Apollo-Soyuz was the first crewed international space mission, carried out jointly by the United States and the Soviet Union in July 1975.

Millions of people around the world watched on television as an American Apollo spacecraft docked with a Soviet Soyuz capsule.

The three American astronauts, Thomas P. Stafford, Vance D. Brand, and Deke Slayton, and two Soviet cosmonauts, Alexei Leonov and Valery Kubasov, performed both joint and separate scientific experiments, including an arranged eclipse of the Sun by the Apollo module to allow instruments on the Soyuz to take photographs of the solar corona.

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.

[citation needed] 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.

[3] Kennedy interested Khrushchev in a joint crewed Moon landing,[4] 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.

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.

[5] 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.

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.

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.

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.

[9] ASTP was particularly significant for the USSR's policy of keeping the details of their space program secret from the Soviet people and the world at large, especially Americans.

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.

[2] Some Soviet publicists called American critics of the mission "demagogues who stand against scientific cooperation with the USSR".

[10] Jack Swigert had originally been assigned as the command module pilot for the ASTP prime crew, but he was removed before the official announcement as punishment for his involvement in the Apollo 15 postal covers incident.

The ASTP entailed the docking of an American Apollo command and service module (CSM) with a Soviet Soyuz 7K-TM spacecraft.

The docking module was designed as both an airlock — as the Apollo was pressurized at about 5 psi (34 kPa) using pure oxygen, while the Soyuz used a nitrogen/oxygen atmosphere at sea level pressure (about 15 psi (100 kPa)) — 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.

The two-man crew on the Soyuz was a result of the modifications needed to allow the cosmonauts to wear the Sokol space suit during launch, docking, and reentry.

The ASTP-class Soyuz 7K-TM spacecraft used was a variation of the post-Soyuz 11 two-man design, with the batteries replaced by solar panels enabling "solo" flights (missions not docking to one of the Salyut space stations).

It was designed to operate, during the docking phase, at a reduced nitrogen/oxygen pressure of 10.2 psi (70 kPa), allowing easier transfers between the Apollo and Soyuz.

And the sixth craft was available as a "cold" backup; it was later used on the last "solo" Soyuz flight in 1976, but with the APAS docking adapter replaced by the MKF-6 multispectral camera.

Three hours later, the two mission commanders, Stafford and Leonov, exchanged the first international handshake in space through the open hatch of the Soyuz.

NASA had calculated that the historic handshake would have taken place over the British seaside resort of Bognor Regis,[17] but a delay resulted in its occurrence being over the city of Metz in France.

The only serious problem was during reentry and splashdown of the Apollo craft, during which the crew were accidentally exposed to toxic monomethylhydrazine and nitrogen tetroxide fumes, caused by unignited reaction control system (RCS) hypergolic propellants venting from the spacecraft and reentering a cabin air intake.

[citation needed] A derivative (but mechanically incompatible) docking collar, APAS-89, was launched as part of the Kristall module of the Soviet Mir space station.

The first PMA, PMA-1, remains in use as the interface connecting the Russian-built, NASA-owned Zarya module to the US segment of the ISS (USOS), and so the APAS continues in use to this day (2024).

The remaining crew's most recent reunion was on 16 July 2010, when Leonov, Kubasov, Stafford, and Brand met at an Omega timepiece store in New York City.