There have been hundreds - perhaps thousands - of spy satellites launched by dozens of nations since Sputnik. While the vast majority of reconnaissance satellites and any information concerning them are strictly classified, some information (such as that concerning the US Corona program) has been declassified with the end of the Cold War.
The primary purpose of most spy satellites is to monitor visible ground activity. While resolution and clarity of images has improved greatly over the years, this role has remained essentially the same. Some other uses of satellite imaging have been to produce detailed 3D maps for use in operations and missile guidance systems, and to monitor normally invisible information such as the growth levels of a country's crops or the heat given off by certain facilities.
To counter the threat posed by these 'eyes in the sky', several nations (the United States and the USSR/Russia at least) have developed systems for destroying enemy spy satellites (either with the use of another 'killer satellite', or with some sort of earth or air launched missile).
The spy satellite game has taken a strange new twist with the recent availability of high resolution, colour images from commercial satellite imaging companies - allowing any country (or any individual for that matter) to get in on the action.
Aerial intelligence goes back hundreds of years. Long in the past (the American Civil War for example) hot air balloons were used to observe enemy formations long in the distance. The use of fixed balloons survived into World War I, when it was accompanied by observation from airships (zeppelins) and the newly invented airplane.
Low- and high-flying planes have been used all through the last century to gather intelligence about the enemy. At the start of the Cold War, foreseeing the need to observe the enemy in peacetime as well as war, the US started developing extremely fast, highflying spy planes. The first such plane, the Lockheed U-2, is still in service, as is the newer and faster SR-71 Blackbird. These planes have the advantage over satellites that they can usually produce more detailed photographs and can be maneuvered over the target more quickly and cheaply, but have the obvious disadvantage that they can be shot down.Satellites
Aerial