Comet

This article is about the astronomical object. For other uses, see Comet (disambiguation).

A comet is an icy small Solar System body or interstellar object. When it passes close to the Sun, it warms up and starts to release gases. This process is called outgassing. The gas forms a large, loose cloud called a coma around the comet's center. Sometimes a tail of gas and dust is also blown out from the coma. These effects happen because of solar radiation and the solar wind acting on the comet's nucleus. Comet nuclei range from a few hundred meters to tens of kilometers across. They are made of loose mixtures of ice, dust, and small rocky pieces. The coma may be up to 15 times as wide as Earth. The tail may stretch beyond one astronomical unit. A comet that is close and bright enough can be seen from Earth without a telescope. It can stretch across up to 30° of the sky. People in many cultures and religions have watched and recorded comets since ancient times.

Comets usually travel in very stretched-out, eccentric oval paths called orbits. Their orbital periods vary a lot, from several years to possibly several million years. Short-period comets come from the Kuiper belt or its nearby scattered disc. These areas lie beyond the orbit of Neptune. Long-period comets are thought to come from the Oort cloud. The Oort cloud is a ball-shaped cloud of icy bodies that reaches from beyond the Kuiper belt about halfway to the nearest star. Long-period comets are pulled toward the Sun by gravitational perturbations from passing stars and the galactic tide. Hyperbolic comets may pass through the inner Solar System only once before they are flung out into space between the stars. The appearance of a comet is called an apparition.

Comets are different from asteroids. Comets contain materials that turn straight from solid to gas as they get close to the Sun. This creates the comet's tail. Asteroids are more like rocks and do not form a tail. Asteroids are thought to have a different origin. They formed inside the orbit of Jupiter, not in the outer Solar System. However, scientists have found main-belt comets and active centaur minor planets. These discoveries have made the distinction between asteroids and comets harder to see. In the early 21st century, scientists found some small bodies with long-period comet orbits that had features of asteroids from the inner Solar System. These were called Manx comets. They are still classed as comets, such as C/2014 S3 (PANSTARRS). Twenty-seven Manx comets were found between 2013 and 2017.

As of November 2021, there were 4,584 known comets. This is only a very small part of all the comets that could exist, because the Oort cloud is thought to hold about one trillion comet-like bodies. About one comet per year can be seen with the naked eye, though many of them are faint and not very impressive. Especially bright comets are called "great comets". Some comets have been visited by spacecraft without people on board. NASA's Deep Impact made a crater on Comet Tempel 1 to study its inside. The European Space Agency's Rosetta was the first spacecraft to land a robot on a comet.

Etymology

The word comet comes from the Old English cometa. That word came from the Latin comēta or comētēs. Latin took it from the Greek word κομήτης, which means 'wearing long hair'. The Oxford English Dictionary notes that in Greek, the phrase (ἀστὴρ) κομήτης already meant 'long-haired star, comet'. The Greek word comes from κομᾶν (koman), meaning 'to wear the hair long'. That word comes from κόμη (komē), meaning 'the hair of the head'. The word was also used to mean 'the tail of a comet'.

The astronomical symbol for comets is a small disc with three hairlike lines. It is shown in Unicode as U+2604 ☄ COMET.

Physical characteristics

The solid center of a comet is called the nucleus. It is made of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia. Because of this mix, comets are often called "dirty snowballs." After a NASA probe hit Comet 9P/Tempel 1 in 2005, some scientists said "icy dirtballs" may be a better name. The surface of the nucleus is usually dry and dusty, and it is very dark, so it soaks up heat. This heat helps drive outgassing, the process where gas and dust escape from the comet. Comet nuclei are usually a few kilometers to about 20 kilometers across. Because they are so light, they do not form round shapes under their own gravity, so they are irregular.

Coma

Main article: Coma (comet)

As a comet gets close to the Sun, the gas and dust released from the nucleus form a huge but very thin atmosphere called the coma. The push of sunlight and the solar wind then form a large tail that points away from the Sun. Most of the material in the coma is water and dust. Sometimes a comet bursts with a sudden release of gas and dust, and its coma can grow very large for a while.

Bow shock

As a comet nears the Sun, UV light breaks up gases in the coma. The solar wind slows down in a large area around the comet, which forms a bow shock. Comets have no magnetic field, but they still act a little like planets that do.

Tails

A comet may have a dust tail, an ion tail, both, or neither. Sunlight turns material from the comet into gas, and both tails begin this way. Radiation pressure from the Sun pushes the dust into a gentle, curved arc. The ion tail points straight away from the Sun, following magnetic field lines.

Jets

Uneven heating can make new gases burst out through weak spots on the nucleus, like a geyser. These streams can make the nucleus spin or even split apart. In 2010, scientists found that dry ice can power jets of material flowing out of a comet nucleus.

