When you look up at the night sky, what do you see? There are other【B1】___bodies out there besides the moon and stars. One of the most【B2】___of these is a comet(彗星).

Comets were formed around the same time the Earth was formed. They are【B3】___ice and other frozen liquids and gases.【B4】___these “dirty snowballs” begin to orbit the sun, just as the planets do.

As a comet gets closer to the sun, some gases in it begin to unfreeze. They【B5】___dust particles from the comet to form a huge cloud. 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【B6】fuzzy(模糊的)atmosphere around a comet are【B7】that can help identify this【B8】___in the night sky.

In any given year, about a dozen known comets come close to the sun in their orbits. The average person can’t see them all, of course. Usually there is only one or two a year bright enough to be seen with the【B9】___eye. Comet Hale-Bopp, discovered in 1995, was an unusually bright comet. Its orbit brought it【B10】___close to the Earth, within 122 million miles of it. But Hale-Bopp came a long way on its earthly visit. It won’t be back for another four thousand years or so.

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