What should a nation do when its adversary has a nuclear weapon pointed its way? Spy, of course.
During the Cold War, the Soviet Union and United States found themselves in exactly this situation. Both sides had nuclear missiles able to reach and flatten any city across the world in half an hour [source: Boot]. Both sides also had sent nuclear missiles on submarines [source: Boot].
The United States began the Discoverer spy satellite program to watch how fast the Soviet Union was making missiles and where they were being sent. Discoverer 4, the first U.S. camera-carrying satellite, launched in 1959 but didn't reach orbit, but Discoverer 14 went up in 1960 and returned photos [sources: NASA: Discover 4, NASA: Discoverer 14].
In all, the "very suspicious" United States used satellites to track Soviet missile preparations and make sure the Soviets weren't going to invade Europe, says Roland. The Soviet Union also had satellites trained on the United States.
"Reconnaissance satellites allowed both sides to look at the strategic assets of the other side and convince themselves that a sneak attack wasn't being prepared," says Roland. "These satellites gave everyone confidence that they knew what the enemy was doing. That kept tensions to a minimum. The satellites may well have prevented World War III."
Modern spy satellites can do more than take photos. They can collect telephone, radio and Internet signals, adding to the ways commanders can guess about what's happening on the ground [source: Britannica]. For more military uses of satellites, continue to our final game-changing military technology.