Eli5: How do objects entering the earth atmosphere catch fire? is it from travelling to quick?
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So…”catching fire” is a term of art, here, obviously, bit it has a real meaning that’s important to contrast against “getting really hot”. Everyone else has done a fairly good job of explaining why stuff falling down from space gets really hot. Kudos to them. However, stuff falling down from space doesn’t really “catch fire”. Reason being is that it’s moving too fast while it’s hot enough and it’s cold once it’s moving slow enough. To make fire, you need 3 things: oxidizer, fuel and heat. Oxidizer is usually pretty easy: in the atmosphere oxygen is pretty much everywhere. But, at high enough speeds, the objects aren’t exposed to any particular oxygen long enough to combust with it; they just fly past each other. Fuel: this is tye object itself when we’re talking about stuff falling in from space. No problems there. Heat: and here’s where it gets tricky. Nearly everything has an “auto-ignition” temperature or a temperature at which you don’t have to apply a flame to it to get it to start combustion, its just hot enough to combust on its own. And, for a lot of things, Atmospheric reentry temperatures get into this range, but…the thing is moving too fast to “collect” the oxygen to combust with. As it slows down, it becomes more able to react with the oxygen, but it’s also shed so much of the heat from reentry that it’s no longer above the auto-ignition temperature. So…all we really get is stuff falling in that emits tons of thermal radiation (is hot and glowy), but that isn’t actually “on fire”. The next chance for it to be on fire is literally when it hits the ground, and well…the odds ain’t great.
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