Comet
A comet is an icy, small Solar System body or interstellar object that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, and sometimes a tail of gas and dust gas blown out from the coma. These phenomena are due to the effects of solar radiation and the outstreaming solar wind plasma acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit. If sufficiently close and bright, a comet may be seen from Earth without the aid of a telescope and can subtend an arc of up to 30° across the sky. Comets have been observed and recorded since ancient times by many cultures and religions.
Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun by gravitational perturbations from passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.
Extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. However, the discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. In the early 21st century, the discovery of some minor bodies with long-period comet orbits, but characteristics of inner solar system asteroids, were called Manx comets. They are still classified as comets, such as C/2014 S3. Twenty-seven Manx comets were found from 2013 to 2017.
, there are 4,584 known comets. However, this represents a very small fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System is about one trillion. Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular. Particularly bright examples are called "great comets". Comets have been visited by uncrewed probes such as NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior, and the European Space Agency's Rosetta, which became the first to land a robotic spacecraft on a comet.
Etymology
The word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a romanization of the Greek κομήτης 'wearing long hair', and the Oxford English Dictionary notes that the term κομήτης already meant 'long-haired star, comet' in Greek. Κομήτης was derived from κομᾶν 'to wear the hair long', which was itself derived from κόμη 'the hair of the head' and was used to mean 'the tail of a comet'.The astronomical symbol for comets is, consisting of a small disc with three hairlike extensions.
Physical characteristics
Nucleus
The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are popularly described as "dirty snowballs" after Fred Whipple's model. Comets with a higher dust content have been called "icy dirtballs". The term "icy dirtballs" arose after observation of Comet 9P/Tempel 1 collision with an "impactor" probe sent by NASA Deep Impact mission in July 2005. Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. Nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, ethane, and perhaps more complex molecules such as long-chain hydrocarbons and amino acids. In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission. In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components may have been formed on asteroids and comets.
The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0%; by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.
Comet nuclei with radii of up to have been observed, but ascertaining their exact size is difficult. The nucleus of 322P/SOHO is probably only in diameter. A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than across. Known comets have been estimated to have an average density of. Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.
Roughly six percent of the near-Earth asteroids are thought to be the extinct nuclei of comets that no longer experience outgassing, including 14827 Hypnos and 3552 Don Quixote.
Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on Rosetta determined that electrons produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma. Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which have been detected for the first time on a comet.
| Name | Dimensions | Density | Mass | Refs |
| Halley's Comet | 15 × 8 × 8 | 0.6 | 3 | |
| Tempel 1 | 7.6 × 4.9 | 0.62 | 7.9 | |
| 19P/Borrelly | 8 × 4 × 4 | 0.3 | 2.0 | |
| 81P/Wild | 5.5 × 4.0 × 3.3 | 0.6 | 2.3 | |
| 67P/Churyumov–Gerasimenko | 4.1 × 3.3 × 1.8 | 0.47 | 1.0 |
Coma
The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma". The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.The coma is generally made of water and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within of the Sun. The parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry. Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.
Although the solid nucleus of comets is generally less than across, the coma may be thousands or millions of kilometers across, sometimes becoming larger than the Sun. For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 had a coma roughly the diameter of the Sun. Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail. Ion tails have been observed to extend one astronomical unit or more.
Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects sunlight directly while the gases glow from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.
In 1996, comets were found to emit X-rays. This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. Thomas E. Cravens was the first to propose an explanation in early 1997. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons.