十万个为什么·科学启蒙30篇（3）
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Why Do Comets Have Tails?
彗星为什么会有尾巴
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Comets are icy bodies that orbit the Sun in long, stretched-out paths.
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When a comet approaches the Sun, its surface warms up and releases gas and dust.
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Solar radiation and the solar wind push this material away from the nucleus.
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The glowing gas forms an ion tail, which always points directly away from the Sun.
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The dust tail curves slightly because tiny particles respond to both sunlight and gravity.
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These tails can stretch millions of kilometers but remain extremely thin and faint.
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A comet only develops visible tails when it is inside the orbit of Mars.
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Once it moves farther out again, the ice freezes solid and the tails vanish.
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This process repeats every time the comet returns on its orbital journey.
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So the tail isn’t left behind—it’s freshly made each time the comet gets warm.