# Eli5: How do objects entering the earth atmosphere catch fire? is it from travelling to quick?

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Eli5: How do objects entering the earth atmosphere catch fire? is it from travelling to quick?

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As far as I understand, yes. Specifically entering the atmosphere at seen causes shock waves that heats up the object.

Not the friction of the air, but the concussion waves caused by pounding into the atmosphere.

It is because they’re going too fast, but that doesn’t explain why going fast makes things hot. Sometimes people say friction, but that’s not right.

Air has very little density, so little that you mostly can’t feel it while moving through it, but it’s still molecules with mass. If you’re going at the speeds objects reach while dropping from orbit, that mass matters. The heat is mostly from slamming into air faster than air can easily get out of the way, called “adiabatic compression.”

Objects coming in from space aren’t just falling, they’re orbiting the Earth and/or the Sun at a phenomenal speed, tens of thousands of miles an hour.

When you slam into the atmosphere at hypersonic speed the gas molecules in the way can’t just move aside, they’re getting rammed into a compression wave ahead of the object.

When you compress a gas it gets warmer, and when you compress a gas at hypersonic speed it gets *really* warm – hot enough to incinerate small objects.

They don’t necessarily catch fire, but they do get very hot (and could catch fire, melt, explode, or do whatever they do when they get that hot).

When something falls from space, it can build up a LOT of speed because until it hits the atmosphere, there’s nothing in the way to slow it down. Eventually, though, it hits the atmosphere and there’s a bunch of gas in the way trying to slow it down. The object is moving so fast, that the air can’t move out of the way fast enough and bunches up in front of it and causes lots of friction and shockwaves (sonic booms like supersonic fighter jets), and all this rubbing against the object heats it really fast (just rub your hands together to feel them warm up; imagine being able to move those hands at tens of thousands of miles an hour).

So, all that energy the object is using to punch through the air becomes heat, and the object can get very hot. It could break apart, catch fire, explode … whatever an object might do if it gets really hot, is being tossed about, and is punching it’s way through a wall of massively compressed air.