# Rockets in space

## Main Question or Discussion Point

Once rockets leave the earth's atmosphere they would have a high velocity
so how come rockets use their engines when they're in space if there's no air resistance? The only reason I can see why they would need to use them is if they wanted to change direction.

## Answers and Replies

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The quick answer: It is not pushing against the air. It is pushing against its own fuel! This is due to the conservation of momentum.

Imagine you are on a frictionless ice rink with a big rock. (this is space... no air resistance = frictionless) Now, if you throw the rock to the west, you will slide to the east! The bigger the rock, and the harder you throw it to the west, the more you'll slide in the opposite direction. You can imagine the same effect in space.

You can try this with an office chair and something heavy, like a medicine ball, your little brother, or the giant book of pokemon cards everyone has from when they were popular. Now pick your feet off the ground, and put yourself in an open area, and throw the ball. You move too! You eventually slow down because of friction between the wheels and the ground, but thats the premise for how rockets fly in space.

Alexandre Colavin

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If you're referring to the Space Shuttle, the Space Station, or other low orbiting rockets, they're at the fringes of the outer atmoshpere and experience some drag. If not for occasional boosts from the rocket engines, their orbits would eventually decay and they would return to earth (or burn up).

DaveC426913
Gold Member
Once rockets leave the earth's atmosphere they would have a high velocity
so how come rockets use their engines when they're in space if there's no air resistance? The only reason I can see why they would need to use them is if they wanted to change direction.
Rockets have to reach escape velocity or they will fall back to Earth. That's about 25 times the speed of sound. They must achieve this velocity before they run out of fuel (which happens pretty quickly). That is the primary reason why they thrust, whether or not they are in atmo or in space.

BTW, it is easier to achieve that speed when in space since there's no air resistance. If they tried to achieve that speed while still in the atmo, they'd melt! So, in one sense, it would be more efficient to use their engines in space than in the atmo!

Question for clarity: what rockets have you been watching in particular? There are rockets that go sub-orbital, rockets that go into low orbit, rockets that go into high orbit, rockets that go to the Moon, and rockets that leave the Earth Moon system altogether. They all have different requirements, and the answer to your question depends on which ones you refer to.

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russ_watters
Mentor
Once rockets leave the earth's atmosphere they would have a high velocity
so how come rockets use their engines when they're in space if there's no air resistance? The only reason I can see why they would need to use them is if they wanted to change direction.
It really depends on what they want to do. Once the Space Shuttle or a satellite reaches space, it isn't in a proper orbit (basically, the orbit still intersects the atmosphere), so it needs to use its engines to reshape the orbit. For a probe going to the outer solar system, engines are used again for course correction and speed - "high velocity" is a relative term and the distances to be traveled are great, so more velocity is always desirable for deep space missions.

For a probe going to the outer solar system, engines are used again for course correction and speed - "high velocity" is a relative term and the distances to be traveled are great, so more velocity is always desirable for deep space missions.
Do the probes turn off their engines once the desired velocity is reached?

russ_watters
Mentor
Do the probes turn off their engines once the desired velocity is reached?
Most, yes. But we do have a few that keep their engines on in order to accelerate during the entire trip. Those use ion propulsion, which provides very low thrust for a very long time.

D H
Staff Emeritus