The reason it’s hard to get to orbit isn’t that space is high up. It’s hard to get to orbit because you have to go so fast.

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The speed you need to stay in orbit is about 8 kilometers per second. Only a fraction of a rocket’s energy is used to lift up out of the atmosphere; the vast majority of it is used to gain orbital (sideways) speed.

~ Randall Munroe from, http://what-if.xkcd.com/58/

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