The Apollo-Soyuz Test Project (ASTP) was the first international space project involving piloted spaceflight. After three long years of preparations, a U.S. Apollo spacecraft and a Soviet Soyuz capsule linked up in Earth orbit in July 1975. It was an important milestone in the history of space exploration since it proved for the first time that political adversaries could work together to implement a complex space project.

 
The Apollo Soyuz Test Project Saturn 1B launch vehicle thunders away from KSC's Launch Complex 39B at 3:50 p.m. on July 15, 1975. Aboard the Apollo Command Module are ASTP astronauts Thomas Stafford, Vance Brand and Donald Slayton. The astronauts would rendezvous and dock with a Soyuz spacecraft, launched that morning from the Baikonur launch facility in the Soviet Union, with arriving Soviet cosmonauts Aleksey Leonov and Valerly Kubasov.

ASTP had its origins in October 1970 when representatives from the National Aeronautics and Space Administration (NASA) and the USSR Academy of Sciences met in Moscow to discuss the possibility of a joint piloted space mission.

Both sides had compelling reasons to participate in such a venture. For the United States, the Apollo Moon program was to end by 1972. Flights to the Skylab space station would follow in 1973 and 1974, but after that there would be a hiatus of nearly five years before the introduction of a new-generation space transportation system (which later became the Space Shuttle). A joint flight in the period between would provide NASA with valuable spaceflight experience. For the Soviets, a joint mission would be a useful public relations exercise by demonstrating that their space technology was on par with that of the United States. After losing the race to the Moon, such a joint flight would present a competitive Soviet space program-a view that had been difficult to support given its recent track record of unremarkable space missions.

President Richard M. Nixon and Soviet Council of Ministers Chairman Alexey N. Kosygin signed a formal document on May 24, 1972 that called for the docking of an Apollo to a Soyuz in Earth orbit in July 1975. The Americans called the project the Apollo-Soyuz Test Project (ASTP) while the Russians referred to it as the Apollo-Soyuz Experimental Flight (or “EPAS” in its Russian abbreviation). Planning for the mission began in July 1972 with the arrival of Soviet scientists at NASA's Lyndon B. Johnson Space Centre near Houston, Texas.

 
Apollo Commander, Astronaut Thomas P. Stafford (in foreground) and Soyuz Commander, Cosmonaut Alexei A. Leonov make their historic handshake in space during the joint
Russian/American docking mission known as the ASTP, or Apollo Soyuz Test Project. The handshake took place after the hatch to the Universal Docking Adapter (UDA) was opened. Stafford is inside the UDA and Leonov is inside the Soyuz.

Crews for the project were announced by the spring of 1973. Space veteran Thomas P. Stafford, would command the flight accompanied by two rookies, Vance D. Brand and Donald K. “Deke” Slayton. Slayton was one of the “original seven” Mercury astronauts selected in 1959 but had been grounded for a decade due to a minor heart problem. On the Soviet side, commander Alexei A. Leonov, the colourful cosmonaut who had made the first ever spacewalk in 1965, was joined by flight engineer Valeri N. Kubasov, a veteran of a 1969 Soyuz flight. Both Leonov and Kubasov had narrowly escaped death in 1971 when they had been slated to fly a mission to the Soviet Salyut space station. Just before launch, they had been replaced by a reserve crew, who subsequently were killed in space due to a valve failure.

Since the Apollo and Soyuz were vastly different spacecraft with incompatible docking systems, American and Soviet planners decided designed and built an interface known as the Docking Module. The 3-meter-long, 1.5-meter-wide cylinder not only served as a docking interface but also as an airlock module between the different atmospheres of the two spacecraft. The Docking Module, carried into orbit with Apollo, used a docking system that would lay the foundation for a standard international docking system.