Orbital characteristics

Main articles: List of numbered comets and List of Halley-type comets

See also: List of long-period comets, List of near-parabolic comets, and List of parabolic and hyperbolic comets

Main article: Exocomet

Most comets are small Solar System bodies that travel in long, stretched-out elliptical orbits. They come close to the Sun for part of their path, then travel far out into the outer Solar System. The point where a comet is closest to the Sun is called perihelion, and its farthest point is aphelion. Some comets are mostly pulled by the gravity of Jupiter when they are near their closest point. Comets are grouped by how far they travel from the Sun, using names based on their orbital periods. Comets with parabolic or hyperbolic paths pass the Sun once and then leave the Solar System for good. The gravity of giant planets can change their paths. Some comets are near-Earth objects (NEOs). Newly found main-belt comets move in rounder paths inside the asteroid belt.

Encke's Comet has an orbit that does not reach Jupiter's orbit, so it is called an Encke-type comet. Short-period comets, also called periodic comets, usually travel in the same direction as the planets, close to the ecliptic plane. Their farthest point often reaches the region of the outer planets, such as Neptune. The gravity of giant planets can bend their paths. Short-period comets are thought to come from the centaurs and the Kuiper belt. Long-period comets are thought to come from the distant Oort cloud, an idea that Jan Hendrik Oort proposed. Long-period comets have very stretched orbits, with eccentricity and periods of at least 200 years. Fewer than a dozen are seen each year. Jan Oort discovered key facts about them, and they help scientists learn about the Oort cloud.

Three objects, 1I/ʻOumuamua, 2I/Borisov, and 3I/ATLAS, have paths that are very open, much more than a closed orbit. This suggests they came from outside the Solar System. ʻOumuamua probably is a comet because its path changed in a way that suggests outgassing. 2I/Borisov is considered the first detected interstellar comet. Comet C/1980 E1 once had a very long orbit, but a close encounter with Jupiter changed it. Comets from interstellar space move at speeds close to nearby stars, and they have a positive specific orbital energy and velocity at infinity. Scientists think there might be about four hyperbolic comets per century within Jupiter's orbit. Comets beyond the Solar System, called exocomets, may be common in the Milky Way. The first was found around Beta Pictoris, a young A-type main-sequence star, in 1987. Astronomers use the absorption spectrum of gas from these comets. The Kepler space telescope and later the TESS telescope helped find them, with help from citizen scientists.

Effects of comets

When a comet gets hot near the Sun, its icy parts outgas and release solid pieces. These pieces are too big to be pushed away by radiation pressure and the solar wind. If Earth's orbit passes through this trail of mostly tiny, rocky grains, there may be a meteor shower. Thick trails make short, strong showers. Thin trails make longer, weaker ones. The Perseid meteor shower happens every year between 9 and 13 August, when Earth passes through the orbit of Comet Swift–Tuttle. Halley's Comet is the source of the Orionid shower in October.

Many comets and asteroids hit the early Earth. Many scientists think that comets that struck the young Earth about 4 billion years ago brought the vast quantities of water that now fill its oceans, or at least some of it. Other scientists have doubted this idea. Comets also contain polycyclic aromatic hydrocarbons. This has led people to wonder whether comets or meteorites may have brought the building blocks of life to Earth. In 2013, it was suggested that crashes between rocky and icy surfaces could create amino acids, which make up proteins, through shock synthesis.

Comet impacts may have brought a lot of water to the Moon over long periods. Some of it may have survived as lunar ice. Comet and meteoroid impacts are thought to be responsible for tektites and australites.

People in Europe once feared comets as acts of God and signs of doom, especially from 1200 to 1650 CE. After the Great Comet of 1618, Gotthard Arthusius wrote that it was a sign that the Day of Judgment was near. He listed many disasters he linked to comets. By 1700, most scholars concluded that such events happened whether or not a comet was seen.

In 1711, William Whiston used records kept by Edmond Halley to claim that the Great Comet of 1680 was responsible for the worldwide flood in the Book of Genesis. This revived fear of comets as direct threats. In 1910, scientists found the poisonous gas cyanogen in the tail of Halley's Comet. People became worried and bought gas masks and useless "anti-comet pills" and "anti-comet umbrellas."

Formation

Comets are thought to be made of material left over from the Solar Nebula, the gas cloud that also formed the planets. Scientists have proposed two ideas for how tiny dust and ice pieces joined together to make comet nuclei. In the first, pairs of small particles stick together when they crash into each other, getting bigger each time. This works for objects up to about one meter, but comet nuclei are much larger. The second idea starts the same way, but at about 1 cm, the size of pebbles, hydrodynamic streaming gathers the pebbles together as gravity pulls them in. This idea cannot explain how the ball of pebbles stays cold and loose while heat from radioactive decay and pressure from gravity act on it.

