The satellite’s orbital speed plus the pull of Earth’s gravity. The same principle keeps Earth and the other planets circling the Sun – only in that case it is the Sun’s gravity and the planets’ velocity that keep the planets in their orbits.
Our Earth sails along the same invisible track century after century because its velocity counterbalances the Sun’s gravitational grip, or inward pull. The same thing happens to a man-made satellite – except that it gradually loses speed as it sweeps through the Earth’s thin atmosphere. When the speed of the artificial moon drops below 28,900 kilometers per hour, the Earth’s gravity wins the battle and pulls the satellite back to Earth.