Apollo–Soyuz was the first crewed international space mission, conducted jointly by the United States and the Soviet Union in July 1975. Millions watched on television as an American Apollo spacecraft docked with a Soviet Soyuz capsule. The mission and its symbolic "handshake in space" became an emblem of détente during the Cold War.
The Americans referred to the flight as the Apollo–Soyuz Test Project (ASTP), while the Soviets called it Experimental flight "Soyuz"–"Apollo" (Russian: Экспериментальный полёт «Союз»–«Аполлон», romanized: Eksperimentalniy polyot "Soyuz"–"Apollon") and designated the spacecraft Soyuz 19. The unnumbered Apollo vehicle was a leftover from the canceled Apollo missions program and was the final Apollo module to fly.
The crew consisted of American astronauts Thomas P. Stafford, Vance D. Brand, and Deke Slayton, and Soviet cosmonauts Alexei Leonov and Valery Kubasov. They carried out joint and independent experiments, including an arranged solar eclipse created by the Apollo spacecraft to allow Soyuz instruments to photograph the solar corona. Preparations for the mission provided experience for later joint American–Russian space flights, such as the Shuttle–Mir program and the International Space Station.
Apollo–Soyuz was the last crewed U.S. spaceflight for nearly six years until STS-1, the first launch of the Space Shuttle on 12 April 1981.
The purpose and catalyst of Apollo–Soyuz was the policy of détente between the two Cold War superpowers: the United States and the Soviet Union. Tensions ran high between the two world superpowers while the United States was engaged in the Vietnam War. Meanwhile, the Soviet press was highly critical of the Apollo space missions, printing "the armed intrusion of the United States and Saigon puppets into Laos is a shameless trampling underfoot of international law" over a photograph of the Apollo 14 launch in 1971. Although Soviet leader Nikita Khrushchev made the Soviet Union's policy of détente official in his 1956 doctrine of peaceful coexistence at the 20th Congress of the Communist Party of the Soviet Union, the two nations seemed to be in perpetual conflict.
After John Glenn's 1962 orbital flight, an exchange of letters between President John F. Kennedy and Soviet Premier Nikita Khrushchev led to a series of discussions led by NASA Deputy Administrator Hugh Dryden and Soviet scientist Anatoly Blagonravov. Their 1962 talks led to the Dryden–Blagonravov agreement, which was formalized in October of that year, the same time the two countries were in the midst of the Cuban Missile Crisis. The agreement was formally announced at the United Nations on 5 December 1962. It called for cooperation on the exchange of data from weather satellites, a study of the Earth's magnetic field, and joint tracking of the NASA Echo II balloon satellite. Kennedy interested Khrushchev in a joint crewed Moon landing, but after the assassination of Kennedy in November 1963 and Khrushchev's removal from office in October 1964, the competition between the two nations' crewed space programs heated up, and talk of cooperation became less common, due to tense relations and military implications.
On 19 April, 1971, the USSR launched the first piloted orbital space station, Salyut 1. Meanwhile, the United States had launched the Apollo 14 mission several months prior, the third mission to land on the Moon. Each side gave the other little coverage of their achievements.
With the close of the Vietnam War, relations between the United States and the USSR began to improve, as did the prognosis for a potential cooperative space mission. Apollo–Soyuz was made possible by the thaw in these relations, and the project itself endeavoured to amplify and solidify the improving relations between the United States and the Soviet Union. According to Soviet leader Leonid Brezhnev: The Soviet and American spacemen will go up into outer space for the first major joint scientific experiment in the history of mankind. They know that from outer space our planet looks even more beautiful. It is big enough for us to live peacefully on it, but it is too small to be threatened by nuclear war. Thus, both sides recognized ASTP as a political act of peace.
In October 1970, Soviet Academy of Sciences president Mstislav Keldysh responded to NASA Administrator Thomas O. Paine's letter proposing a cooperative space mission, and there was subsequently a meeting to discuss technical details. At a meeting in January 1971, U.S. President Richard Nixon's Foreign Policy Adviser Henry Kissinger enthusiastically espoused plans for the mission, and expressed these views to NASA administrator George Low: "As long as you stick to space, do anything you want to do. You are free to commit – in fact, I want you to tell your counterparts in Moscow that the President has sent you on this mission".
Both sides had severe criticisms of the other side's engineering. Soviet spacecraft were designed with automation in mind; the Lunokhod 1 and Luna 16 were both uncrewed probes, and each Soyuz spacecraft had been designed to minimize risk due to human error by having fewer manual controls with which human operators would have to contend during flight. By contrast, the Apollo spacecraft was designed to be operated by humans and required highly trained astronauts to operate. The Soviet Union criticized the Apollo spacecraft as being "extremely complex and dangerous".
The Americans also had concerns about Soviet spacecraft. Christopher C. Kraft, director of the Johnson Space Center, criticized the design of the Soyuz: "We in NASA rely on redundant components – if an instrument fails during flight, our crews switch to another in an attempt to continue the mission. Each Soyuz component, however, is designed for a specific function; if one fails, the cosmonauts land as soon as possible. The Apollo vehicle also relied on astronaut piloting to a much greater extent than did the Soyuz machine." The American astronauts had a very low opinion for the Soyuz spacecraft as it was a craft that was designed to be controlled from the ground. This was in contrast with the Apollo module that was meant to be flown from the capsule. Eventually Glynn Lunney, the Manager of the Apollo–Soyuz Test Program, warned them about talking to the press about their dissatisfaction as they had offended the Soviets. NASA was worried that any slight would cause the Soviets to pull out and the mission to be scrapped.
American and Soviet engineers settled their differences for a possible docking of American and Soviet spacecraft in meetings between June and December 1971 in Houston and Moscow, including Bill Creasy's design of the Androgynous Peripheral Attach System (APAS) between the two ships that would allow either to be active or passive during docking.
By April 1972, both the United States and the USSR signed an Agreement Concerning Cooperation in the Exploration and Use of Outer Space for Peaceful Purposes, committing both the USSR and the United States to the launch of the Apollo–Soyuz Test Project in 1975.
The ASTP was the first Soviet space mission to be televised live during the launch, while in space, and during the landing. Soyuz 19 was also the first Soviet spacecraft to which a foreign flight crew had access before flight; the Apollo crew were permitted to inspect it and the launch and crew training site, which was an unprecedented sharing of information with Americans about any Soviet space program.
Not all reactions to ASTP were positive. Many Americans feared that ASTP was giving the USSR too much credit in their space program, putting them on equal footing with the sophisticated space exploration efforts of NASA. More feared that the apparent peaceful cooperation between the USSR and the United States would lull people into believing there was no conflict at all between the two superpowers. Some Soviet publicists called American critics of the mission "demagogues who stand against scientific cooperation with the USSR". In general, tensions between the United States and the USSR had softened, and the project set a precedent for future cooperative projects in space.