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Soyuz-20 pushes forward presence in space

In November 1975, the unpiloted Soyuz-20 spacecraft was launched to the Salyut-4 space station, which itself had no crew onboard at the time. Instead other passengers, such as turtles, were reportedly riding inside Soyuz-20, for a presumed endurance mission, although many of its details remained a mystery for decades.

Testing of docking interfaces between Soyuz spacecraft and a Salyut space station.

Soyuz-20 mission at a glance:

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Third launch to Salyut-4

After the departure of its second expedition on July 26, 1975, the Salyut-4 space station was conducting an unpiloted solo flight. According to Soviet sources, the spacecraft continued running some of the onboard experiments, for example, a specially developed extension program supported astrophysics studies, including operation of the Filin X-ray telescope-spectrometer, which was trained on a powerful X-ray source in the Monoceros constellation. Simultaneously, the same source was observed with optical sensors from multiple ground stations of the Shternberg State Astronomy Institute. (152)

However, in the West, observers of the Soviet space program debated what to expect from the secretive Soviet space program. Some predicted a third expedition heading to the station, before its retirement. Others interpreted Soviet sources mentioning the fact that Salyut-4 had to support crews for two months longer than planned due to a crew launch failure in April, as a tacit admission of problems with the station's life-support system. (If the second expedition arrived as planned, the occupation of the lab would be concluded in early June). There were some rumors that at the end of the Soyuz-18 crew stay, the windows aboard Salyut-4 were completely fogged and the interior walls covered with mold, because of problems with humidity control inside the station. There were also unconfirmed speculations that planned spacewalks by the second expedition had to be cancelled, because by the time the second crew arrived, the station's life-support system was too depleted. In 1977, after Soviet spacewalk activities had been renewed aboard Salyut-6 after a long break, a Soviet publication did say that some unspecified spacewalks aboard one of the earlier Salyuts had been planned but not implemented due to excessive workload on the crews.

In any case, during much of Fall 1975, the uninhabited Salyut-4 was gradually losing its altitude, while a classified Soviet Kosmos launch on September 2, correctly suspected to be a Soyuz (INSIDER CONTENT) entered an orbit clearly precluding a rendezvous with Salyut-4. (50)

At the end of October 1975, a short official update on Salyut-4 reported the station in a 338 by 358-kilometer orbit, but between Nov. 3 and Nov. 4, 1975, tracking data revealed an orbit-raising maneuver, hinting at a move to a potential rendezvous orbit.

Behind the scene, Soviet engineers were preparing a Soyuz transport with production No. 64 for launch to Salyut-4. All the ships from Series 60 were originally intended for the Almaz military space station project, but Vehicle No. 64 was apparently "borrowed" by the Salyut project in order to certify the crew ship for three-month stays at the station, which would be a month-long extension from the previous longest flight. In exchange, Vehicle No. 41, originally assigned to the Salyut project, but probably behind Vehicle No. 64 in the production pipeline, was transferred to the Almaz program. (1127)

Soyuz-20 flies three-month mission

A fresh Soyuz spacecraft lifted off from Tyuratam on Nov. 17, 1975. It was successfully delivered into a 199.7 by 263.5-kilometer orbit with an inclination 51.6 degrees toward the Equator. (2)

Somewhat unusually, it was announced as Soyuz-20, only the second spacecraft in a series disclosed to be an unpiloted Soyuz at launch! (52)

After a series of automated maneuvers, Soyuz-20 climbed to a 343.9 by 365.7-kilometer orbit for rendezvous with Salyut-4.

Soyuz-20 successfully docked at the station on Nov. 19, 1975, at 19:20 UTC. It was the third vehicle to reach the station in its 11 months of orbital flight.

The use of the 34-orbit, two-day rendezvous profile in the Soyuz-20 flight looked unusual to independent observers, because previous missions would normally approach the station after one day or 17 orbits in autonomous flight. Because the two-day rendezvous would eventually be made routine by Progress missions, it was assumed that Soyuz-20 was also testing a fully automated docking system for the yet-to-be developed cargo ships. Possibly, a potential operation for in-orbit replacement of Soyuz at the station, using either a second back-up port, or even a single docking mechanism on the host spacecraft, was also being tested in the Soyuz-20. But all of this remained speculations.

The Soviet announcements at the time said only that the objectives of the Soyuz-20 mission included testing of the structural elements, aggregates and onboard systems of Soyuz and Salyut spacecraft in a joint flight.

However, six days after the Soyuz-20 docking with Salyut-4 came the launch of the Kosmos-782 satellite, which was soon identified as a biological research satellite on a three-week orbital mission which had some foreign involvement. The long-running flight program of biological missions would eventually be christened Bion.

After Kosmos-782 entered orbit, the Soviet sources revealed that Soyuz-20 carried an array of biological experiments as well involving plants and live organisms. These included turtles, designed to predict changes in the biological species and fruit flies, intended to see genetic changes during growth and development of organisms in weightlessness.

Specially designed chambers (aboard Soyuz-20) contained around 20 species of plants, including cactus, gladiolus bulbs, various vegetable seeds, corn and legumes. These experiments aimed to study changes in the molecular structure of the plants under the influence of space radiation.

While Soyuz-20 was in orbit, similar experiments went underway aboard the Kosmos-782, which had a much shorter mission.

Soyuz-20 remained docked at Salyut-4 until Feb. 16, 1976, when the transport ship was reported departing the station and successfully deorbiting on the same day. (152) There were no details on the reentry and landing sequence at the time, but it was later revealed that the undocking between Soyuz-20 and Salyut-4 took place at 23:04 UTC on February 16.

There was a report that post-landing evaluation of the capsule revealed deterioration in some undisclosed systems, which was attributed to repeated thermal fluctuations during the orbital flight. As a result, the 90-day limit was apparently imposed on the flight duration of Soyuz vehicles for the time being. (1128)

In the meantime, the Salyut-4 space station, remained in orbit for another year, but made no detectable orbit adjustments until its deorbiting maneuver over the Pacific Ocean on Feb 3, 1977. (52)

Next page: Salyut-5

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

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

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Landing of a Descent Module attributed to the Soyuz-20 mission.