# Propulsion in Space

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I know this is probably blatently obvious, but how does propulsion from engines work in space. Does propulsion (such as in the space shuttle) come from the exhaust/stuff coming out the back pushing against the air and ground molecules (or ground when it takes off) and then action/reaction causes it to be pushed forward?

If so, in space, there are much fewer particles so how does the propulsion work?

As a side note : in space how many particles are there usually (or is it a complete empty vacuum?)

I'm confused.

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You dont need to "push against" anything. In the case of the space shuttle for example, it fires its rockets, and propells burning gass out at high speed. Since every action has an equal and opposite reaction (Newtons 3rd law) this pushes the shuttle forward.

Another example is a gun. You are holding a rifle, and you shoot it, bang. Bullet flies out the barrell at high velocity. The gun flies backwards with an equal force, which is why you feel kickback.

Rockets dont need to "push" against the air, the ground or anything else to function.

As for how many particles are in space it depends on how close to a planet you are. 100km up is generally considered "space" but there would be far more particles there than in the space between stars.

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']You dont need to "push against" anything. In the case of the space shuttle for example' date=' it fires its rockets, and propells burning gass out at high speed. Since every action has an equal and opposite reaction (Newtons 3rd law) this pushes the shuttle forward.

Another example is a gun. You are holding a rifle, and you shoot it, bang. Bullet flies out the barrell at high velocity. The gun flies backwards with an equal force, which is why you feel kickback.

Rockets dont need to "push" against the air, the ground or anything else to function.

As for how many particles are in space it depends on how close to a planet you are. 100km up is generally considered "space" but there would be far more particles there than in the space between stars.[/quote']

Oh I see. The burning gasses is the action, not the push on other molecules. If you were, say, between galaxies, would there ever be no particles at all or are there always some small amount?

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'']Another example is a gun. You are holding a rifle, and you shoot it, bang. Bullet flies out the barrell at high velocity. The gun flies backwards with an equal force, which is why you feel kickback.

In fact, if you were to fire a gun in space, you would be blown backwards uncontrollably--there'd be nothing to stop you.

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i was just told in a thread i started, that apparently particle/anit-particle pairs are constantly being created and destroyed in the vacuum of space, so i suppose that means that there are a certain number of particles wherever you are.

Just thinking about it, if a particle has enough energy to escape earths gravity, then it will travel off into space, i am sure that must be happening quite a lot, so i suppose there must be particles from some source in deepest space.

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