The Apollo-Soyuz Test Project (ASTP) was the first international crewed spaceflight — a mission that literally bridged the gap between Cold War rivals. On July 15, 1975, a Soviet Soyuz 19 spacecraft carrying cosmonauts Alexei Leonov and Valeri Kubasov launched from the Baikonur Cosmodrome in Kazakhstan. Seven and a half hours later, an American Apollo spacecraft carrying astronauts Thomas Stafford, Vance Brand, and Deke Slayton lifted off from Kennedy Space Center’s Launch Complex 39B atop the last Saturn IB rocket — and the last Saturn rocket of any kind — ever launched.

Two days later, on July 17, the Apollo spacecraft docked with Soyuz 19 over the Atlantic Ocean using a specially designed docking module that served as an airlock between the two vehicles’ different atmospheres. Then came the moment the world had been waiting for: Commander Thomas Stafford opened the hatch and shook hands with Commander Alexei Leonov. The “Handshake in Space” was broadcast live on television in both nations, symbolizing the end of the Space Race and the beginning of a new era of international cooperation in space. The crews exchanged flags, shared meals, and conducted five joint experiments over two days of docked operations.

The mission held deep personal significance for Deke Slayton, one of the original Mercury Seven astronauts selected in 1959. Slayton had been grounded in 1962 due to an irregular heartbeat (atrial fibrillation) and spent 13 years working as NASA’s Director of Flight Crew Operations, watching his colleagues fly while waiting for his chance. When he was finally cleared for flight status in 1972, ASTP became his one and only spaceflight — making him the last of the Mercury Seven to reach orbit.

After undocking, the Apollo crew remained in orbit for three more days conducting solo experiments, including an artificial solar eclipse created by having Soyuz fly in front of the Sun as seen from Apollo. The mission ended on July 24, 1975, with splashdown in the Pacific Ocean and recovery by the USS New Orleans — though the landing was marred when toxic reaction control system propellants entered the cabin, requiring the crew to be briefly hospitalized. ASTP was the last Apollo spacecraft to fly and the last American crewed mission for nearly six years, until the Space Shuttle Columbia launched on STS-1 in April 1981. The cooperative spirit of ASTP would eventually bear fruit in the Shuttle-Mir program and the International Space Station.