Comet

Comet Tempel collides with Deep Impact's impactorComet 67P/Churyumov–Gerasimenko orbited by Rosetta
Comet 17P/Holmes and its blue ionized tailComet Wild 2 visited by Stardust probe
Hale–Bopp seen from Croatia in 1997Comet Lovejoy seen from orbit
Comets – nucleus, coma and tail:

A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures.

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.[1] Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by 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.

Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). However, 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.[2] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[3][4] The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets.

As of November 2014 there are 5,253 known comets,[5] a number that is steadily increasing as they are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion.[6][7] Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.[8] Particularly bright examples are called "great comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first ever to land a robotic spacecraft on a comet,[9] and NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.

Etymology

The word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης ("wearing long hair"), and the Oxford English Dictionary notes that the term (ἀστὴρ) 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".[10][11]

The astronomical symbol for comets is (in Unicode U+2604), consisting of a small disc with three hairlike extensions.[12]

Other Languages
Afrikaans: Komeet
Alemannisch: Komet
Ænglisc: Comēta
العربية: مذنب
aragonés: Cometa
অসমীয়া: ধূমকেতু
asturianu: Cometa
Avañe'ẽ: Jaguaveve
azərbaycanca: Kometa
বাংলা: ধূমকেতু
Bân-lâm-gú: Tn̂g-boé-chheⁿ
башҡортса: Комета
беларуская: Камета
беларуская (тарашкевіца)‎: Камэта
भोजपुरी: पुच्छल तारा
български: Комета
Boarisch: Komet
bosanski: Kometa
brezhoneg: Steredenn-lostek
català: Cometa
Чӑвашла: Вутлă хӳре
čeština: Kometa
Cymraeg: Comed
dansk: Komet
Deutsch: Komet
ދިވެހިބަސް: ގިނިހިލަ
eesti: Komeet
Ελληνικά: Κομήτης
español: Cometa
Esperanto: Kometo
euskara: Kometa
Fiji Hindi: Jhaarru (Taara)
français: Comète
Frysk: Komeet
furlan: Comete
Gaeilge: Cóiméad
Gàidhlig: Reul earballach
galego: Cometa
ગુજરાતી: ધૂમકેતુ
客家語/Hak-kâ-ngî: So-pá-sên
한국어: 혜성
Հայերեն: Գիսաստղ
हिन्दी: धूमकेतु
hrvatski: Komet
Ido: Kometo
Ilokano: Bandus
Bahasa Indonesia: Komet
interlingua: Cometa
íslenska: Halastjarna
italiano: Cometa
עברית: שביט
Basa Jawa: Komèt
ಕನ್ನಡ: ಧೂಮಕೇತು
Kapampangan: Kometa
ქართული: კომეტა
қазақша: Комета
Kiswahili: Nyotamkia
Kreyòl ayisyen: Komèt
Кыргызча: Комета
лезги: Комета
Latina: Cometes
latviešu: Komēta
Lëtzebuergesch: Koméit
lietuvių: Kometa
Limburgs: Komeet
Livvinkarjala: Komiettu
lumbaart: Cometa
magyar: Üstökös
македонски: Комета
മലയാളം: ധൂമകേതു
Malti: Kometa
मराठी: धूमकेतू
მარგალური: კომეტა
مصرى: مزنب
Bahasa Melayu: Komet
Mìng-dĕ̤ng-ngṳ̄: Chēng-chiū-sĭng
မြန်မာဘာသာ: ကြယ်တံခွန်
Nāhuatl: Popocacitlalin
Nederlands: Komeet
नेपाल भाषा: तःहाःनगु
日本語: 彗星
нохчийн: Комета
norsk: Komet
norsk nynorsk: Komet
Nouormand: Conmète
Novial: Komete
occitan: Cometa
oʻzbekcha/ўзбекча: Kometa
ਪੰਜਾਬੀ: ਪੂਛਲ ਤਾਰਾ
Patois: Kamit
Piemontèis: Comëtta
Plattdüütsch: Komet
polski: Kometa
português: Cometa
Qaraqalpaqsha: Kometa
Ripoarisch: Komeet
română: Cometă
Runa Simi: Aquchinchay
русиньскый: Комета
русский: Комета
саха тыла: Комета
संस्कृतम्: धूमकेतुः
Scots: Comet
Seeltersk: Komete
shqip: Kometa
sicilianu: Cumeta
සිංහල: වල්ගාතරු
Simple English: Comet
slovenčina: Kométa
slovenščina: Komet
کوردی: کلکدار
српски / srpski: Комета
srpskohrvatski / српскохрватски: Kometa
Basa Sunda: Komét
suomi: Komeetta
svenska: Komet
Tagalog: Kometa
татарча/tatarça: Комета
తెలుగు: తోకచుక్క
тоҷикӣ: Комета
Türkmençe: Kometa
українська: Комета
ئۇيغۇرچە / Uyghurche: قۇيرۇقلۇق يۇلتۇز
vèneto: Cometa
Tiếng Việt: Sao chổi
Winaray: Kometa
吴语: 彗星
ייִדיש: קאמעט
粵語: 彗星
žemaitėška: Šloutžvaizdė
中文: 彗星