These days, you hear a lot about satellites for communications and scientific research. Some satellites, though, have far less public uses. Reconnaissance satellites are used to spy on other countries. They provide intelligence information on the military activities of foreign countries. These satellites can even detect missile launches or nuclear explosions in space. Reconnaissance satellites can pick up and record radio and radar transmissions while passing over a country. Finally, they can be used as an orbital weapon by placing warheads on a low orbit satellite to be launched at a ground target. (This is not a recommended or frequent use of satellites.)

There are basically four types of reconnaisance satellites. First, there are the optical-imaging satellites that have light sensors that detect missile launches and "see" enemy weapons on the ground. Next are the radar-imaging satellites. They are able to observe the Earth using radar technology through cloud cover. Third, there are signals-intelligence or ferret satellites that are essentially super-sophisticated radio receivers that capture the radio and microwave transmissions emitted from any country on Earth. Finally, there are the relay satellites that make military satellite communications around the globe much faster by transmitting data from spy satellites to stations on Earth.

Starting in the 1960's, the United States began launching many series of reconaissance satellites. The first series was called Discover, and as these satellites circled the Earth in polar orbits, they took pictures. The next series of U.S. spy satellites was given the code name Keyhole, or KH for short. They mostly did routine surveillance or weapons targeting. In a low, 80-mile orbit, they either took wide-area pictures of large land masses or took close-up pictures of special interest objects.

KH-9 was a noteworthy satellite from that series. Weighing 30, 000 pounds, it was nicknamed Big Bird because of its extraordinarily large size. Big Bird, however, was unable to penetrate the thick cloud cover that often obscured the areas it was targeting. Sophisticated for its time, the camera used by Big Bird took area-surveillance images and close-up photos.

In 1976, KH-11, code-named Kennan, was launched. From Kennan's sun-synchronous orbit, the sun was at a constant angle with respect to objects on Earth. This choice of orbit helped to detect the movement and size of objects observed by Kennan by using the shadows recorded from space. Kennan also used a special piece of technology called a charge-coupled device (CCD) to obtain images.

To learn about one of NASA's more recent spy satellites, Lacrosse, click here.