)	2025	1 km~	58 million~	-3.3	Third brightest comet in the 2020's. Its peak magnitude may still differ as it moves away from perihelion, with it possibly being brighter then Tsuchinshan-ATLAS.	♠

This table may have several inaccuracies and is subject to change. Please use other sources to relate to if your going to use this table as a reference.

Classifications

There are many types of comets that have been digressed by our civilization. These types can vary from its type of orbit, type of nucleus, or the characteristics its tails. The most common way of separating comets into categories is by its orbit and orbital period, using several calculations to evaluate what type of orbit it has, and how long it takes to traverse it. They also calculate where the path leads it, if it is a sungrazer or parabolic. Comets can also be separated based on its stability, whether the comet may be stable enough to traverse its orbit for thousands of years, or whether its next approach will lead it to doom. However, some of these calculations could be inaccurate when the time comes. A good example of this is C/2024 S1 (ATLAS), one of the three comets (12P/Pons-Brooks, Tsuchinshan-ATLAS (A3) and ATLAS (S1)) expected to become great, however, it fragmented and melted away, likely because it was a remnant comet, a comet which has used itself up, and as such it became extinct.

Orbital period groups

These subsections are the types of comets described by its orbital period.

Periodic

67P/Churyumov–Gerasimenko, a periodic comet that was subject of the Rosetta mission.

Periodic comets are comets that take less then 200 years to orbit, which makes them faster then the average comet (non-periodic). These comets are typically faint, due to their ion and ice shafts melting off rather quickly. They are usually designated by '(n)P/(Name)', with the 'P' meaning periodic. Halley's Comet, the most well-known, has the title for the most great appearances, with its simple yet far orbit of 76 years, meaning the body has time to regenerate some of its icy and ionic surface, causing a spectacular show every time it appears. 67P/Churyumov-Gerasimenko, a Jupiter family comet, was subject to the Rosetta mission, causing it to become one of the most known. It was also discovered that 67P had a moon, dubbed Churyumoon. 12P/Pons-Brooks is a very rapid, yet still neat comet to look out for, with its not-so bright yet still dominant light. Comets 19P/Borrelly and Encke always range close with their very short orbital periods.

Non-periodic

Non-periodic comets are comets that take more then 200 years to orbit, however they are not hyperbolic or parabolic. There are two types of non-periodic comets, non-parabolic and near-parabolic. They are typically bright, mainly because they have a lot of ions and ice to burn off during their approach. They are usually designated as 'C/(Year) (Month) (Number)', with the 'C' meaning comet, year being its year of discovery, month being a singular letter to denominate one half of a month (A is the 1st half of January, B is the 2nd half, C is the 1st half of February, etc.), and the number being the placement of when it was discovered (say 'E3' would be 1st half of May, and the 3rd comet discovered in that period). Example comets of this group are Hale-Bopp, one of the most famous comets known, Hyakutake, one of the most infamous comets known, with it having the longest tail of any comet as found by the Ulysses probe, West, known for its brightness and with it exceeding expectations, and NEOWISE, so far the brightest comet of the 21st century.

C/2023 A3 (Tsuchinshan-ATLAS) dominating the sky in the Midwest during October.

Parabolic/Hyperbolic

Parabolic and hyperbolic comets are comets that have enough speed to leave the Sun's gravity as a whole and leave the solar system. Just like the non-periodic comets, they are typically bright because of their surface. They are also designated the same way as non-periodic comets. A non-periodic or even a periodic comet may transition into a parabolic comet due to an influx of speed, or general highly eccentric orbit, which typically occurs with near-parabolic comets. As such, when such transition happens, they will be sent off flying into space where they will likely be never detected again. There have been multiple parabolic comets in history. One of them, C/2023 A3 (Tsuchinshan-ATLAS), was predominantly bright and is probably the brightest parabolic comet we have seen throughout human history. Other comets include Comet Viscara, a comet known for its unique yellow trail, The Great Southern Comet of 1947, known for becoming extremely bright, and Caeser's Comet, known for being the first parabolic comet known.

Miscellaneous

These next set of classifications are miscellaneous. All of them do not have such relation to one and another and are used universally.

Sungrazer

Sungrazers are comets that come near or come into contact with the Sun. They are typically parabolic or periodic and are brought close to the Sun for numerous reasons. Most typically don't survive and usually fragment and die, however, one bright exception, Comet Lovejoy, became a great comet before plunging into the Sun's corona. It somehow managed to survive, heavily damaged, but it was later found that it had an alcohol coating, which likely helped it breeze through the extreme temperatures.

Kreutz sungrazer

This subsection of the sungrazers are a form of sungrazers that likely formed and originated from one parent sungrazer, the Great Comet of 1106. The remnants of the Great Comet likely became smaller comets, which today are the Kreutz sungrazers.

Interstellar

Interstellar comets are extremely rare. To date, there is only 1 comet that has been designated an interstellar comet: Borisov. Interstellar comets are comets whose origin are outside the solar system. Interstellar comets and objects in general are given the (n)I/(Name) designation.

Remnant

Remnant comets are comets which are in the process of dying. This process may be caused by fragmentation, its resources being used up, and complete loss of ions and ice, the latter would make the comet a regular asteroid.

Extinct

Extinct comets are comets which have died and are no longer in the range to be assessed as a comet. This may be caused by major fragmentation, sungrazing end, its resources being used up, and complete loss of ions and ice, the latter, as mentioned above, would make the extinct comet a regular asteroid.

Lost

Lost comets are comets that have been lost. This may happen because of an unexpected extinction, or it not being able to be detected. Lost (and destroyed comets as well) are given the (n)D/(Name) designation.