Handshake in Orbit: The Apollo–Soyuz Test Project

On July 15, 1975, two rockets roared into the sky from opposite sides of the globe. At the Kennedy Space Center in Florida, an American Saturn IB rocket carried three astronauts inside an Apollo spacecraft. Half a world away, at the Baikonur Cosmodrome in Soviet Kazakhstan, a Soyuz rocket propelled two cosmonauts into orbit. While millions of people still remembered the fierce competition of the 1960s Space Race—when the Apollo 11 lunar landing in 1969 established American technological supremacy—this new mission had a very different purpose. Known as the Apollo–Soyuz Test Project (ASTP), it was designed not to beat a rival, but to join hands above the Earth.

Achieving that orbital rendezvous required solving complex engineering puzzles. The United States and the Soviet Union had spent nearly two decades designing independent space programs. Consequently, their hardware was entirely incompatible. The Soviet Soyuz craft operated with an atmospheric mixture similar to normal sea-level air—about 78 percent nitrogen and 21 percent oxygen at standard atmospheric pressure. In contrast, the American Apollo capsule used pure oxygen at a much lower pressure, roughly one-third of sea level. If an astronaut had opened an ordinary hatch directly between the two ships, the sudden difference in pressure and gas mixture would have caused severe physical harm, including decompression sickness.