Apollo-Soyuz Test Project (ASTP). Illustration of the historic 1975 Apollo-Soyuz rendezvous in space, where the Soviet Union's Soyuz spacecraft (right) docked with the United States's Apollo. Intended initially as a political gesture, this was the world's first international manned space flight. Apollo and Soyuz were launched on July 15 and successfully docked two days later at an altitude of 225 km, thanks to a joint-designed docking mechanism. The crews of two cosmonauts (Leonov and Kubasov) and three astronauts (Stafford, Slayton and Brand) visited each other's craft and conducted joint experiments.