As the small particles grew, the Solar Nebula changed and spread out as the orbits of the giant planets shifted. This spreading led to groups of objects in the Kuiper belt, the Scattered disk, and the Oort cloud, as well as objects called Trojans trapped around Jupiter and Neptune. These distant regions act as cold storage. The Kuiper belt and Scattered disk, which lie around 35–100 AU away, are about 40 K. The Oort cloud, which lies beyond 3×10³ AU, is about 10 K.

Reservoirs

Comets that we can see up close have unstable orbits, so they last only a few hundred thousand years. Inactive comets spend most of their time far from the Sun, in several named areas.

Oort cloud

Main article: Oort cloud

The Oort cloud is thought to be a huge region. It starts somewhere between 2,000 and 5,000 AU from the Sun and reaches out as far as 50,000 AU. It surrounds the parts of the Solar System that lie between the Sun and the outer edge of the Kuiper Belt. Its outer part is held only weakly by the Sun. It sends long-period comets, which take a very long time to orbit, toward the inner Solar System, past the orbit of Neptune. The inner part is called the Hills cloud, after Jack G. Hills, who proposed it in 1981. It may hold many more comet nuclei than the outer part. It may slowly refill the outer cloud as its comets are used up.

Kuiper Belt

Main article: Kuiper Belt

Closer to the Sun, the Kuiper Belt is another home for inactive comets.

Fate

Most comets are eventually thrown out of the Solar System. Other comets break apart on their own, get pulled apart by the gravity of large objects, lose the icy material on their surface, or crash into big objects in the Solar System. A comet that travels fast enough may leave the Solar System. These are called hyperbolic comets. Comets from our Solar System are only known to be sent out by interacting with another object in the Solar System, such as Jupiter. For example, Comet C/1980 E1 was moved from an orbit of 7.1 million years around the Sun onto a hyperbolic path after it passed close to Jupiter in 1980. Interstellar comets such as 1I/ʻOumuamua, 2I/Borisov and 3I/ATLAS never orbited the Sun, so they did not need another object to push them out of the Solar System.

Main article: Extinct comet

Dormant or extinct comets have lost their icy surface material. Over time, most of the material inside a comet's nucleus evaporates. This leaves a small, dark lump of rock or rubble that can look like an asteroid. Many extinct comets are expected to exist in the Solar System. Some comets also break apart, which shows that their nuclei can be fragile. Comet Shoemaker–Levy 9 is a major example. It was discovered in 1993. A close pass in July 1992 broke it into pieces. In July 1994, those pieces fell into Jupiter's atmosphere. This was the first time astronomers had seen two objects in the Solar System collide. Other comets that split apart include 3D/Biela in 1846 and 73P/Schwassmann–Wachmann from 1995 to 2006. The Greek historian Ephorus wrote that a comet split apart as far back as the winter of 372–373 BCE. Scientists think comets may split because of heat stress, gas pressure inside them, or impacts. Comets 42P/Neujmin and 53P/Van Biesbroeck may be pieces of one parent comet. Comets West and Ikeya–Seki have been seen to break up near the Sun. Biela's Comet split into two pieces in 1846. Spectacular meteor showers were seen in 1872 and 1885, when the comet should have been visible. The Andromedids, a minor meteor shower, happen every November because Earth crosses the orbit of Biela's Comet. The Liller comet family, which may also come from breakups, includes C/2023 V5 (Leonard). P/2013 R3 also broke apart in 2014. Some comets come to a more dramatic end by falling into the Sun or crashing into a planet or other body.

Nomenclature

Main article: Naming of comets

The first comet linked to an astronomer was Halley's Comet. Edmond Halley found that the comets seen in 1531, 1607, and 1682 were the same comet. He correctly predicted that it would come back in 1759 by working out its path. The second and third known comets that return regularly, Encke's Comet and Biela's Comet, were named after the astronomers who worked out their paths. They were not named after the people who first saw them. Later, comets that returned regularly were usually named after the people who discovered them. Comets that appeared only once were still named by the year they were seen.

In 1995, the International Astronomical Union adopted a naming system. It is based on the way minor planets are named. It uses letters. C/ is used for comets that appear only once, and P/ is used for comets that return regularly. The letters are followed by the year and a special date code. For example, in 2019, astronomer Gennadiy Borisov saw a comet that seemed to come from outside the solar system. It was named 2I/Borisov after him.

History of study

Main article: Observational history of comets

People have noticed comets for thousands of years, as shown on ancient Chinese oracle bones. Until the 1500s, many people thought comets were bad omens. They believed comets warned of coming trouble. Aristotle was the first known scientist to build a clear theory about comets. He thought they were part of the air near Earth. Later, Seneca the Younger questioned this idea. In 1301, the Italian painter Giotto painted a comet in his work Adoration of the Magi more accurately than anyone had before. In the 1500s, Tycho Brahe and Michael Maestlin showed that comets must be outside Earth's atmosphere.

