duration of the joint mission. The Apollo mission was not a numbered mission of the Apollo program, and similarly bore the call sign "Apollo". Despite this, the press and NASA have referred to the mission as "Apollo 18", but this should not be confused with the canceled lunar mission. The Apollo spacecraft was launched with a docking module specially designed to enable the two spacecraft to dock with each other, used only once for this mission. The Saturn IB launch vehicle and CSM were surplus material. Like the Apollo Lunar Module, the docking module had to be retrieved from the S-IVB upper-stage of the Saturn IB rocket after launch. The docking module was designed as both an airlock — as the Apollo was pressurized at about using pure oxygen, while the Soyuz used a nitrogen/oxygen atmosphere at sea level pressure (about ) — and an adapter, since the surplus Apollo hardware used for the ASTP mission was not equipped with the APAS docking collar jointly developed by NASA and the Academy of Sciences of the Soviet Union for the mission. One end of the docking module was attached to the Apollo using the same "probe-and-drogue" docking mechanism used on the Lunar Module and the Skylab space station, while its other end had the APAS docking collar, which Soyuz 19 carried in place of the standard Soyuz/Salyut system of the time. The APAS collar fitted onto Soyuz 19 was releasable, allowing the two spacecraft to separate in case of malfunction. The Apollo flew with a three-man crew on board: Tom Stafford, Vance Brand, and Deke Slayton. Stafford had already flown into space three times, including within eight nautical miles of the lunar surface as Commander of Apollo 10, and was the first general officer to fly into space. He was a brigadier general in the United States Air Force at the time of the flight; he would retire with three stars in 1979. Slayton was one of the original Mercury Seven astronauts selected in 1959, but an irregular heartbeat grounded him until 1972. He became head of NASA's astronaut office and, after a lengthy medical program, selected himself for this mission. At the time, Slayton was the oldest person to fly in space and the one with the longest gap between selection as an astronaut and first flight into space. Brand, meanwhile, had trained with the Apollo spacecraft during his time