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Soyuz 7K-TM

Soyuz 7K-TM No. 71 (Kosmos-638)

INSIDER CONTENT

Soyuz 7K-TM No. 72

INSIDER CONTENT

Soyuz-16

Origin of the Apollo-Soyuz project

In the early 1970s, Soviet and US officials began work on the joint development of technology for a potential docking in space which led to a formal agreement on a docking mission in 1972.

Previous chapter: Soyuz-18-1 aborted launch

Artist impression of the Soyuz spacecraft docking with the Skylab space station.

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The Cold War competition and the arms race, which drove the development of piloted spacecraft in the USSR and the US in the 1960s, naturally excluded the possibility for cooperation, even if calls for joining forces in such a complex and expensive endeavor came from top leaders, most notably President Kennedy's 1963 idea for a US-Soviet lunar exploration program. Only with the de-facto conclusion of the Moon Race in 1969, did various studies and proposals for the future direction in space flight on both sides of the Atlantic begin seriously reexamining options for international efforts in piloted space flight. This process coincided with a detente in US-Soviet relations. In April 1970, the flight emergency aboard the Apollo-13 spacecraft also re-focused attention on the possibility of international rescue in space. It was immediately clear that even with political will, a ready-to-launch spacecraft and favorable orbital mechanics, a hypothetical rescue attempt would not work due to incompatible hardware and procedures.

Just three months later, on July 31, 1970, NASA Administrator Thomas O. Paine sent a letter to President of the Soviet Academy of Sciences Mstislav Keldysh proposing talks on the joint development of a compatible docking mechanism. On September 4 of the same year, Paine also offered the USSR to consider sending a Soyuz spacecraft to the US Skylab space station then planned for launch in 1973. After consultations at the top, Keldysh responded on Sept. 23, 1970, proposing a meeting of experts in Moscow. It took place on Oct. 26 and 27, 1970, where an early concept of the so-called androgynous docking mechanism was presented by a US engineer Caldwell C. Johnson.

On Jan. 16-21, 1971, another round of consultations took place in Moscow, where NASA Acting Administrator George Low proposed to use Soyuz and Apollo spacecraft for testing the new docking mechanism.

In June 1971, the first Soviet Salyut space station was launched (and disclosed to the world), suddenly offering yet another destination for a potential joint mission. During a series of discussions in Houston, Texas, on June 21-25, 1971, the option of sending Apollo to the Salyut-Soyuz combo was considered as primary. However, such a scenario required a new station with two docking ports, which existed only on a drawing board at the time.

As a result, during a series of meetings on April 4-6, 1972, led by the Vice-President of the Soviet Academy of Sciences, Vladimir Kotelnikov and Deputy NASA Administrator George Low, the Soviet representatives told their US colleagues that it would not be possible to involve Salyut and instead proposed returning to a docking between Soyuz and Apollo. Both sides agreed to perform that flight in 1975, subject to approval by their respective governments. (1113)

In May 1972, during a visit to Moscow by US President Richard Nixon, the two sides signed a formal agreement on cooperation in the peaceful exploration of space, which included the joint development of a compatible docking mechanism.

The official goal of what would become the Apollo-Soyuz Test Project, ASTP, was to demonstrate the possibility of rescue in space. The two sides agreed to focus on the Soyuz spacecraft variant, which previously carried crews to the Salyut space stations on the Soviet side and the Command and Service Module of the Apollo spacecraft to be used by NASA. (Ironically, for the US, it was the last Apollo mission, closing a whole chapter in the history of piloted space flight, before a long and difficult transition to what was promised to be a low-cost and routinely flying Space Shuttle.)

In addition to the obvious standardization of docking interfaces, a joint mission would require the two sides to overcome a number of other incompatibilities between the US and Soviet spacecraft, such as rendezvous and communications systems. Perhaps, the most difficult technical problem was posed by the pure-oxygen atmosphere of the Apollo spacecraft which had to be reconciled with the air-mixture atmosphere in Soyuz, operating at a higher pressure.

An early depiction of the Apollo and Soyuz docking.

The retrofitting the existing ships with a common life-support system was deemed impossible, so the developers resorted to adding a special docking module to the Apollo spacecraft, which would serve as an airlock between the two vehicles. It meant that all four habitable compartments on the two ships could not be opened to each other at the same time.

Next page: Managing the ASTP project (INSIDER CONTENT)

The article by Anatoly Zak; last update: July 17, 2025

Page editor: Alain Chabot; last edit: July 17, 2025

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A concept of the Soyuz spacecraft docked with the Apollo-Skylab stack. Credit: NASA

A concept of the Apollo spacecraft docked with the Salyut-Soyuz stack. Credit: NASA

A proposed development timeline for the ASTP project as of 1972.