They eavesdrop and relay messages. They monitor weather patterns and troop movements. Hundreds of satellites have been launched into earth orbit, beginning with Sputnik in 1957, and they serve a multitude of purposes, both military and commercial. The most familiar are communications satellites, able to receive, amplify, and relay signals sent from earth. Telstar I, launched in 1962, transmitted live TV signals and telephone conversations across the Atlantic. Today, there is a growing commercial market for high-resolution satellite images, once the exclusive province of government intelligence agencies.