In 1972, the United States and USSR agreed to meet in space.
Three years later, Soyuz 19 and an Apollo capsule, the last to be used in the space program, docked and for 2 days exchanged crews and good will.
In preparation for this historic, albeit mostly symbolic, mission, many problems had to be overcome.
Among them, the Apollo and Soyuz programs used different docking mechanisms and air, Apollo used pure oxygen under low pressure; Soyuz used oxygen and nitrogen under higher pressure.
To solve the problems, the two countries developed a docking module in which the air quality could be altered slowly enough to avoid physical distress.
The docking went off without a hitch and both Apollo and Soyuz returned home triumphantly.
The Apollo-Soyuz Test Project was a symbol of détente that the two superpowers were pursuing at the time, and it ended the tension of the Space Race.