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.
彗星是冰冷的小型太阳系天体，当它靠近太阳时，会变暖并开始释放气体，这个过程称为脱气。 这会产生可见的气氛或彗发，有时还会产生彗尾。 这些现象是由于太阳辐射和作用在彗星核上的太阳风的影响。 彗核的范围从几百米到几十公里不等，由冰，尘埃和小的岩石颗粒的松散集合组成。 彗发可能高达地球直径的15倍，而彗尾可能会拉伸一个天文单位。 如果足够明亮，则无需借助望远镜就可以从地球上看到一颗彗星，并且彗星可能会在天空中呈30°弧度（60个卫星）。 自古以来，许多文明都对彗星进行了观测和记录。
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 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.
彗星通常具有高度偏心的椭圆形轨道，并且轨道周期范围很广，从几年到可能的数百万年不等。 短周期彗星起源于柯伊伯带或其相关的散布盘，它们位于海王星的轨道之外。 人们认为长周期彗星起源于奥尔特云，这是一个球形的冰状体，从柯伊伯带外延伸到最近的恒星的一半。 长周期彗星由于经过恒星和银河潮汐引起的引力扰动而从奥尔特云向太阳移动。 双曲线彗星可能仅一次穿过内部太阳系后，就抛向星际空间。 彗星的出现称为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.
彗星与小行星的区别在于它们的中心核周围存在着延伸的，不受重力约束的大气。 这种大气具有被称为彗发的部分（紧紧围绕核的中央部分）和彗尾的部分（通常是类似线的部分，由太阳的轻压力或太阳风等离子从彗发中吹出的灰尘或气体组成）。 然而，已经多次接近太阳的灭绝彗星失去了几乎所有的挥发性冰和尘埃，并且可能像小行星一样。
Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. 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 (PANSTARRS). 27 Manx comets were found from 2013 to 2017.
人们认为小行星的起源与彗星不同，前者是在木星轨道内部而不是太阳系外部形成的。  主带彗星和活跃的半人马座小行星的发现模糊了小行星和彗星之间的区别。 在21世纪初期，人们发现了一些具有长周期彗星轨道的小天体，但它们具有内部太阳系小行星的特征，被称为Manx彗星。 它们仍被归类为彗星，例如C / 2014 S3（PANSTARRS）。 从2013年到2017年，发现了27颗Manx彗星。
As of July 2018 there are 6,339 known comets, 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. 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 unmanned probes such as the European Space Agency’s Rosetta, which became the first ever to land a robotic spacecraft on a comet, and NASA’s Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.
截至2018年7月，已知彗星有6,339个，被发现的彗星数正在稳步增长。 但是，这仅占潜在彗星总数的一小部分，因为估计太阳外部太阳系（在奥尔特云中）的类似彗星体的储量为1万亿。  每年大约有一颗肉眼可见的彗星，尽管其中许多彗星微弱且不引人注目。 特别明亮的例子称为“大彗星”。 彗星已经被无人驾驶的探测器所访问，例如欧洲航天局的罗塞塔号（Rosetta），这是有史以来第一个将宇宙飞船降落在彗星上的太空飞船，和美国宇航局的深度撞击，它炸毁了坦普尔1号彗星的陨石坑以研究其内部。
A comet was mentioned in the Anglo-Saxon Chronicle that allegedly made an appearance in 729 AD.
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”.
彗星一词源自拉丁语comēta或comētēs的古英语彗星。 反过来，这是希腊语κομήτης（“戴长发”）的拉丁语，牛津英语词典指出，（ἀστὴρ）κομήτης在希腊语中已经意味着“长发恒星，彗星”。 Κομήτης取自κομᾶν（“长发”），其本身又取自κόμη（“头的头发”），用来表示“彗星的尾巴”。 
彗星的天文符号是（在Unicode ☄ U + 2604中），由一个小盘和三个类似头发的扩展名组成。
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. However, many comets have a higher dust content, leading them to be called “icy dirtballs”. 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.
彗星的固体核心结构称为核。 彗核是由岩石，尘埃，水冰和冷冻的二氧化碳，一氧化碳，甲烷和氨气混合而成的。 因此，在弗雷德·惠普尔（Fred Whipple）的模型之后，它们通常被描述为“脏雪球”。 然而，许多彗星的尘埃含量较高，因此被称为“冰雪土球”。 2014年进行的研究表明，彗星就像“油炸冰淇淋”，因为它们的表面是由稠密的结晶冰和有机化合物混合而成，而内部的冰却较冷，密度较小。
The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and 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 (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.
核的表面通常是干燥，多尘或多岩石的，这表明冰藏在几米厚的地壳下面。 除了已经提到的气体外，核还包含各种有机化合物，其中可能包括甲醇，氰化氢，甲醛，乙醇和乙烷，也许还有更复杂的分子，例如长链烃和氨基酸。 [ 20] 2009年，已确认在美国国家航空航天局（NASA）的“星尘”任务回收的彗星尘中发现了氨基酸甘氨酸。 2011年8月，根据NASA对地球上发现的陨石的研究，发表了一份报告，暗示小行星和彗星可能已经形成了DNA和RNA成分（腺嘌呤，鸟嘌呤和相关有机分子）。 
Comet Borrelly exhibits jets, but has no surface ice.
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.
彗核的外表面反射率很低，是太阳系中反射最少的物体之一。 乔托（Giotto）太空探测器发现哈雷彗星的核反射了约4％的光，而深空1号发现博雷利彗星的表面反射不到3.0％； 相比之下，沥青可以反射了7%。 核的深色表面材料可能由复杂的有机化合物组成。 太阳能加热驱散了较轻的挥发性化合物，留下了往往很暗的较大有机化合物，例如焦油或原油。 彗星表面的低反射率导致它们吸收驱动脱气过程的热量。
Comet nuclei with radii of up to 30 kilometres (19 mi) have been observed, but ascertaining their exact size is difficult. The nucleus of 322P/SOHO is probably only 100–200 metres (330–660 ft) 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 100 metres (330 ft) across. Known comets have been estimated to have an average density of 0.6 g/cm3 (0.35 oz/cu in). Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.
