A joint US–Soviet mission from 15 to 24 July 1975, in which the Apollo 18 and Soyuz 19 spacecrafts docked (17–19 July). They were connected by a special airlock/docking module so the two crews—three astronauts and two cosmonauts—could visit one another. The module was required because the atmosphere of the NASA vehicle was pure oxygen at less than sea-level pressure, while the Soviets used oxygen–nitrogen at sea-level pressure.