Why don’t satellites fall down?

A stable orbit is determined by just the right combination of satellite speed and distance from the Earth. The speed must be great enough so gravity doesn’t bring the satellite back to Earth, but not so great that the satellite escapes gravity out into space.

A simple example shows why that is so. Think of a stone tied to the end of a piece of string held in hand and whirled in a circle. The stone simulates the satellite, and the hand is the Earth. Centrifugal force pulls outward, but the taut string holds the stone in its circular orbit. If the speed of the stone is too low, the stone doesn’t move in a circle, but falls toward the hand holding the string.

There’s no air resistance in space, so as soon as a satellite has gained the right speed, it retains that speed. Satellites fly in stable orbits, for which satellite speed and distance from the Earth are calculated accurately. A satellite in a stable orbit won’t fall down of its own accord, so there’s almost no chance of being hit by one.

Occasionally, a satellite must correct its orbit to keep its position. When a satellite has reached the end of its useful life, it can be moved to a “graveyard orbit” or sent back toward Earth to burn up in the atmosphere.