In 1705, Edmond Halley studied older comet sightings. He realized that three of them were the same comet returning. He predicted it would come back in 1758–59, and it did. It became known as Halley's Comet. Isaac Newton explained in his Principia Mathematica how comets move under gravity. In 1950, Fred Lawrence Whipple proposed that comets are icy objects with some dust and rock. This idea became known as the "dirty snowball" model. In 1986, spacecraft flew through the coma of Halley's Comet and took photographs of its nucleus. In 2014, the Rosetta probe's lander, Philae, landed on a comet's surface for the first time in history.

See also: List of comets visited by spacecraft and List of missions to comets

Classification

Astronomers can sort comets by looking at the planets that control their perihelion and aphelion, and their Tisserand orbital parameters. There are also simpler ways to sort comets.

Main article: Great comet

See also: Great Comet of 1577

About once every ten years, a comet becomes bright enough for an ordinary person to see. These are called great comets. It is very hard to predict which comets will become great, because many things can change how bright a comet is. Comet Kohoutek in 1973 had all the conditions that were expected to make it spectacular, but it was not. Comet West appeared three years later. People did not expect much from it, but it became an extremely impressive comet. The Great Comet of 1577 is a well-known example. It passed near Earth as a non-periodic comet, and many people saw it, including astronomers Tycho Brahe and Taqi ad-Din. Observations of it, together with the nova of 1572, made people question the Aristotelian idea of an unchanging universe. Comet Hyakutake appeared in 1996, followed by Hale–Bopp in 1997. The first great comet of the 21st century was C/2006 P1 (McNaught), which could be seen without telescopes in January 2007.

Main article: Sungrazing comet

A sungrazing comet passes extremely close to the Sun at perihelion. Small ones can completely evaporate, while larger ones can survive many passes. However, the strong tidal forces they feel often break them apart. About 90% of the sungrazers seen with SOHO belong to the Kreutz group. Other related groups include the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups seem to be related to Comet 96P/Machholz, which is the parent of two meteor streams, the Quadrantids and the Arietids.

Some comets have unusual features. Comet Encke orbits from outside the asteroid belt to just inside the orbit of Mercury. The 29P/Schwassmann–Wachmann comet travels in a nearly circular orbit between Jupiter and Saturn. 2060 Chiron has an unstable orbit between Saturn and Uranus. It was first classed as an asteroid until a faint coma was noticed. Comet Shoemaker–Levy 2 was also first labeled as an asteroid.

See also: § Fate of comets, and Swastika § Comet

Main article: Centaur (minor planet)

Centaurs usually have features of both asteroids and comets. Some are classed as comets, such as 60558 Echeclus and 166P/NEAT. 166P/NEAT was discovered while it had a coma, so it is classed as a comet despite its orbit. 60558 Echeclus was discovered without a coma but later became active, so it is classed as both a comet and an asteroid. One plan for Cassini involved sending it to a centaur, but NASA decided to destroy it instead.

Observation

In the outer Solar System, comets stay frozen and inactive. They are very hard or impossible to see from Earth because they are so small. Most comets are too faint to see without a telescope, but a few each decade become bright enough to see with the naked eye. Even without special equipment, an amateur astronomer can find a comet that passes close to the Sun by downloading images from satellite observatories such as SOHO.

Comets are named by the Minor Planet Center of the International Astronomical Union. Each name starts with a prefix that tells about the comet's orbit. P/ is used for a comet that goes around the Sun in 200 years or less. C/ is used for other comets. X/ means the orbit is unknown. D/ is used for a periodic comet that has disappeared. After the prefix comes the year of discovery and a letter for the half-month. This naming system began in 1995.

In popular culture

See also: Comets in fiction and Category:Fiction about impact events

Comets appear often in popular culture. For a long time in the Western world, people saw comets as signs of bad luck or big changes. Each time Halley's Comet returned, it led to many exciting stories in newspapers and books. Some famous people were born or died around the time it visited. One was writer Mark Twain, who correctly guessed that he would "go out with the comet" in 1910. Another was Eudora Welty. The singer Mary Chapin Carpenter wrote the song "Halley Came to Jackson" about Welty's life.

In the past, bright comets often scared people, who thought they were bad omens. In 1910, Earth passed through the tail of Halley's Comet. Wrong news reports made people afraid that cyanogen in the tail could poison many people. In science fiction, the impact of comets is shown as a danger that people can overcome with technology and bravery. Two examples are the 1998 movies Deep Impact and Armageddon. Some stories show comets causing huge disasters around the world, such as Lucifer's Hammer (1979). Others show comets bringing zombies, such as Night of the Comet (1984). In Jules Verne's Off on a Comet, people are stuck on a comet that travels around the Sun.