After careful maneuvering, the two craft linked up around noon on July 18. Some 225 kilometers (140 miles) above Earth, the astronauts and cosmonauts visited each other’s craft, performed joint experiments, and made further tests of the new docking system. Following the undocking Saturday, _Apollo_ fired its engines briefly and moved away from _Soyuz_. _Soyuz_ descended from orbit and landed in the south-central USSR early Monday morning, July 21. Astronauts Stafford, Slayton, and Brand remained in orbit conducting research and making science demonstrations. Splashdown into the Pacific Ocean occurred in late afternoon on Thursday, July 24. The historic ASTP mission was accomplished by using existing systems and a new docking module. The _Apollo_ spacecraft was made available when the lunar-landing program was curtailed. Since the command module was built with a docking system designed to work only with U.S. spacecraft, a method of incorporating the new docking system had to be devised. A second important problem was the difference between the spacecraft atmospheres. The _Apollo_ used a pure oxygen atmosphere at about one-third of the atmospheric pressure on earth’s surface; _Soyuz_ used a nitrogen-oxygen mixture at normal atmospheric pressure. To permit crews to pass from _Soyuz_ to _Apollo_ without suffering from the “bends” (a painful condition experienced when nitrogen gas bubbles form in the body fluids), engineers had to design an airlock to equalize the pressure. [Illustration: 78. The Soviet _Soyuz_ atop a three-stage launch vehicle lifts off July 15, 1975, to begin the joint US-USSR space mission.] [Illustration: 79. Overhead view of _Soyuz_ in orbit, photographed from the _Apollo_ spacecraft during the joint mission. The three major components of the _Soyuz_ are the spherical Orbital Module, the bell-shaped Descent Vehicle, and the cylindrical Instrument-Assembly Module from which two solar panels protrude.] [Illustration: 80. View of _Apollo_ spacecraft as seen in Earth-orbit from _Soyuz_. The Command/Service Module and Docking Module are contrasted against a black-sky background and the horizon of the Earth is below.] The docking module, 3 meters long and 1.5 meters in diameter (10 feet long and 5 feet in diameter), also solved the problem of incompatible docking mechanisms by carrying the new docking system on one end and a system compatible with _Apollo_ on the other. Prime contractor for Apollo Command Module, Service Module, and Docking Module was Rockwell International. The _Apollo_ hardware is from the National Aeronautics and Space Administration, and the _Soyuz_ spacecraft is on loan from the USSR Academy of Sciences. _Apollo_