Astronomy

Despite The Fireball, Meteorites Aren't Hot

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What happens when a meteorite hits Earth? If movies are to be believed, a meteorite is a fiery death ball that hurtles through the air with alarming speed until it slams into the ground in an explosion of molten rock and flaming debris that creates an impact crater where it finally rests, red-hot and smoldering. According to science, however, the reality is a little less exciting: meteorites aren't even hot when they hit the ground.

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What's Really Going On

First things first: space is cold. Really cold. Barely three degrees above absolute zero cold. That means that the chunks of rock we know as meteoroids are in a deep freeze. When that meteoroid begins hurtling through our atmosphere (and becomes a meteor), they begin to glow with such brilliance that they're visible for hundreds of miles. But it's not the meteor itself that glows; it's the air around it, through something called ram pressure—that is, the compression in the air caused by the meteor's forward motion. Compression makes air heat up, and that's why a falling meteor looks red-hot.

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Still, hot air must heat up the meteor, right? Right! But only the outside. The outer layers can get so hot that they melt, but they're quickly blown off in a phenomenon known as ablation. That leaves behind un-melted rock that's just warm, at best. There's also the fact that although a meteor's speed peaks at thousands of kilometers per hour, all that air pressure slows it down considerably before it hits the ground, giving it—and the air around it—plenty of time to cool down. Once it lands (and becomes a meteoroid), it's a comfortably lukewarm temperature.

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Why This Matters

Knowing what happens to a meteor as it enters Earth's atmosphere helps you understand what happens to a spacecraft when it does the same thing. Spacecraft actually take advantage of the same ablation phenomenon that meteorites experience to protect their delicate equipment from getting too hot. Modern heat shields are large rounded surfaces covered in a material designed to burn off like the outer layers of a meteor, leaving the cargo and people inside just like that meteor—comfortably warm, not molten hot.

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