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Second expedition to Salyut-4

The two-month trip of the Soyuz-18 crew to a space station in 1975, which broke a Soviet flight-duration record, is also widely credited with setting standards and procedures for progressively longer stays of Soviet cosmonauts aboard Earth-orbiting outposts for years to come.

Soyuz-18 mission at a glance:

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Preparing second expedition to Salyut-4

When the Soyuz-16 spacecraft successfully returned to Earth on Dec. 8, 1974, a Soviet correspondent promised a "very busy Summer of 1975." That was the first hint, that besides the Apollo-Soyuz project, developed jointly with the United States, the USSR had more missions up its sleeve. (50)

However, the eventual decision to have a crew aboard the Salyut-4 space station simultaneously with the joint Soviet-American docking mission did not come without controversy. Even though the launches of two Soyuz crew vehicles could be spread out in time and managed effectively, the two parallel missions, involving a Salyut space station and an American spacecraft were obviously extraordinary and put additional workload on ground control and other support facilities.

But on the political side, the ongoing expedition aboard Salyut-4 with two Soviet cosmonauts compensated for the smaller two-person crew heading to a joint flight with the three-seat American spacecraft, so the idea would be looked at favorably in the Kremlin.

The management of the Soviet space program was split on the issue. Valentin Glushko, who led NPO Energia, and Konstantin Bushuev, the head of the Apollo-Soyuz project, argued against the simultaneous manning of Salyut. They were also supported by leading engineers Aleksei Yeliseev (responsible for mission control), Mikhail Ryazansky and G.I. Voronin. Another group of managers and engineers, including Yuri Semenov, who led the Salyut program, as well as Konstantin Feoktistov, Gai Severin, Armen Mnatsakyan and Sergei Okhapkin wanted a crew aboard Salyut. The latter group prevailed thanks to the blessing of Sergei Afanasiev, the head of the Ministry of General Machine-Building, MOM, which oversaw the rocket industry on behalf of the Kremlin. (52)

Another challenge for the planners was to settle all the issues stemming from a launch accident during the previous attempt to put a crew aboard Salyut-4 on April 5, 1975. It was also important to return Soyuz to flight before the Apollo-Soyuz mission.

Without an extra crew vehicle available within the station's projected life span, the program managers considered whether the second expedition could utilize the entire two-month cache of supplies still remaining aboard the station after the first expedition, instead of the original plans for a total of three visits to Salyut-4. In case of a second crew launch at the end of May 1975, the two-month expedition would also overlap with the Apollo-Soyuz flight that July.

Apparently, the final decision for extending the second Salyut-4 stay from 28 to 62 days was left until after the launch, based on the performance and health status of the second crew during the actual flight.

Following the April 5 launch failure, two additional pairs of cosmonauts were in training for work aboard Salyut-4 — the first included Petr Klimuk and Vitaly Sevastyanov and the other — Vladimir Kovalenok and Yuri Ponomarev. (231) Klimuk and Sevastyanov had served as back-up crew in the aborted mission in April, so it was naturally their turn.

To form its rendezvous orbit with Soyuz, Salyut-4 was commanded to perform two orbit corrections. (152) According to Western tracking, the station first maneuvered between May 12 and 14, 1975, followed by another small orbit correction on May 22, 1975. It was a clear hint to Western observers that another attempt to put a crew aboard Salyut-4 was imminent. (50)

Klimuk practices use of a space station treadmill aboard an aircraft simulating weightlessness.

Soyuz-18 travels to Salyut-4

The Soyuz 7K-T No. 50 crew vehicle, with Petr Klimuk and Vitaly Sevastyanov aboard, lifted off on May 24, 1975, at 17:58:10 Moscow Time from Site 1 in Tyuratam.

This time, everything apparently went smoothly and the spacecraft entered a 193 by 247-kilometer parking orbit. Soyuz-18 performed two planned orbit corrections on the first day of the flight (around 12 hours after launch) and another maneuver was made on May 25, 1975. At one point, Soyuz-18 ended up in a 322 by 384-kilometer orbit, or somewhat higher than Salyut-4's 344 by 356-kilometer orbit.

According to the Soviet press, the autonomous rendezvous began at 18:11 GMT on May 25, 1975, as Soyuz-18 was closing in on Salyut-4 with a relative speed of 12 meters per second. At a distance of 1.5 kilometers between the objects, the automated system determined that the approach was too fast and commanded a slowdown. At 18:21, both spacecraft entered night time while they were 800 meters from each other. After some effort to confirm their relative position, Klimuk switched to manual control at a distance of 100 meters from the station and brought the closing rate to just 0.3 meters per second. He then completed docking at 18:44 GMT on May 25, 1975, while out of range of Soviet ground stations, so it took some time to confirm it on the ground. In the early hours of May 26, 1975, the cosmonauts successfully entered Salyut-4. (50)

For the first time in the Soviet space flight, cosmonauts were able to re-occupy a habitable vehicle in orbit.

Subsequent operations aboard the station also went largely as planned, finally demonstrating the maturity of the Soviet space station program.

Soyuz-18 crew works aboard Salyut-4

After activating onboard systems, the crew turned to scientific experiments, starting with medical studies, using the Polinom multi-function monitoring unit, which included electrocardiography measurements. They also could not avoid taking blood samples intended for the return to Earth.