已经观察到半径不超过30公里（19英里）的彗星核，但很难确定其确切大小。 322P / SOHO的核直径可能只有100-200米（330-660英尺）。 尽管仪器灵敏度提高，但仍未检测到较小的彗星，这使一些人暗示确实存在小于100米（330英尺）的彗星。 据估计，已知彗星的平均密度为0.6 g / cm3（0.35 oz / cu in）。 由于彗星核的质量低，它们在自身重力作用下不会变成球形，因此形状不规则。
Comet 81P/Wild exhibits jets on light side and dark side, stark relief, and is dry.
大约6％的近地小行星被认为是不再喷气的彗核，包括14827 Hypnos和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 (within 1 km (0.62 mi) above the comet nucleus) 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 (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.
Rosetta和Philae航天器的结果表明，67P / Churyumov–Gerasimenko的原子核没有磁场，这表明磁性可能在行星的早期形成中没有作用。  此外，罗塞塔号上的ALICE光谱仪确定，由太阳辐射使水分子光电离而产生的电子（在彗核上方1公里（0.62英里以内））产生的电子（而不是先前认为的来自太阳的光子）引起了水的降解 和从彗核释放的二氧化碳分子进入昏迷状态。  菲莱着陆器上的仪器至少在彗星表面发现了十六种有机化合物，其中四种（乙酰胺，丙酮，异氰酸甲酯和丙醛）是首次在彗星上被发现。  
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 H2O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun. The H2O 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.
昏迷通常由水和灰尘组成，当彗星位于太阳的3至4个天文单位（450,000,000至600,000,000公里； 280,000,000至370,000,000英里）以内时，水最多构成从核流出的挥发物的90％。  与光化学相比，H2O母体分子主要通过光离解和较小程度的电离作用被破坏，而太阳风在水的破坏中起着较小的作用。 沿着彗星的轨道路径会留下较大的尘埃颗粒，而较小的尘埃颗粒会在轻压力下被推离太阳进入彗星的尾巴。
Although the solid nucleus of comets is generally less than 60 kilometres (37 mi) across, the coma may be thousands or millions of kilometres 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 also 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 1.5 astronomical units (220,000,000 km; 140,000,000 mi) 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 (150 million km) or more.
尽管彗星的实核通常不超过60公里（37英里），但昏迷可能跨越数千或数百万公里，有时甚至比太阳还大。 例如，在2007年10月爆发大约一个月后，彗星17P /霍姆斯短暂地具有比太阳大的微弱尘埃大气。 1811年的大彗星也有大约太阳直径的昏迷。 即使昏迷可能变得很大，但它的大小仍可以在它越过火星轨道约1.5天文单位（220,000,000 km； 140,000,000 mi）时减小。 在这一距离处，太阳风变得足够强大，足以将气体和灰尘吹离昏迷，并因此而使尾巴增大。 已观察到离子尾巴延伸一个天文单位（1.5亿公里）或更多。
C/2006 W3 (Chistensen) emitting carbon gas (IR image)
C / 2006 W3（Chistensen）散发出碳气（红外图像）
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.
彗发和彗尾都被太阳照亮，当彗星穿过内部太阳系时，它们可能会变得可见；当气体从电离中发光时，灰尘直接反射太阳光。 大多数彗星太暗太小，没有望远镜就无法看到，但是每十年有几颗变得足够明亮，肉眼也能看到。 有时候，彗星可能会突然爆发出巨大的气体和尘埃，在此期间彗星的尺寸会大大增加。 这发生在2007年的福尔摩斯彗星上。
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. 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.
1996年，发现彗星发出X射线。 这使天文学家大为惊讶，因为X射线发射通常与非常高温的物体有关。 X射线是由彗星和太阳风之间的相互作用产生的：当高电荷的太阳风离子飞过彗星大气时，它们与彗星原子和分子发生碰撞，从而以一种称为“窃取”的方式从原子中窃取一个或多个电子，叫做“电子交换”。 电子向太阳风离子的交换或转移之后，通过发射X射线和远紫外光子将其去离子为基态。
弓形激波 Bow shock
Bow shocks form as a result of the interaction between the solar wind and the cometary ionosphere, which is created by ionization of gases in the coma. As the comet approaches the Sun, increasing outgassing rates cause the coma to expand, and the sunlight ionizes gases in the coma. When the solar wind passes through this ion coma, the bow shock appears.
弓形激波是由于太阳风和彗星电离层之间相互作用而形成的，而彗星电离层是由彗发中的气体电离而产生的。 随着彗星接近太阳，除气速率的增加会导致彗发膨胀，而阳光会使彗发中的气体电离。 当太阳风通过该离子彗发时，会出现弓形激波。
The first observations were made in the 1980s and 90s as several spacecraft flew by comets 21P/Giacobini–Zinner, 1P/Halley, and 26P/Grigg–Skjellerup. It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at, for example, Earth. These observations were all made near perihelion when the bow shocks already were fully developed.
最初的观测是在1980年代和90年代进行的，当时几架航天器由21P / Giacobini–Zinner， 1P / Halley，和26P / Grigg–Skjellerup 彗星飞行。 然后发现，与例如在地球上看到的剧烈的行星弓形激波相比，彗星的弓形激波更宽，梯度更大。 所有这些观察都是在近日点附近进行的，这时彗星的弓形激波已经非常完全了。
The Rosetta spacecraft observed the bow shock at comet 67P/Churyumov–Gerasimenko at an early stage of bow shock development when the outgassing increased during the comet’s journey toward the Sun. This young bow shock was called the “infant bow shock”. The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.
罗塞塔号飞船在彗星向太阳飞行的过程中排气量增加时，在弓激波发展的早期阶段观察到彗星67P /丘留莫夫-格拉西缅科的弓激波。 这种年轻的弓形休克被称为“初生弓形激波”。 初生弓激波是不对称的，相对于到核的距离，它比完全发展的弓激波更宽。
In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope but these detections have been questioned. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.
在外层太阳系中，彗星由于体积小，一直保持冻结和不活动状态，极难或不可能从地球上探测到。 哈勃太空望远镜的观测报告了对柯伊伯带无活动彗核的统计检测 ，但这些检测受到质疑。  当彗星接近内部太阳系时，太阳辐射会使彗星内的挥发性物质蒸发并从核中流出，从而带走尘埃。
The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet’s orbit in such a manner that it often forms a curved tail called the type II or dust tail. At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. On occasions—such as when the Earth passes through a comet’s orbital plane, the antitail, pointing in the opposite direction to the ion and dust tails, may be seen.
