# Eli5: why do all objects entering the earths atmosphere from space catch fire?

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Jus wondering like why asteroids and those detachable things from spaceships catch fire when entering earth

In: 5

So, space is more or less empty. No air to slow things down. Gravity, meanwhile, stretches very far away and objects pick up a lot of speed falling through all of that nothingness.

By the time they hit Earth’s atmosphere, they are moving so doggone fast that they *SLAM* into our atmosphere. They hit the air so hard that it gets super hot *just from how violently it was squished*. This superheated air scoops up some of the material from the object and creates a plume of “fire” around it.

When an object moves through the air, it pushes against the air particles and creates a force called drag. The faster an object moves, the more drag it experiences. As an object enters the Earth’s atmosphere from space, it is moving at a very high speed and encounters a lot of drag. This causes the object to heat up, which can sometimes cause it to catch fire. This process is called re-entry. The heat and fire caused by re-entry usually only last for a short time and will not cause any damage to the object as long as it is designed to withstand the heat.

Have you ever rubbed your hand together when you’re cold? it produce heat and feel good.

Well, it’s not only your hands. Whenever two things rub strongly against each other, it makes heat. On the other hand, it slows down the two things rubbing against each other: it turns the kinetic energy (movement) into calorific energy (heat). That’s what friction does.

The issues is that in space, it’s a big emptiness. So there is nothing to cause friction. So objects that move just keep their speed and produce no heat from friction. Once that object hits the atmosphere though, it’s really, really fast and now that it’s rubbing against all that air, it makes a lot of heat and start burning.

This explains it pretty well. I’m not even gonna assume I can add to it.

https://what-if.xkcd.com/28/

Particles in a gas or liquid are constantly bouncing around.

What we call “heat” is a measurement of how fast they’re bouncing around.

When something pushes them together, they hit each other more and bounce around faster.

Stuff hitting the air in Earth’s atmosphere hits lots of air particles really hard, which pushes them together really hard, which makes them bounce around really fast, which is another way of saying they get really hot.

This is an oversimplification but I think it conveys the basic idea: heat is a measurement of the kinetic energy of particles and compressing a given volume of particles increases that kinetic energy.

They don’t.

Many of them get really hot.

But getting hot is not the same as on fire.

The reason they get hot is that objects in space tend to move really fast compared to Earth and if you go really fast in the atmosphere friction will heat you up.

If you rub your hands together quickly they will heat up.

If you rub a rock moving many times the speed of sound against air it will heat up too.

Objects in space tend to be quite cold if they have been there for a while. So once they hit earth they may not actually be hot despite getting heated while coming down.

For things in orbit the main issue is that orbit means going really really fast around the world.

The International Space Station flies at extremely high speed around the planet. down here on Earth this would be bout two dozen times the speed of sound.

Any spacecraft departing from the space station or a similar obit will need to slow down a lot, before they can land.

You can slow a spacecraft with rockets, but that costs fuel and extra fuel is something you want to avoid with rockets.

So most spacecraft do most of their slowing down by hitting the planets atmosphere and letting the air slow it down.

If you are familiar with automobiles, you will know that breaks get hot when you use them.

Using the air to break works just like that. The air and the parts of the space craft it is rubbing against get heated up.

A lot!

This is why the shuttle had this big heat shield on its underside.

It could heat up with out transferring the heat to the astronauts inside and cooking them.

The whole rubbing against air while going really fast thing will not just heat up things, it will also rip them apart if they are not build quite right.

A rock being heated up and getting ripped apart by going through air really fast, may look like it is “burning up”, but there usually isn’t real fire involved.