Getting to space is easy. Staying in space is hard.

http://what-if.xkcd.com/58/

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.

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

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