灰尘和气体流各自形成各自不同的尾巴，指向略有不同的方向。 尘埃的尾部以通常形成II型或尘埃尾的弯曲尾部的方式留在彗星的轨道上。 同时，由气体构成的离子或I型尾巴总是直接指向太阳，因为这种气体比磁场沿轨道线而不是轨道轨迹受太阳风的影响要强于灰尘。 ] 在某些情况下，例如地球穿过彗星的轨道平面时，可能会看到反尾，其指向与离子和尘埃尾巴相反的方向。
The observation of antitails contributed significantly to the discovery of solar wind. The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an “induced magnetosphere” around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called “pick-up ions”) congregate and act to “load” the solar magnetic field with plasma, such that the field lines “drape” around the comet forming the ion tail.
对反尾的观察大大促进了太阳风的发现。 离子尾巴是通过太阳紫外线照射彗发中的粒子进行电离而形成的。 一旦粒子被离子化，它们就会获得净正电荷，进而在彗星周围产生“感应磁层”。 彗星及其感应磁场形成了向外流动的太阳风粒子的障碍。 因为彗星和太阳风的相对轨道速度是超音速的，所以在彗星上游沿太阳风的流动方向会形成弓形激波。 在这种弓形激波中，高浓度的彗星离子（称为“拾取离子”）聚集并起到向等离子体中“加载”太阳磁场的作用，从而使磁力线“悬垂”在彗星周围，形成离子尾巴。 70]
If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a “tail disconnection event”. This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke’s Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.
如果离子尾部负载足够大，则将磁力线挤压在一起，直到沿着离子尾部一定距离会发生磁重连。 这导致“尾巴断开事件”。 这在很多情况下都可以观察到，2007年4月20日记录了一个值得注意的事件，当时恩克的彗星的离子尾巴被完全割断，而彗星则通过了日冕物质抛射。 STEREO空间探测器观察到了这一事件。
2013年，欧洲航天局（ESA）科学家报告说，金星的电离层以类似于在类似条件下从彗星流出的离子尾巴的方式向外流出。”  
Gas and snow jets of 103P/Hartley
Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet’s nucleus, like a geyser. These streams of gas and dust can cause the nucleus to spin, and even split apart. In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus. Infrared imaging of Hartley 2 shows such jets exiting and carrying with it dust grains into the coma.
加热不均匀会导致新产生的气体像间歇泉一样从彗星核表面的薄弱部位破裂。 这些气体和灰尘流会导致原子核旋转，甚至分裂。 2010年，有消息显示，干冰（冻结的二氧化碳）可以为从彗核流出的物质提供动力。 Hartley 2的红外成像显示出这样的喷射流，并带着尘埃到达彗发。
Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder. Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.
Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years. They usually orbit more-or-less in the ecliptic plane in the same direction as the planets. Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley’s Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet’s orbit are called its “family”. Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.
周期性彗星或短周期彗星通常被定义为轨道周期小于200年的那些。 它们通常在与行星相同的方向上在黄道平面内或多或少地绕轨道运行。 它们的轨道通常将它们带到顶峰处的外行星区域（木星及以后）。 例如，哈雷彗星的顶峰稍微超出了海王星的轨道。 短视距在主要行星轨道附近的彗星称为其“家族”。 人们认为，这些家族是由于行星将原先长周期的彗星捕获到较短的轨道而产生的。
At the shorter orbital period extreme, Encke’s Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called traditional Jupiter-family comets (JFCs). Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs). As of 2019, 85 HTCs have been observed, compared with 664 identified JFCs.
在较短的轨道周期极端，恩克彗星的轨道没有达到木星的轨道，因此被称为恩克型彗星。 轨道周期小于20年且向黄道倾斜度低（最高达30度）的短周期彗星称为传统木星家庭彗星（JFCs）。  像哈雷这样的卫星，其轨道周期在20至200年之间，并且倾斜度从零延伸到90度以上，被称为哈雷型彗星（HTC）。  截至2019年，已观察到85个HTC，以及已查明的664个JFC。
Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations. Short-period comets have a tendency for their aphelia to coincide with a giant planet‘s semi-major axis, with the JFCs being the largest group. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.
由于它们的椭圆轨道经常使它们靠近巨型行星，因此彗星会受到进一步的引力扰动。 短周期彗星有一种特征，即其短视角与巨型行星的半长轴重合，而JFC则是最大的群。 显然，由于近距离接触，从奥尔特云中出来的彗星的轨道常常受到巨型行星引力的强烈影响。 木星是最大扰动的来源，其质量是其他所有行星总和的两倍以上。 这些扰动可以使长周期彗星变成较短的轨道周期。 
Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc —a disk of objects in the trans-Neptunian region—whereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence). Vast swarms of comet-like bodies are thought to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable.
根据它们的轨道特征，认为短周期彗星起源于半人马天体群和柯伊伯带/散盘（海王星区域的一个物体盘），而长周期彗星的来源被认为来自是距离更远的球形奥尔特云（在荷兰天文学家扬·亨德里克·奥尔特猜测其的存在之后）。 人们认为，大量类似彗星的物体在这些遥远的区域以大致圆形的轨道绕太阳运行。 有时，外行星（对于柯伊伯带天体而言）或附近的恒星（对于奥尔特云物体而言）对万有引力的影响可能会将其中一个物体抛向椭圆形轨道，从而使其向内朝太阳形成可见的 彗星。 与周期性彗星的返回不同，后者的轨道是由先前的观测确定的，通过这种机制，新彗星的出现是不可预测的。
Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years. An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System. For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the eccentricity drops below 1 as it moves farther from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System.
长周期彗星的轨道高度偏心，周期从200年到数千年不等。 当接近近日点时，偏心率大于1并不一定意味着彗星将离开太阳系。 例如，2007年1月，麦克诺德彗星的日心偏心率在近日点通过时代附近为1.000019，但由于它离太阳越来越远，所以偏心率下降到1以下，因此与太阳的轨道大约为92,600年。 当在离开行星区域后的一个纪元计算出近轨道，并相对于太阳系的质心进行计算时，就可以正确地获得长周期彗星的未来轨道。
By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having “periods”. The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have aphelion distances of nearly 70,000 AU with orbital periods estimated around 6 million years.
