Does a falling stone choose its path?
Obviously not. It follows the trajectory dictated by the pull of Earth’s gravity. At first sight, you might wonder what this fact has to do with satellite orbits. But think again…

Contrary to what we see in science fiction movies and books, a spacecraft is not free to move as it likes once it is beyond the barrier of Earth’s atmosphere.
It can’t just turn on the spot or come to a dead stop in space. Behind its apparent tranquillity, the Earth keeps a firm hand on anything in its vicinity as a result of the force of universal gravitation to which all celestial bodies are subjected.

Any object in space—be it a satellite or probe, or even a planet, comet or asteroid—is a projectile travelling at breathtaking speed on a continuous and inescapable path. This path is called its orbit.

^{Last updated: January 2004}