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This question already has an answer here:

Why is space a vacuum? Also, why isn't the air from the Earth escaping due to the vacuum in space? The ozone layer is only gas and a magnetic field, so why doesn't the air escape from Earth into space?

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marked as duplicate by ACuriousMind, Community Aug 24 '15 at 17:16

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Space isn't a true vacuum, just really really sparsely populated with atoms. $\endgroup$ – Kyle Kanos Aug 24 '15 at 0:15
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    $\begingroup$ While certainly related, the proposed duplicate is asking about the drag of objects in space while this question is asking about the escape of particles due to the supposed vacuum. $\endgroup$ – Kyle Kanos Aug 24 '15 at 13:13
  • $\begingroup$ I deleted a couple comments that were actually attempts to answer the question. $\endgroup$ – David Z Aug 24 '15 at 19:03
  • $\begingroup$ @DavidZ Please restore my comment, and in the future, don't second-guess commenters' judgement as to whether or not their comments actually answer the question. (Actually, in the future, just stop deleting comments. The "ephemeral" comment threads are the single worst thing about SE as it is right now. :-P) $\endgroup$ – zwol Aug 25 '15 at 16:38
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    $\begingroup$ @zwol nope, not going to happen. It's actually irrelevant whether or not the comments answered the question; comments are always ephemeral. That's part of the core principle of the SE network, so if you want to contribute somewhere where all your comments will stick around, I'd suggest looking for another site, like a traditional forum. (If you want access to the text of the comment so you can post it as an answer, ask on Physics Meta, just don't make a habit of it.) $\endgroup$ – David Z Aug 25 '15 at 23:36
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Gravity.

You can think of planets like wells or deep holes in the ground (gravity wells). Denser things fall to the bottom (rocks), less dense things rest on top of that (water), even less dense things on top of that (air), and finally the least dense thing on top of everything (vacuum).

The air, for the most part, isn't leaving the planet for the same reason the water isn't flying out of the ocean, gravity is holding it down.

It's important to remember that space doesn't suck. It's not that kind of vacuum. There is not a force pulling things into space in the same way there is not a force pulling air out of a hole in a balloon.

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The typical speed of an air molecule is a few hundred meters per second, while escape velocity from Earth is over 10,000 meters per second. So almost all the air molecules just fall back down. They're affected by gravity just like everything else!

We do lose some air molecules this way, though. In particular, hydrogen and helium are lighter, so they move faster and escape significantly more often. This is why our atmosphere has very little of those two. But Jupiter, which has much stronger gravity, has an atmosphere mostly of hydrogen and helium.

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    $\begingroup$ The solar wind also has a say in stripping the Earth's atmosphere - it can easily accelerate air molecules to escape velocities. Of course, it affects light molecules the most, and the lightest tend to be the highest as well, both conspiring to hydrogen and helium leaking a lot more than the heavier gasses. $\endgroup$ – Luaan Aug 24 '15 at 7:42
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    $\begingroup$ @Luaan fortunately our magnetic field blocks most of the solar wind. $\endgroup$ – Paŭlo Ebermann Aug 24 '15 at 16:26
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Air fails to escape into space for the same reason you fail to: gravity. As noted in Kevin's answer, occasionally some do get going fast enough to escape. You would too, if enough stuff hit you hard enough. :)

Space is a vacuum (for some definition of vacuum), because vacuum is simply the absence of air/gas pressure, and there aren't enough gas molecules in space to create a noticeable pressure.

That said, note that there are particles in space; they are moving, and they do exert pressure on things they hit. See e.g. solar wind and heliopause. So depending on what/where you're measuring, space is far from a vacuum.

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To give a simple answer:

Space is an (almost) vacuum, since there is simply not enough air left to be there.

Where did all this air go? Due to gravity the air is attracted to large objects, like planets and exactly this gravity is also keeping the air close to the object - preventing it from "flowing back into space"

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  • $\begingroup$ So gravity keeps air in by dragging it down doesn't that mean that other gasses and star dust from the solar system can enter earth in the same way? And if so will it increase the mass of the planet in time? $\endgroup$ – Malachi Daniel Oct 27 '15 at 11:07
  • $\begingroup$ Correct, but space is really empty, so there is almost no gas no enter earth. On the other hand some air is able to exit the earths atmosphere (see Kevins answer) $\endgroup$ – Nijin22 Oct 28 '15 at 11:45
  • $\begingroup$ So after some time will the earth be starved from air because air escapes the atmosphere? (And trees that provide the oxygen are being chopped down every day) $\endgroup$ – Malachi Daniel Oct 28 '15 at 11:57

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