根据定义，长周期彗星仍与太阳有引力作用； 那些由于近距离掠过巨大行星而从太阳系中抛出的彗星不再被认为具有“周期”。 长周期彗星的轨道使它们远远超出了的外行星，并且它们的轨道平面不必位于黄道附近。 诸如彗星西和C / 1999 F1之类的长周期彗星的近日点距离可以达到近70,000 AU，轨道周期约为600万年。
Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun. The Sun’s Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)). Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.
单向或非周期彗星与长周期彗星相似，因为当它们位于太阳系内部的近日点附近时，它们也具有抛物线形或双曲线形。 但是，来自巨型行星的引力扰动会导致其轨道发生变化。 单向彗星具有双曲或抛物线的密合轨道，使它们在经过一次太阳后就被永远抛出太阳系。 太阳的希尔球的最大边界不稳定，为230,000 AU（1.1秒差距（3.6光年））。 在近日点附近，只有几百颗彗星到达双曲线轨道（e> 1），这表明使用日心中心无扰动的两体最佳拟合表明它们可能逃脱了太阳系。
As of 2019, only two objects have been discovered with an eccentricity significantly greater than one: 1I/ʻOumuamua and 2I/Borisov, indicating an origin outside the Solar System. While ʻOumuamua, with an eccentrcity of about 1.2, showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectory—which suggests outgassing—indicate that it is probably a comet. On the other hand, 2I/Borisov, with an estimated eccentricity of about 3.36, has been observed to have the coma feature of comets, and is considered the first detected interstellar comet. Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet. Comets not expected to return to the inner Solar System include C/1980 E1, C/2000 U5, C/2001 Q4 (NEAT), C/2009 R1, C/1956 R1, and C/2007 F1 (LONEOS).
截至2019年，仅发现了两个偏心率明显大于一个的物体：1I /ʻOumuamua和2I / Borisov，表明其起源于太阳系之外。尽管`Oumuamua的中心大约为1.2，但在其于2017年10月通过内部太阳系时未显示出彗星活动的光学迹象，但其轨迹发生了变化（表明存在脱气现象），表明它可能是一颗彗星。另一方面，观测到的2I / Borisov偏心率约为3.36，被认为具有彗发特征，被认为是最早发现的星际彗星。  C / 1980 E1彗星在1982年近日点通过之前的轨道周期约为710万年，但是1980年与木星的相遇加速了彗星的生长，使它成为任何已知双曲线彗星中最大的离心率（1.057）。不期望返回内部太阳系的彗星包括C / 1980 E1，C / 2000 U5，C / 2001 Q4（NEAT），C / 2009 R1，C / 1956 R1和C / 2007 F1（LONEOS）。
Some authorities use the term “periodic comet” to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets), whereas others use it to mean exclusively short-period comets. Similarly, although the literal meaning of “non-periodic comet” is the same as “single-apparition comet”, some use it to mean all comets that are not “periodic” in the second sense (that is, to also include all comets with a period greater than 200 years).
一些机构使用“周期性彗星”一词来指代任何具有周期性轨道的彗星（即，所有短周期彗星加上所有长周期彗星），而另一些机构则使用它专门指短周期彗星。  同样，尽管“非周期性彗星”的字面含义与“单向彗星”相同，但有人用它来表示第二种意义上不是“周期性”的所有彗星（也就是说，还包括所有周期大于200年的彗星）。
Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). If such objects entered the Solar System, they would have positive specific orbital energy and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter’s orbit, give or take one and perhaps two orders of magnitude.
早期的观察已经揭示了一些真正的双曲线（即非周期）轨迹，但最多不过是木星扰动所能解释的。 如果彗星弥漫在星际空间中，它们将以与太阳附近恒星的相对速度（每秒几十公里）相同的速度运动。 如果这些物体进入太阳系，它们将具有正比轨道能量，并且将观察到它们具有真正的双曲线轨迹。 粗略的计算表明，木星的轨道上每个世纪可能有四个双曲彗星，给定或取一个或两个数量级。【？？】
奥尔特云和希尔云Oort cloud and Hills cloud
The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly). The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0.32–0.79 ly), and a doughnut-shaped inner cloud, the Hills cloud, of 2,000–20,000 AU (0.03–0.32 ly).
奥尔特云被认为占据了一个广阔的空间，从太阳到2000到5,000 AU（0.03到0.08 ly）到50,000 AU（0.79 ly）。 一些估计将外边缘置于100,000至200,000 AU之间（1.58至3.16 ly）。 该区域可细分为20,000-50,000 AU（0.32-0.79 ly）的球形外部奥尔特云和2,000-20,000 AU（0.03-0.32 ly）的甜甜圈形内云Hills Hills。
The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of Neptune. The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981. Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo; it is seen as a possible source of new comets that resupply the relatively tenuous outer cloud as the latter’s numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.
外层云只与太阳弱结合，并提供落入海王星轨道内部的长周期（可能是哈雷型）彗星。 内部的奥尔特云也被称为希尔斯云，以J. G.希尔斯（J. G. Hills）的名字命名，他于1981年提出存在它的概念。 模型预测，内部云的彗核数应是外部光晕的数十倍或数百倍；   它被视为可能是新彗星的来源，这些彗星为相对较弱的外部云提供了补充。 后者的数量逐渐枯竭。 希尔斯云解释了数十亿年后奥尔特云的持续存在。
Main article: Exocomet
Exocomets beyond the Solar System have also been detected and may be common in the Milky Way. The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987. A total of 10 such exocomet systems have been identified as of 2013, using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.
太阳系外的外彗星也已被发现，在银河系中很常见。 1987年检测到的第一个外彗星系统是Beta Pictoris周围，这是一颗非常年轻的A型主序星。  截至2013年，已利用彗星靠近恒星时发出的大量气体云所引起的吸收光谱，确定了总共10个这种外彗星系统。 
彗星的影响Effects of comets
与流星雨的关系Connection to meteor showers
As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind. If the Earth’s orbit sends it through that debris, there are likely to be meteor showers as Earth passes through. The Perseid meteor shower, for example, occurs 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.
