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.
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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.
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.
Great explanations so far, but to put things into perspective: the speed required for an object (or satellite or spacecraft) to remain in earth orbit is 18,000 mph. That speed’s obtainable in the near-vacuum of earth orbit, since there’s no air resistance to speak of. When a craft re-enters, it speeds up even more since it has let gravity capture it more effectively.
The world-record re-entry speed IIRC was Apollo 10, returning from the moon, and not just earth orbit – it hit the atmosphere at just shy of 24,000 mph. Those are the kinds of speeds none of us even have a mental benchmark for, it’s tough to intuitively understand just how fast that is. The highest velocity rifle bullets are under 3,000mph, not much compared to re-entry speeds.
Most of those objects are moving VERY fast and slowing down VERY quickly. All of that kinetic energy has to go somewhere. Most of it heats up the gas behind the shock, some is turned into radiation (light), and a few percent end up deposited on the surface. Even that few percent is enough to make most things burn when there is oxygen present.
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