In 1973, the US launched the Skylab sortie space station and inhabited it for 171 days with three crews ferried aboard Apollo spacecraft. During that time, President Richard Nixon and Soviet general secretary Leonid Brezhnev were negotiating an easing of Cold War tensions known as détente. As part of this, they negotiated the Apollo-Soyuz program, in which an Apollo spacecraft carrying a special docking adapter module rendezvoused and docked with Soyuz 19 in 1975. The American and Russian crews shook hands in space, but the purpose of the flight was purely symbolic.