放气会导致彗星在尾流中留下一堆固体碎片，这些碎片太大而无法被辐射压力和太阳风吹走。 如果地球恰好经过它们的轨道，它们就可能因为地球引力作用形成流星雨。 例如，每年英仙座流星雨发生在每年8月9日至13日之间，这时地球经过了Swift–Tuttle彗星的轨道。 哈雷彗星是10月“猎户座”流星雨的来源。
彗星对生命的影响Comets and impact on life
Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill the Earth’s oceans, or at least a significant portion of it. Others have cast doubt on this idea. The detection of organic molecules, including polycyclic aromatic hydrocarbons, in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of life—or even life itself—to Earth. In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis. In 2015, scientists found significant amounts of molecular oxygen in the outgassings of comet 67P, suggesting that the molecule may occur more often than had been thought, and thus less an indicator of life as has been supposed.
许多彗星和小行星在早期与地球相撞。 许多科学家认为，大约40亿年前，彗星轰击了年轻的地球，带来了如今充斥着地球海洋或至少其中很大一部分的大量水。 其他人对此想法表示怀疑。 对彗星中大量有机分子的检测，包括多环芳烃，导致人们猜测，彗星或陨石可能将生命的前体，甚至是生命本身带到了地球。 2013年，有人建议在岩石表面和冰表面之间的撞击（例如彗星）有可能通过激波合成产生构成蛋白质的氨基酸。 2015年，科学家在67P彗星的放气中发现了大量的分子氧，这表明该分子的出现频率可能比人们想象的要高，因此比预期的生命指标要少。
It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth’s Moon, some of which may have survived as lunar ice. Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.
对彗星的恐惧Fear of comets
Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650. The year after the Great Comet of 1618, for example, Gotthard Arthusius published a pamphlet stating that it was a sign that the Day of Judgment was near. He listed ten pages of comet-related disasters, including “earthquakes, floods, changes in river courses, hail storms, hot and dry weather, poor harvests, epidemics, war and treason and high prices”. By 1700 most scholars concluded that such events occurred whether a comet was seen or not. Using Edmund Halley’s records of comet sightings, however, William Whiston in 1711 wrote that the Great Comet of 1680 had a periodicity of 574 years and was responsible for the worldwide flood in the Book of Genesis, by pouring water on the Earth. His announcement revived for another century fear of comets, now as direct threats to the world instead of signs of disasters. Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley’s Comet, causing panicked buying of gas masks and quack “anti-comet pills” and “anti-comet umbrellas” by the public.
从公元1200年到1650年，认为彗星是上帝的惩罚的思想在欧洲空前流行。例如，在1618年大彗星之后的第二年，Gotthard Arthusius出版了一本小册子，指出这是审判日临近的标志。他列出了十页与彗星有关的灾难，包括“地震，洪水，河道变化，冰雹风暴，炎热干燥的天气，丰收，流行病，战争和叛国罪和高昂的价格”。到1700年，大多数学者得出结论，无论是否看见彗星，此类事件都会发生。威廉·惠斯顿（William Whiston）在1711年使用埃德蒙·哈雷（Edmund Halley）的彗星观测记录，写道，1680年的大彗星具有574年的周期性，并通过在地球上注水造成了《创世纪》中的全球洪水。他的声明复活是因为人们对彗星再有一个世纪的恐惧，如今它已成为对世界的直接威胁而不是灾难的迹象。 1910年的光谱分析发现，在哈雷彗星尾巴中有毒的氰化氢，引起公众恐慌地购买了防毒面具和嘎嘎叫的“防彗星药丸”和“防彗星雨伞”。
彗星的命运Fate of comets
从太阳系被抛出Departure (ejection) from Solar System
If a comet is traveling fast enough, it may leave the Solar System. Such comets follow the open path of a hyperbola, and as such they are called hyperbolic comets. To date, comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter. An example of this is thought to be Comet C/1980 E1, which was shifted from a predicted orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter.
如果彗星的行进速度足够快，它可能会离开太阳系。 这些彗星遵循双曲线的开放路径，因此被称为双曲线彗星。 迄今为止，已知彗星只能通过与太阳系中的另一个物体（如木星）相互作用而被抛出。 一个例子是彗星C / 1980 E1，它在1980年经过木星行星的近距离掠过后，从预计的绕太阳轨道710万年转变为双曲线轨道。
Main article: Extinct comet
Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages. Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid. Some asteroids in elliptical orbits are now identified as extinct comets. Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.
木星族彗星和长周期彗星似乎遵循完全不同的衰落定律。 JFC在约10,000年或约1,000个轨道的生命周期中处于活动状态，而长周期彗星的衰落速度则快得多。 长周期彗星中只有10％可以存活50次以上，到达小近日点，而只有1％存活2000次以上。 最终，彗核中所含的大部分挥发性物质蒸发掉了，彗星变成了一块小而暗的惰性块状岩石或瓦砾，类似于小行星。 椭圆形轨道上的一些小行星现在被确认为灭绝彗星。    大约百分之六的近地小行星被认为是绝种的彗核。
分裂和碰撞Breakup and collisions
The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart. A significant cometary disruption was that of Comet Shoemaker–Levy 9, which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter’s atmosphere—the first time astronomers had observed a collision between two objects in the Solar System. Other splitting comets include 3D/Biela in 1846 and 73P/Schwassmann–Wachmann from 1995 to 2006. Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC. Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.
一些彗星的核可能是脆弱的，这一结论得到观察到的彗星分裂的支持。 一次重大的彗星破坏是在1993年发现的Shoemaker-Levy 9彗星。1992年7月的一次紧密相遇将其破碎成碎片，并且在1994年7月的六天内，这些碎片落入了木星的大气层- 天文学家第一次观察到太阳系中两个物体之间的碰撞。  其他分裂彗星包括1846年的3D / Biela和1995年至2006年的73P / Schwassmann–Wachmann。 希腊历史学家埃弗勒斯（Ephorus）报道说，彗星早在公元前372-373年冬天就分裂了。 怀疑彗星由于热应力，内部气压或冲击而分裂。
Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.
Some comets have been observed to break up during their perihelion passage, including great comets West and Ikeya–Seki. Biela‘s Comet was one significant example, when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela’s Comet.
已经观察到一些彗星在近日点时会破裂，包括大彗星West和Ikeya–Seki。 比耶拉的彗星就是一个重要的例子，它在1846年穿过近日点时破裂成两半。这两个彗星在1852年被分别看到，但此后再也没有出现过。 取而代之的是在1872年和1885年看到了壮观的流星雨，当时彗星本应可见。 每年11月发生一次较小的流星雨，即Andromedids，这是地球越过Biela彗星的轨道时引起的。
Some comets meet a more spectacular end – either falling into the Sun or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet Shoemaker–Levy 9 broke up into pieces and collided with Jupiter.
