Comets
2012级（4）班[毕业]
陈天明
2012-12-09 00:11
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英语作文
Comets are small bodies of nonvolatile grains and frozen gases.Their orbits bring them very close to the Sun and deep into space, often beyond the orbit of Pluto.They have different structures,but they all develop a surrounding cloud of material that usually grows in size and brightnessas they approach the Sun.Usually,there is a bright nucleus in the middle of this material.The material and the nucleus form the head of a comet.As comets approach the Sun,comes develop tails of luminous material that extend for millions of kilometers from the head.