61 A Light Blazing Across the Sky

When you look up at the night sky, what do you see? There are other heavenly bodies out there besides the moon and stars. One of the most fascinating of these is a comet.

Comets were formed around the same time the Earth was formed. Usually about five to ten miles across, they are made up of ice and other frozen liquids and gases. Now and then these “dirty snowballs” begin to orbit the sun, just as the planets do. Scientists know of over one hundred comets in orbits that sometimes bring them fairly close to Earth.

As a comet gets closer to the sun, some of the gases in it begin to unfreeze. The gases combine with dust particles from the comet to form a cloud thousands of miles across. As the comet gets even nearer to the sun, a solar wind blows the cloud behind the comet, thus forming its tail. The tail and the generally fuzzy atmosphere around a comet are characteristics that can help you to identify this phenomenon in the night sky.

In any given year, about a dozen known comets come close to the sun in their orbits. There may also be a dozen or more newly discovered comets. The average person can’t see them all, of course. Usually there are only one or two a year bright enough to be seen with the naked eye. Comet Hale-Bopp, discovered in 1995, was an unusually bright comet. Its orbit brought it relatively close to Earth—within 122 million miles of it. But Hale-Bopp came a long way on its earthly visit. It won’t be back in this vicinity for another four thousand years or so.

当你仰望夜空时，你会看到什么？除了月亮和星星，还有其他天体存在，其中最迷人的之一就是彗星。

彗星的形成时间与地球大致相同。它们的直径通常在5到10英里之间，由冰、其他冻结的液体和气体组成。这些“脏雪球”偶尔会像行星一样开始绕太阳运行。科学家们已知有超过一百颗彗星的轨道有时会让它们离地球相当近。

当彗星靠近太阳时，它内部的一些气体会开始解冻。这些气体与彗星上的尘埃颗粒结合，形成一个直径达数千英里的云团。当彗星离太阳更近时，太阳风会将云团吹到彗星后方，从而形成彗尾。彗尾和彗星周围通常模糊的大气层，是能帮助你在夜空中识别这种现象的特征。

在任何一年里，大约有十二颗已知的彗星会在轨道上靠近太阳，也可能会有十二颗或更多新发现的彗星。当然，普通人不可能看到所有彗星——通常每年只有一两颗亮到能用肉眼看到。1995年发现的海尔-波普彗星是一颗异常明亮的彗星，它的轨道让它离地球相对较近——距离地球不到1.22亿英里。但海尔-波普彗星这次造访地球的旅程很远，它要再过大约四千年才会回到这一区域。