一些彗星的结局更为壮观-要么掉入太阳要么砸入行星或其他物体。 彗星与行星或卫星之间的碰撞在早期的太阳系中很常见：例如，月球上许多陨石坑中的某些可能是由彗星引起的。 最近，一颗彗星与一颗行星相撞发生在1994年7月，当时鞋匠-Levy 9彗星破裂成碎片并与木星相撞。
Brown spots mark impact sites of Comet Shoemaker–Levy 9 on Jupiter
棕色斑点标记着木星彗星– Levy 9在木星上的撞击点
The break up of 73P/Schwassmann–Wachmann within three days (1995)
73P / Schwassmann–Wachmann在三天内分解（1995年）
Ghost tail of C/2015 D1 (SOHO) after passage at the Sun
太阳经过后的C / 2015 D1（SOHO）鬼尾
P / 2013 R3（2014）的解体
Main article: Naming of comets
Halley’s Comet in 1910
The names given to comets have followed several different conventions over the past two centuries. Prior to the early 20th century, most comets were simply referred to by the year when they appeared, sometimes with additional adjectives for particularly bright comets; thus, the “Great Comet of 1680“, the “Great Comet of 1882“, and the “Great January Comet of 1910“.
在过去的两个世纪中，彗星的名称遵循了几种不同的惯例。 在20世纪初期之前，大多数彗星在出现的那一年就被简单地提及，有时还为特别明亮的彗星添加了形容词。 因此，“ 1680年大彗星”，“ 1882年大彗星”和“ 1910年1月大彗星”。
After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759 by calculating its orbit, that comet became known as Halley’s Comet. Similarly, the second and third known periodic comets, Encke’s Comet and Biela’s Comet, were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their appearance.
埃德蒙·哈雷（Edmund Halley）证明1531、1607和1682年的彗星是同一物体，并通过计算其轨道成功预测了1759年的返回之后，该彗星被称为哈雷彗星。 同样，第二和第三种已知的周期性彗星，Encke彗星和Biela彗星以计算其轨道而不是原始发现者的天文学家的名字命名。 后来，周期性的彗星通常以发现者的名字命名，但是只出现过一次的彗星继续以它们出现的年份命名。
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet can be named after its discoverers, or an instrument or program that helped to find it.
研究历史History of study
Main article: Observational history of comets
早期观察及思考Early observations and thought
From ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia. Until the sixteenth century, comets were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.
从古代资源（例如中国甲骨）可以知道，人类已经注意到彗星已有数千年了。 直到十六世纪，彗星通常被是认为是国王或贵族死亡或即将来临的灾难的不祥之兆，甚至被解释为天上人对居民的攻击。 
Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days. Pliny the Elder believed that comets were connected with political unrest and death.
In India, by the 6th century astronomers believed that comets were celestial bodies that re-appeared periodically. This was the view expressed in the 6th century by the astronomers Varāhamihira and Bhadrabahu, and the 10th-century astronomer Bhaṭṭotpala listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were.
在印度，到6世纪，天文学家认为彗星是天体，会定期重新出现。 这是六世纪天文学家Varāhamihira和Bhadrabahu所表达的观点，而十世纪的天文学家Bhaṭṭotpala列出了某些彗星的名称和估计时期，但尚不清楚这些数字的计算方式或精确程度。[ 155]
In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth’s atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.
在16世纪，第谷·布拉赫（Tycho Brahe）通过测量1577年大彗星的视差，从地理上分开的观察者收集的观测数据中证明，彗星必须存在于地球大气之外。 在测量的精度范围内，这意味着彗星的距离必须至少是从地球到月球的距离的四倍。 
Further information: Observational history of comets § Orbital studies
Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of gravity must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet’s path through the sky to a parabolic orbit, using the comet of 1680 as an example.
艾萨克·牛顿（Isaac Newton）在1687年的《数学原理》（Principia Mathematica）中证明，在重力作用下移动的物体必须描绘出一个形状像圆锥曲线截面的轨道，并且他演示了如何使彗星穿过天空到达抛物线轨道， 以1680年的彗星为例。
In 1705, Edmond Halley (1656–1742) applied Newton’s method to twenty-three cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation caused by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758–9. Halley’s predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet’s 1759 perihelion to within one month’s accuracy. When the comet returned as predicted, it became known as Halley’s Comet (with the latter-day designation of 1P/Halley). It will next appear in 2061.
1705年，埃德蒙·哈雷（Edmond Halley，1656–1742年）将牛顿方法应用于1337年至1698年之间发生的23次彗星幻影。他指出，其中的三种，即1531年，1607年和1682年的彗星，具有非常相似的轨道元素， 他还能够解释由于木星和土星引起的引力扰动，它们的轨道略有不同。 他确信这三个幻影是同一颗彗星的三个外表，因此他预测它将在1758–9年再次出现。 哈雷的预计返回日期后来由三位法国数学家组成的小组进行了完善：亚历克西斯·克莱洛特，约瑟夫·拉兰德和妮可·雷尼·勒帕特，他们将彗星1759年近日点的日期预测在一个月的准确度之内。  当彗星如预期般返回时，它就被称为哈雷彗星（后来的名称为1P / Halley）。 它将在2061年出现。
物理特征的研究Studies of physical characteristics
Further information: Observational history of comets § Studies of physical characteristics
Isaac Newton described comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun. Newton suspected that comets were the origin of the life-supporting component of air.
艾萨克·牛顿（Isaac Newton）将彗星描述为紧凑且坚固的固体，它们在倾斜轨道中运动，而其尾部则是由其原子核发射，被太阳点燃或加热的稀薄蒸汽流。 牛顿怀疑彗星是维持生命的空气的起源。
As early as the 18th century, some scientists had made correct hypotheses as to comets’ physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion. In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley’s Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet’s orbit, and he argued that the non-gravitational movements of Encke’s Comet resulted from this phenomenon.
