For example, if you place a satellite at approximately 22,300 miles above the equator and set it in motion with a speed that matches the Earth’s rotation, then it just sits there (at least from the point of view of the Earth). Looked at from below, it’s stationary—or geosynchronous (GEO), to use the technical term—with a little less than half the world in its field of view (the exceptions are the polar areas). That means it can receive line-of-sight signals from Earth and beam them back to a receiver located anywhere else in the world that is in its field of view. Hooking these two stations up would mean a lot of copper wire; plus, you can send transmissions to intermediate stations in between. Best of all, you can send the radio beam to another satellite, also hovering over the equator, and that one can send it to another. Either of these can send the message down to a station within its field of view. In that way, you can contact anyplace on Earth (except at its very top and very bottom).

This orbit is best for communications satellites. It is also the home of our Cold War infrared satellites, such as the ones that detected the ballistic missiles launched at Riyadh or Israel.

If you want your satellite to listen to radio broadcasts from some station on Earth—say, KRNT radio in Des Moines, Iowa—it turns out that GEO may not provide a view of an area you want covered. That means you want your satellite to be closer to the station. The problem is that a GEO orbit is the only place in which the satellite will stand still over the surface of the earth; and besides, since you don’t care to listen to ABC in Perth, Australia, on the other side of the world, you want your orbiting satellite to float over Des Moines and whiz by Perth. That means you’ll want to create an egg-shaped orbital path, with the Earth located at the bottom of the small part of the oval. In that way, your satellite will spend most of its time looking down on Iowa and very little time looking down on down-under. This kind of elliptical orbit is called a “Molniya” orbit (the word is Russian; I don’t know what it means).

If you want to use a satellite to look closely at the surface of the earth, then you’ll want to fly it as close to it as you can get, because the closer you are, the better you can see things. So you put your satellite in as low an orbit as possible. The problem now is that if you get too low, you start scraping air, and your bird slows down and eventually falls to earth. This orbit is called Low Earth Orbit, or LEO (about 60 to 300 miles up).