A satellite requires a speed of 17,450 miles per hour in order to maintain a low Earth orbit. Satellites in higher orbits travel more slowly; for example, a geostationary satellite only orbits at 6,858 miles per hour.
When satellites orbit, they are falling around the planet because the gravitational pull keeps the satellite in motion. The closer they are, the greater the pull is. Orbit around Earth was first thought of by Isaac Newton with his cannonball thought experiment. It stated that if a cannon on top of a mountain fired a ball, the faster it shot, the farther it would travel. He correctly reasoned that a speed could be reached that would allow an object to continue traveling around the planet.