早在18世纪，一些科学家就彗星的物理成分做出了正确的假设。 1755年，伊曼纽尔·康德（Immanuel Kant）假设彗星由某种挥发性物质组成，它们的蒸发会导致它们在近日点附近的明亮显示。 1836年，德国数学家弗里德里希·威廉·贝塞尔（Friedrich Wilhelm Bessel）在1835年哈雷彗星出现期间观察到蒸汽流后，提出蒸发物质的喷射力可能足以显着改变彗星的轨道，他认为 Encke 彗星的引力运动是由这种现象引起的。
In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. This “dirty snowball” model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency‘s Giotto probe and the Soviet Union’s Vega 1 and Vega 2) that flew through the coma of Halley’s Comet in 1986, photographed the nucleus, and observed jets of evaporating material.
1950年，弗雷德·劳伦斯·惠普尔（Fred Lawrence Whipple）提出，彗星不是含有冰和冰的岩石，而是含有灰尘和岩石的冰冻物体。 这种“肮脏的雪球”模型很快就被接受，并得到了1986年飞过哈雷彗星昏迷状态的舰队舰队（包括欧洲航天局的Giotto探测器和苏联的Vega 1和Vega 2）的观察的支持。 ，拍摄了原子核，并观察到蒸发物质的喷射。
On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet Ceres, the largest object in the asteroid belt. The detection was made by using the far-infrared abilities of the Herschel Space Observatory. The finding is unexpected because comets, not asteroids, are typically considered to “sprout jets and plumes”. According to one of the scientists, “The lines are becoming more and more blurred between comets and asteroids.” On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).
2014年1月22日，欧洲航天局的科学家报告了在确定的时间内首次在小行星地带最大的天体矮星谷神星上发现水蒸气的情况。 利用赫歇尔太空天文台的远红外能力进行了探测。 这一发现是出乎意料的，因为通常将彗星而不是小行星视为“发芽的喷气流”。 一位科学家认为，“彗星和小行星之间的界线越来越模糊。”  2014年8月11日，天文学家首次使用阿塔卡马大型毫米/亚毫米阵列（ALMA）发布了研究，  详细介绍了HCN，HNC，H2CO和尘埃在彗星C / 2012 F6（Lemmon）和C / 2012 S1（ISON）彗星内的分布。 
See also: List of comets visited by spacecraft
- The Halley Armada describes the collection of spacecraft missions that visited and/or made observations of Halley’s Comet 1980s perihelion.
- 哈雷舰队（Halley Armada）描述了访问和/或观察哈雷1980年代彗星近日点的航天器飞行任务的集合。
- Deep Impact. Debate continues about how much ice is in a comet. In 2001, the Deep Space 1 spacecraft obtained high-resolution images of the surface of Comet Borrelly. It was found that the surface of comet Borrelly is hot and dry, with a temperature of between 26 to 71 °C (79 to 160 °F), and extremely dark, suggesting that the ice has been removed by solar heating and maturation, or is hidden by the soot-like material that covers Borrelly. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. The mission yielded results suggesting that the majority of a comet’s water ice is below the surface and that these reservoirs feed the jets of vaporised water that form the coma of Tempel 1. Renamed EPOXI, it made a flyby of Comet Hartley 2 on 4 November 2010.
- 深度撞击。 关于彗星中有多少冰的争论还在继续。 2001年，“深空1号”航天器获得了Borrelly彗星表面的高分辨率图像。 发现Borrelly彗星的表面炎热干燥，温度在26至71°C（79至160°F）之间，并且非常暗，表明冰已通过太阳加热和蒸发而除去，或者 被类似覆盖住Borrelly的类似煤灰的物质所掩盖。 2005年7月，“深度撞击”探测器在坦普尔1号彗星上炸了一个坑，以研究其内部。 该任务产生的结果表明，彗星的大部分水冰都在地表以下，这些水库为形成坦普尔1号昏迷的汽化水流提供了养分。 它更名为EPOXI，于2010年11月4日飞越了哈特利2号彗星。
- Stardust. Data from the Stardust mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been “born in fire”, at extremely high temperatures of over 1,000 °C (1,830 °F). Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk. As a result, comets also contain crystalline grains that formed in the early, hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the “comet dust resembles asteroid materials”. These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.
- 星尘。 来自“星尘”任务的数据表明，从“野生2号”尾部取回的材料是结晶的，只能在超过1000°C（1,830°F）的极高温度下“生于火中”。  尽管彗星在外太阳系中形成，但人们认为在太阳系早期形成过程中物质的径向混合已经在整个原行星盘中重新分布了物质。 结果，彗星还包含在早期高温内部太阳系中形成的晶粒。 这在彗星光谱以及样品返回任务中都可以看到。 更近些时候，检索到的材料表明“彗星尘埃类似于小行星物质”。 这些新结果迫使科学家重新思考彗星的性质及其与小行星的区别。
- Rosetta. The Rosetta probe orbited Comet Churyumov–Gerasimenko. On 12 November 2014, its lander Philae successfully landed on the comet’s surface, the first time a spacecraft has ever landed on such an object in history.
- 罗塞塔。 罗塞塔号探测器绕过楚留莫夫–格拉西缅科彗星。  2014年11月12日，它的着陆器Philae成功降落在彗星表面上，这是航天器首次降落在彗星表面上。
Main article: Great comet
Woodcut of the Great Comet of 1577
Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as great comets. Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet’s brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so. Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.
大约每十年一次，一颗彗星就会变得足够明亮，以至于偶然的观察者会注意到，导致这种彗星被指定为大彗星。 众所周知，预测彗星是否会变成大彗星非常困难，因为许多因素可能导致彗星的亮度与预测值大相径庭。 广义上讲，如果一颗彗星具有大而活跃的核，它将靠近太阳，并且在最亮时从地球上看不到太阳，那么它就有可能成为一颗大彗星。 但是，1973年的柯欧特克彗星满足了所有条件，并有望成为壮观，但未能做到。 三年后出现的西彗星，期望值低得多，但却成为令人印象深刻的彗星。
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession—Comet Hyakutake in 1996, followed by Hale–Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.
20世纪末期出现了很长的间隙，没有出现任何大彗星，随后又相继出现了两颗彗星——Comet Hyakutake（1996年彗星），Hale-Bopp，在两年前被发现的1997年达到最大亮度。 21世纪的第一颗大彗星是C / 2006 P1（麦克诺德），于2007年1月被肉眼观察者看到。这是40多年来最亮的彗星。
Comet C/2006 P1 (McNaught) taken from Victoria, Australia 2007
Main article: Sungrazing comet
A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres. Although small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.
