Many people assume that satellites orbit the Earth far above its surface, but the numbers tell a different story. Most satellites orbit at less than 300km above the ground. Compared with the radius of the Earth, 6400km, this is very small. Their orbits just skim the top of the atmosphere. We can expect, therefore, that the acceleration of gravity on such a satellite will not be very different from what it is near the ground. How then can it happen that the satellite doesn't fall to the ground like our cannonballs in the first chapter?

1. ▷ Communications and many weather satellites, which must be in “geostationary” orbits, are an important exception, being in distant orbits. We will return to these orbits in Chapter 4.

The answer is that it tries to, but the ground falls away as well. Imagine firing a cannon over a cliff. Eventually the ball will fall back to the height from which it was fired, but the ground is no longer there. The Earth has been cut away at the cliff, so the ball must fall further in order to reach the ground.

1. ▷ The picture behind the words on this page is the Hubble Space Telescope (HST), a satellite launched by the National Aeronautics and Space Administration (NASA), with participation from the European Space Agency (ESA) as well. […]