Comets are among the most interesting and unpredictable bodies in the solar system. They are made of frozen gases (water vapor, ammonia, methane, carbon dioxide, and carbon monoxide) that hold together small pieces of rocky and metallic materials. Many comets travel in very elongated orbits that carry them far beyond Pluto. These long-period comets take hundreds of thousands of years to complete a single orbit around the Sun. However, a few short-period comets (those having an orbital period of less than 200 years), such as Halley’s Comet, make a regular encounter with the inner solar system.

When a comet first becomes visible from Earth, it appears very small, but as it approaches the Sun, solar energy begins to vaporize the frozen gases, producing a glowing head called the coma. The size of the coma varies greatly from one comet to another. Extremely rare ones exceed the size of the Sun, but most approximate the size of Jupiter. Within the coma, a small glowing nucleus with a diameter of only a few kilometers can sometimes be detected. As comets approach the Sun, some develop a tail that extends for millions of kilometers. Despite the enormous size of their tails and comas, comets are relatively small members of the solar system.

The observation that the tail of a comet points away from the Sun in a slightly curved manner led early astronomers to propose that the Sun has a repulsive force that pushes the particles of the coma away, thereby forming the tail. Today, two solar forces are known to contribute to this formation. One, radiation pressure, pushes dust particles away from the coma. The second, known as solar wind, is responsible for moving the ionized gases, particularly carbon monoxide. Sometimes a single tail composed of both dust and ionized gases is produced, but often two tails—one of dust, the other, a blue streak of ionized gases—are observed.

As a comet moves away from the Sun, the gases forming the coma recondense, the tail disappears, and the comet returns to distant space. Material that was blown from the coma to form the tail is lost from the comet forever. Consequently, it is believed that most comets cannot survive more than a few hundred close orbits of the Sun. Once all the gases are expelled, the remaining material—a swarm of tiny metallic and stony particles—continues the orbit without a coma or a tail.

Comets apparently originate in two regions of the outer solar system. Most short-period comets are thought to orbit beyond Neptune in a region called the Kuiper belt, in honor of the astronomer Gerald Kuiper. During the past decade over a hundred of these icy bodies have been discovered. Most Kuiper belt comets move in nearly circular orbits that lie roughly in the same plane as the planets. A chance collision between two comets, or the gravitational influence of one of the Jovian planets—Jupiter, Saturn, Uranus, and Neptune—may occasionally alter the orbit of a comet in these regions enough to send it to the inner solar system and into our view.

Unlike short-period comets, long-period comets have elliptical orbits that are not confined to the plane of the solar system. These comets appear to be distributed in all directions from the Sun, forming a spherical shell around the solar system, called the Oort cloud, after the Dutch astronomer Jan Oort. Millions of comets are believed to orbit the Sun at distances greater than 10,000 times the Earth-Sun distance. The gravitational effect of a distant passing star is thought to send an occasional Oort cloud comet into a highly eccentric orbit that carries it toward the Sun. However, only a tiny portion of the Oort cloud comets have orbits that bring them into the inner solar system.

The most famous short-period comet is Halley’s Comet, named after English astronomer Edmond Halley. Its orbital period averages 76 years, and every one of its 30 appearances since 240 B.C. has been recorded by Chinese astronomers. When seen in 1910, Halley’s Comet had developed a tail nearly 1.6 million kilometers (1 million miles) long and was visible during daylight hours. Its most recent approach occurred in 1986.

彗星是太阳系中最有趣和最不可预测的天体之一。它们由冷冻气体(水蒸气、氨气、甲烷、二氧化碳和一氧化碳)组成，这些气体把小块岩石和金属材料粘合在一起。许多彗星以非常狭长的轨道运行，它们的轨道远远超出冥王星。这些长周期彗星需要数十万年时间才能完成围绕太阳的单一轨道。然而，一些短期彗星(轨道周期小于200年)，如哈雷彗星，却定期与内太阳系相遇。 当一颗彗星从地球上首先被看到时，它看起来很小，但当它接近太阳时，太阳能开始蒸发冻结的气体，产生一个发光的头部，称为慧发。慧发的大小因彗星而异。极罕见的慧星超过太阳的大小，但大多数接近木星的大小。在慧发里，有时可以探测到直径只有几公里的发光小核。当彗星接近太阳时，一些彗星形成了一条长达数百万公里的尾巴。尽管彗星的尾巴和彗发体积巨大，但它们是太阳系的相对较小的成员。 一颗彗星的尾巴以轻微弯曲的方式指向太阳反方向，这一观察使得早期天文学家提出，太阳有一种排斥力，将慧发的粒子推开，从而形成尾巴。今天，已知有两种太阳力量促成了彗星尾巴的形成。第一种是辐射压力，将尘埃粒子从慧发中推开。第二种被称为太阳风，负责移动电离气体，特别是一氧化碳。有时会产生一条由尘埃和电离气体组成的单一尾巴，但经常会观察到两条尾巴--一条是尘埃，另一条是蓝色条纹状的电离气体--。 当彗星离开太阳时，形成慧发的气体重新凝结，尾巴消失，彗星返回遥远的太空。从慧发中吹来形成尾巴的物质永远从彗星中消失。因此，人们认为，大多数彗星不能生存在太阳的超过几百个的近距轨道。一旦所有的气体被逐出，剩下的物质--一群微小的金属和石头粒子--继续在轨道上运行，而不会有慧发或尾巴。 彗星显然起源于外太阳系的两个区域。大多数短周期的彗星被认为是在海王星以外的一个叫做柯伊伯带的区域的轨道运行，这个区域的是以天文学家杰拉尔德•柯伊伯(Gerald Kuiper)的名字命名。在过去的十年里，这些冰体中有一百多个被发现。大多数柯伊伯带彗星的轨道几乎都是圆形的，与行星的平面大致相同。两颗彗星之间的偶然碰撞，或者木星、土星、天王星和海王星这些类木行星的重力引力影响，可能偶尔会改变彗星在这些区域的轨道，足以将其送入内太阳系并进入我们的视野。 与短周期彗星不同，长周期彗星的椭圆轨道并不局限于太阳系的平面。这些彗星似乎分布在太阳的各个方向，形成了一个围绕太阳系的球壳，称为奥尔特云，以荷兰天文学家简•奥特的名字命名。据信，数百万颗彗星绕太阳运行的距离是地球-太阳距离的10000倍。人们认为，一颗遥远的经过的星星的引力效应会将一颗偶然的奥尔特云彗星送入高度偏心的轨道，并将其带向太阳。然而，只有一小部分奥尔特云彗星有能将它们带入内太阳系的轨道。 最著名的短周期彗星是哈雷彗星，以英国天文学家爱德蒙•哈雷命名。它的轨道周期平均为76年，自公元前240年以来已经出现30次，每一次都被中国天文学家记录下来。当1910看到哈雷彗星时，它已经形成了一条长近一百六十万公里(一百万英里)的尾巴，在白天也可以看到。最近的一次出现发生在1986年。