When you look up at the night sky, what do you see? There are other（26） bodies out therebesides the moon and stars.One of the most（27） of these is a comet (彗星).
Comets were formed around the same time the Earth was formed.They are（28） ice and otherfrozen liquids and gases.（29）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（30）dust particlesfrom the comet to form a huge cloud.As the comet gets even nearer to the sun, a solar wind blows thecloud behind the comet, thus forming its tail.The tall and the（31）fuzzy (模糊的) atmospherearound a comet are（32）that can help identify this（33）in the night sky.
In any given year, about a dozen known comets come close to the sun in their orbits.The averageperson can't see them all, of course.Usually there is only one or two a year bright enough to be seenwith the（34 ）eye.Comet Hale-Bopp, discovered in 1995, was an unusually bright comet.Its orbitbrought it（35）close to the Earth, within 122 million miles of it.But Hale-Bopp came a long way onits earthly visit.It won't be back for another four thousand years or so.