The Apollo spacecraft was similar to the types used on the Moon missions, consisting of a conical Command Module attached to a cylindrical Service Module. The Soyuz was modified from the 7K-T version used as a ferry to the Salyut space stations. The ASTP Soyuz was known as the 7K-M and was equipped with new solar panels and modified controls. Apollo would be launched by a Saturn 1B left over from the Apollo Moon project, while the Soyuz would be launched by a standard three-stage Soyuz booster based on the old R-7 intercontinental ballistic missile (ICBM).

Between July 1973 and May 1975, Soviet and American delegations visited each other's countries several times. During the exchanges, American astronauts trained at the Russian Star City complex outside of Moscow while the Soviet crews trained at Johnson Space Centre.

In contrast to American openness about their space program, the Soviets were careful to conceal many important aspects of their space program, mostly because such a large portion of their space program had ties to the military. For example, throughout the project, the Soviets never revealed who actually designed and built the Soyuz spaceship (the company now known as RKK Energia). Instead, they used the cover of the USSR Academy of Sciences to give the impression that their space program was run by the Academy.

One of the most difficult obstacles in planning ASTP was the language barrier. In order to ease communication, the American crewmembers learned Russian while the Soviets learned English. Planners agreed that the crews would speak each other's languages as much as possible during rendezvous and docking operations.

The joint mission began exactly on schedule. The Soyuz, known as Soyuz 19, was successfully launched into from the Soviet launch complex at Baikonur on July 15, 1975. About seven-and-a-half hours after the Soyuz launch, Apollo was launched into orbit from the NASA's Kennedy Space Centre in Florida. Soon after, Stafford's crew manoeuvred the Apollo to link up with the Docking Module, which was stowed inside an adaptor above the Saturn 1B upper stage.

After a series of extensive manoeuvres, both Apollo and Soyuz entered similar orbits and began tracking each other by about 9 a.m. (all times in Eastern Time) on July 17. The two spacecraft made physical contact with each other at 12:09 p.m. the same day; cosmonaut Leonov reported to the ground that “Soyuz and Apollo are shaking hands now.” Hard docking between the Apollo/Docking Module and the Soyuz was completed minutes later.

Three hours after docking, astronauts Stafford and Slayton opened their Apollo hatch into the Docking Module. Soon after Leonov opened his hatch into the module and greeted Stafford. Their historic handshake was televised live to the world. During four hours of joint activities, the crews received congratulations from Soviet General Secretary Leonid Brezhnev and U.S. President Gerald Ford. The crews signed international certificates and exchanged commemorative items such as flags and plaques. After these activities, the crews returned to their own spacecraft to perform scientific experiments.

A second period of joint activities followed on July 18 when astronaut Brand entered the Soyuz and Leonov joined Stafford and Slayton in Apollo. The crews provided televised tours of each other's spacecraft and ate lunch together. After a third transfer between spacecraft later in the day and a joint press conference, the crews once again returned to their respective spacecraft.

Apollo and Soyuz separated from each other at 8:02 a.m. on July 19. At this point, the Apollo vehicle blocked the Sun, simulating a solar eclipse; the cosmonauts were thus able to photograph the solar corona. (The Apollo was termed an “occulting” object.) The two ships re-docked briefly (with the Soyuz system in motion rather than the Apollo) and then finally parted ways permanently after about three hours. During their slow drift away, the two crews performed an ultraviolet atmospheric absorption experiment.

Approximately 30 hours after undocking, Leonov and Kubasov returned to Earth, landing in a remote area of Kazakhstan at 6:51 a.m. on July 21. The landing was televised live to the world.

Apollo remained in orbit much longer. During a period of three days, Stafford's crew performed a large number of scientific experiments focused on astronomy, biology, materials science, and Earth observation. The Apollo Command Module splashed down safely at 5:18 p.m. on July 24 about 500 kilometres west of Hawaii in the Pacific Ocean. It was the last water landing for American astronauts.

The only critical event during the whole mission occurred during the Apollo re-entry when noxious nitrogen tetroxide gas from the reaction control thruster system accidentally entered the internal cabin. The crew was briefly exposed to toxic components and experienced severe eye irritation and discomfort in the lungs. There were, however, no long-term effects.