Comets

Comets are small asteroid-like bodies when they are far from the Sun, traveling in highly ellipical orbits about the Sun. When they sweep in close to the Sun, dramatic changes occur as they brighten and develop an extended tail. The nucleus is widely described as a "dirty snowball" composed of ice and some rocky debris.

当彗星远离太阳时，它们看起来像小的陨石体，沿着高度椭圆的轨道绕太阳运行。当它们接近太阳时，会发生剧烈的变化，它们会变亮并形成长长的尾部。彗核通常被描述为一个‘脏雪球’，由冰和一些岩石碎片组成。

The lighter ionized gas atoms of the ion tale cause it to point outward, directly away from the Sun, because the influence of the solar wind is dominant. I take the lower, more focused part of the tail in the image above to be the ion tail. The dust tail is made up of more massive particles and the role of gravity is important. If particles influenced by gravity are moved to an orbit further from the Sun, their radial direction falls behind that of the nucleus of the comet because their orbital period will be longer. The upper part of the Halley image would then appear to be the dust tail - you can see a slight curvature. In its most visible phase close to the Sun, the comet has a small solid nucleus and a ball of gas around it called the coma. Comas have been found to be on the order of 100,000 km in diameter at their maximum size, comparable to the largest planets. Most aof the light reflection is from the coma. Surrounding the coma and the visible tails is a hydrogen envelope which may extend millions of kilometers. The light from comets is purely reflected light; like the planets, the comets produce no light of their own.

较轻的离子化气体原子导致离子尾指向 outward，直接远离太阳，因为太阳风的影响占主导地位。我上面图像中较低、更集中的部分被认定为离子尾。尘埃尾由更重的粒子组成，重力的作用很重要。如果受重力影响的粒子被移动到更远离太阳的轨道，它们的径向方向会落后于彗核的方向，因为它们的轨道周期会更长。因此，Halley彗星图像的上部则可能是尘埃尾——你可以看到轻微的弯曲。当彗星接近太阳时，其最明显的阶段是一个小的固体核心和围绕它的气体球，称为 coma。Coma 的最大直径约为 100,000 公里，与最大的行星相当。大部分光反射来自 coma。围绕 coma 和可见的尾部的是一个氢包层，可能延伸数百万公里。彗星的光是纯粹的反射光；像行星一样，彗星自身不发光。

Current models of the nuclei of comets view them as balls of loosely packed ices, a cold mixture containing gas and dust. The dust is thought to be trapped in a mixture of methane, ammonia, and water ice. The smaller moons of the outer solar system are similar in constitution. Since they spend most of their time far from the Sun, their temperatures are thought to be a few tens of kelvins. Chaisson & McMillan suggest a core temperature of 200K and a surface temperature on the order of 350K for Halley when it made its close approach to the Sun.

目前关于彗核的模型将其视为由松散结合的冰组成的一个球体，这是一种含有气体和尘埃的冷混合物。尘埃被认为被甲烷、氨和水冰的混合物所捕获。外太阳系的较小卫星在构成上类似。由于它们大部分时间远离太阳，因此被认为其温度在几十开尔文范围内。Chaisson & McMillan认为哈雷彗星在接近太阳时，其核心温度为200K，表面温度约为350K。

The short-period comets (less than 200 years) are thought to originate in a region of the solar system out past the orbit of Neptune called the Kuiper belt (30 to 100 AU). Most of them are found to have prograde orbits (in the same orbital direction as the planets) and to be close to the ecliptic plane. The Kuiper belt is described as a region of asteroid-like comets, most of which travel in roughly circular orbits. It may be that occasional close encounters between comets or the cumulative gravitational pull of the outer planets brings one into the higly elliptical orbit which brings it close to the Sun.

短周期彗星（少于200年）被认为起源于太阳系中位于海王星轨道之外的区域，称为柯伊伯带（30到100个天文单位）。大多数彗星的轨道方向与行星相同，且接近黄道平面。柯伊伯带被描述为一个类似小行星的彗星区域，其中大多数彗星的轨道大致是圆形的。可能由于彗星之间的偶尔近距离相遇，或者外行星的累积引力作用，使它们进入高度椭圆的轨道，从而接近太阳。

Other comets, characterized as "long-period comets", are found in random orientations with respect to the ecliptic plane. They are thought to originate in a large "cloud" of objects in a region perhaps 50,000 AU from the Sun called the Oort cloud.

其他彗星，被称为‘长周期彗星’，其轨道相对于黄道面是随机取向的。人们认为它们起源于一个可能位于太阳约50000天文单位之外的广阔‘云’中，这个区域被称为奥尔特云。

虽然最著名的彗星是哈雷彗星，但最近也有一些有趣的相遇，例如与吉奥博尼-欣纳彗星的相遇，以及舒梅克-列维9号彗星撞击木星的事件。