Apollo-Soyuz Test Project docking illustration
NASA

Apollo-Soyuz Command Module

The Apollo command module on display at the California Science Center was flown by American astronauts Tom Stafford, Deke Slayton and Vance Brand to rendezvous with a Russian Soyuz spacecraft parked in orbit around the Earth. Although built to fly to the moon as Apollo 18, its mission was changed when funding was cut for the Apollo program. The Science Center’s Apollo capsule is on loan from the Smithsonian Institution, National Air and Space Museum.

Crew Transfer between Apollo and Soyuz

Crewmembers meet at hatch of ASTP
NASA

Astronaut Deke Slayton and cosmonaut Aleksey Leonov meet in space during the Apollo Soyuz Test Project

When Apollo and Soyuz docked together, it was the first time that the Soviet Union and United States had come together in orbit, and was the first international human space flight mission. This event was the beginning of a new partnership, turning the competition that had previously characterized the space race into cooperation. 

One of the main objectives of this mission was to test rendezvous and docking systems. One obstacle to be overcome was that the atmospheric pressure and gas composition inside the Apollo command module differed from that used inside the Russian Soyuz spacecraft. The American spacecraft used a pure oxygen environment at one-third atmospheric pressure (5 psi). The Soyuz used an 80-percent nitrogen 20-percent oxygen environment at a pressure of one full atmosphere (14.7 psi). In order to allow safe transfer between vehicles, the Russian and American engineering teams jointly created a docking module that was inserted between the Apollo and Soyuz spacecraft. 

Prior to docking with the Apollo command module (that was linked to the docking module) the Russian crew lowered their cabin atmospheric pressure from a full atmosphere to two-thirds atmosphere. After docking with the Soyuz, the American crew transferred from the Apollo spacecraft into the docking module and closed the hatch behind them. They added nitrogen to the pure oxygen environment which raised the pressure inside the docking module from one-third atmosphere to two-thirds atmosphere and resulted in a gaseous composition that matched the Russian Soyuz spacecraft. The astronauts could then safely open the hatch between the docking module and the Soyuz.

Apollo Soyuz Test Project full crew
NASA

The entire ASTP crew included American astronauts Thomas Stafford (standing, left); Donald K. “Deke” Slayton (seated, left) and Vance D. Brand (seated, center) and Russian cosmonauts Aleksey A. Leonov (standing, right) and Valeriy N. Kubasov (seated, right)