Inertia

It took a few more technological advances, but it was largely Newton’s insights from 1697 that have allowed us to put more than 2,000 mechanical messengers and observers in weightless orbit around our planet. He explained that orbiting objects, like our robotic satellites or the moon, are constantly falling to the earth. They are just traveling fast enough that by the time they would have fallen to the ground they have passed, or missed the earth, only to find themselves on another side of the planet falling still.

In order to miss the earth’s surface from the altitude of low earth orbit, the International Space Station, for instance, must travel an amazing 4.76 miles per second (7.6 km/s). Meanwhile, the moon is far enough outside our gravitational pull that it only needs to move little faster than a typical bullet; a mere 0.635 miles per second (1 km/s).