Why burn up on entering Earth's atmosphere?

10 April 2011

Question

Why do things burn up on entering Earth's atmosphere?

Answer

Dave - The main reason why things heat up when they hit the Earth's atmosphere is they've got huge amounts of kinetic energy - they're going incredibly fast. When they bash into the Earth's atmosphere, most of the heating is actually because the air they bash into hasn't got time to get out of the way, so the air gets compressed; and when you compress air, it gets hotter. You may have noticed this if you've ever pumped up a bicycle tyre very, very quickly: the end of the pump gets hot.

So, the air in front of the inbound object - such as a meteor or even an asteroid - heats up, and that starts to erode the surface of the object and you get this tail of hot, burning material, which you see as a shooting star. With very small things, because the friction is so much larger compared to their mass, they tend to lose their speed much more gently very high up in the atmosphere, so they slow down more gently and don't get as hot. And once they slow down enough, they just drift down like dust through the atmosphere. So, it is conceivable that something like bacteria clinging to a small dust grain could survive re-entry from space, whereas a big lump of rock would melt very quickly.

Comments

Speed would have nothing to do with it. Ever heard of terminal velocity? Due to the Earth's gravity, objects will reach a maximum speed which will never increase further. If you were to jump off a 30 story building you would reach terminal velocity before you reach the ground, but you wouldn't burn up in the atmosphere while doing it. Your explanation doesn't make any factual sense.
Bike pumps get hot due to friction. Ever heard of rubbing two sticks together?

Annie, YOU are quite wrong, I'm afraid. The effect being described here is called "adiabatic heating" and is the same principle by which a diesel engine ignites fuel. You might want to go and look that up...

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