晒太阳的彗星是在近日点上非常接近太阳的彗星，通常在几百万公里之内。 尽管在接近太阳的过程中小型放牧者可以完全蒸发，但是大型放牧者可以在许多近日点通行证中幸存。 然而，他们所经历的强大的潮汐力量常常导致其分裂。
About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner Solar System. The remainder contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.
用SOHO观测到的放牧者中约有90％是Kreutz组的成员，它们全部来自一个巨大的彗星，该彗星在其第一次穿过内部太阳系时分裂成许多较小的彗星。 其余的包含一些零星的放牧者，但其中还发现了其他四个相关的彗星组：Krakt，Kracht 2a，Marsden和Meyer组。 Marsden和Kracht组似乎都与96P / Machholz彗星有关，Comet 96P / Machholz也是两个流星流的象限，象限和阿里流经。
Euler diagram showing the types of bodies in the Solar System.
Of the thousands of known comets, some exhibit unusual properties. Comet Encke (2P/Encke) orbits from outside the asteroid belt to just inside the orbit of the planet Mercury whereas the Comet 29P/Schwassmann–Wachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn. 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed. Similarly, Comet Shoemaker–Levy 2 was originally designated asteroid 1990 UL3. (See also Fate of comets, above)
在成千上万的已知彗星中，有些表现出不同寻常的特性。 Encke彗星（2P / Encke）从小行星带外绕行到水星行星的轨道内，而29P / Schwassmann–Wachmann彗星目前正以近似圆形的轨道行进，完全在木星和土星的轨道之间。 2060年Chiron的不稳定轨道位于土星和天王星之间，最初被归类为小行星，直到注意到彗发。 同样，“鞋匠-征途2”彗星最初被定为1990 UL3小行星。 （另请参见上述彗星的命运）
Main article: Centaur (minor planet)
Centaurs typically behave with characteristics of both asteroids and comets. Centaurs can be classified as comets such as 60558 Echeclus, and 166P/NEAT. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet despite its orbit, and 60558 Echeclus was discovered without a coma but later became active, and was then classified as both a comet and an asteroid (174P/Echeclus). One plan for Cassini involved sending it to a centaur, but NASA decided to destroy it instead.
半人马通常具有小行星和彗星的特征。 半人马可以归类为彗星，例如60558 Echeclus和166P / NEAT。 166P / NEAT被发现时有彗发状态，尽管它的轨道不像一般彗星那样但仍被定义为彗星，而60558 Echeclus被发现没有彗发，但后来变得活跃，然后被分类为彗星和小行星（ 174P / Echeclus）。 卡西尼号的一个计划包括将其送入半人马座，但美国宇航局决定改为销毁它。
A comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as SOHO. SOHO’s 2000th comet was discovered by Polish amateur astronomer Michał Kusiak on 26 December 2010 and both discoverers of Hale–Bopp used amateur equipment (although Hale was not an amateur).
彗星可以使用广角望远镜在摄影上发现，也可以通过双筒望远镜在视觉上发现。 然而，即使没有光学设备，业余天文学家仍然有可能通过下载某些卫星观测站（如SOHO）积累的图像来在线发现一个放牧彗星。 波兰业余天文学家MichałKusiak于2010年12月26日发现了SOHO的第2000颗彗星，发现者黑尔-波普也使用了业余设备（尽管黑尔不是业余爱好者）。【？？】
Main article: Lost comet
A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a “new” comet is discovered, and calculation of its orbit shows it to be an old “lost” comet. An example is Comet 11P/Tempel–Swift–LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001. There are at least 18 comets that fit this category.
在前几十年或上个世纪发现的许多周期性彗星现在已消失。 他们的轨道从来都不为人所知，无法预测未来的出现或彗星已经分解。 但是，偶尔会发现“新”彗星，并且对其轨道的计算表明它是旧的“迷失”彗星。 一个例子是彗星11P /坦普尔–斯威夫特–线性彗星，它于1869年发现，但由于木星的扰动而在1908年之后无法观测。 直到2001年LINEAR意外地重新发现它为止。 至少有18个符合此类别的彗星。
大众文化In popular culture
The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change. Halley’s Comet alone has caused a slew of sensationalist publications of all sorts at each of its reappearances. It was especially noted that the birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain (who correctly speculated that he’d “go out with the comet” in 1910) and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song “Halley Came to Jackson“.
流行文化中对彗星的描绘牢固地植根于西方悠久的传统中，即将彗星视为厄运的预兆和改变世界的预兆。 哈雷的彗星在每次再现时都引起了大量轰动性的出版物。 特别要指出的是，一些著名人物的出生和死亡与彗星的出现相吻合，例如作家马克·吐温（马克·吐温（Mark Twain，他正确地推测他会在1910年“与彗星一起离开”）和尤多拉） Welty，Mary Chapin Carpenter为他的一生奉献了“ Halley Came to Jackson”这首歌。
In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley’s Comet in 1910, the Earth passed through the comet’s tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, whereas the appearance of Comet Hale–Bopp in 1997 triggered the mass suicide of the Heaven’s Gate cult.
在过去的时代，明亮的彗星通常会激发普通民众的恐慌和歇斯底里，被认为是不好的预兆。 最近，在1910年哈雷彗星通过时，彗尾传过来地球，错误的报纸报道激起了人们的恐惧，担心尾巴中的氰可能会毒害数百万人，而黑尔-波普彗星在1997年的出现触发了 天门邪教组织的大规模自杀。
In science fiction, the impact of comets has been depicted as a threat overcome by technology and heroism (as in the 1998 films Deep Impact and Armageddon), or as a trigger of global apocalypse (Lucifer’s Hammer, 1979) or zombies (Night of the Comet, 1984). In Jules Verne‘s Off on a Comet a group of people are stranded on a comet orbiting the Sun, while a large manned space expedition visits Halley’s Comet in Sir Arthur C. Clarke‘s novel 2061: Odyssey Three.
在科幻小说中，彗星的影响被描述为技术和英雄主义克服的威胁（如1998年的电影《深度冲击》和《世界末日》），或者是全球启示录的触发者（路西法的哈默，1979年）或僵尸（夜之星）。 彗星，1984年）。 在儒勒·凡尔纳的《彗星起飞》中，一群人被困在绕太阳公转的彗星上，而大型载人太空探险队则访问了亚瑟·克拉克爵士的小说《 2061：奥德赛三号》中的哈雷彗星。