The cosmonauts apparently took most interest in the Oazis plant-growing chamber, which got a makeover after the first expedition and got activated on the sixth day of the expedition. (152)

The upgraded unit was equipped with walls attracting moisture, instead of rejecting it. The planting method for the unit was also modified, greatly improving viability of the seeds. On top of the usual experiments with the seeds, the crew brought a pair of small onions to the station and, then surprised the Oazis team on the ground with the news that the plant had successfully produced shoots between 10 and 15 centimeters long. (231)

On the seventh day of the flight, the crew conducted spectral analysis of the atmosphere and of the Earth's surface. The cosmonauts were reported conducting remote-sensing experiments over the Caucusus and Northern Kazakhstan. (152)

Decades later, it also became know that during the flight, the cosmonauts performed the Svinets-5 military experiment, apparently designed to test methods for tracking ballistic missiles and warheads from space for the Soviet early warning system being developed at 2nd TsNII research institute of the Ministry of Defense. (1122)

In 2021, it was also revealed that Klimuk and Sevastyanov, who were often seen goofy and laughing aboard the station, almost fell out with each other over an alcohol-containing "energy drink," which reportedly was included in their medical kit! The story goes that despite their agreement to share the liquid during the flight, they discovered it almost gone early in the mission. They blamed each other for its disappearance until the eventual realization that the unstable fluid had just evaporated through the poorly sealed container, leaving behind a toxic residue.

Despite all the tensions, the crew activated astrophysics equipments, measuring isotope and chemical composition of galactic and solar radiation. The onboard X-ray telescope was used to image X-ray sources in the Scorpio, Virgo and Cygnus constellations. These observations required re-orienting the entire station in space and keeping steady attitude with the use of celestial navigation tools. On June 3, 1975, the X-ray telescope-spectrometer used by the crew detected considerable changes in radiation flows from sources in Scorpio and Cygnus.

The 11th and 14th days of the flight were dedicated to solar research conducted under guidance from scientists at the Crimean astrophysical observatory. The crew produced several dozen of UV spectra, while a diffraction spectrometer was simultaneously recording the radiation intensity in the same wavelength from the entire solar disk. (152)

Overall, the Soyuz-18 crew was known to operate practically the entire astrophysics arsenal aboard Salyut-4, including the SILYa and KSS spectrometers, the ITS-K infra-red telescope, the Filin X-ray instrument and, perhaps most significantly, the OST solar telescope. (231)

Extending the flight

According to recollections by Vitaly Sevastyanov, he and his commander got the news about the extension of their flight for an additional 35 days some two weeks into the mission. Klimuk reportedly asked Sevastyanov to request ground control to re-send the message, because he was so surprised by that change. The new telegram re-confirmed that their new landing date was now July 26, 1975. The decision was apparently supported by data about the good health of both cosmonauts.

So the crew continued their work. On June 15 and 16, 1975, the cosmonauts tested celestial navigation aboard Salyut-4. Within the scope of these experiments and eyeing future upgrades of navigational systems, they also photographed stars and the Earth's surface in the following two days.

Around the same time, back on Earth, the Crimean observatory, which closely worked with the crew, observed the emergence of a solar storm, as the active area on the solar disk was slowly drifting toward its western edge. On that news, all routine operations on Salyut-4 planned for June 18 were suspended to focus on the work of the OST solar telescope, according to instructions from Andrei Severny, the head of the Crimean observatory.

During a series of spectrographic sessions, the crew imaged a major ejection from the sun.

In the following days, the cosmonauts switched back to navigational tests and also continued studies of galactic radiation sources, including X-ray imaging of the galactic core of a nebula in the constellation Lira, as well as a pulsating source in the Hercules constellation.

From June 25 to around July 1, 1975, the crew was reported conducting more remote sensing work over Ukraine, the Volga Region, Kazakhstan and Middle Asia, and also continued life science experiments, including observations of fruit flies, known for their fast development cycles, thus making it possible to monitor them in special incubators.

The cosmonauts also performed some adjustments in the greenhouse facility.

On July 2 and 3, 1975, the crew captured some rare photos of high-altitude clouds, whose unexpected appearance prompted another change in the experiment schedule. For these operations, the crew changed the orientation of Salyut-4 over the night side of the planet, followed by photography and spectral imaging. The cosmonauts also used the same equipment to target aurora, which they observed over the Antarctic.

On July 4, 1975, the cosmonauts again dropped their planned activities to train their solar telescope on the active area in the Western section of the solar disk, according to instructions from the Crimean observatory. The crew also observed the polarization of solar radiation reflected by the Earth's surface and the atmosphere.

From July 8 to July 11, 1975, the Soyuz-17 crew was reported conducting more remote-sensing activities for "economic and scientific tasks." The cosmonauts used multiple instruments to scan Earth's surface in the European part of the USSR, Kazakhstan, Middle Asia, the Far Eastern Coast, the Kuril Islands, the Pamir Mountains, Sakhalin Island and the Eastern section of the Baikal-Amur Railroad, BAM, the official reports said. During these imaging sessions, the new Kaskad attitude-control system, which had been tested by the previous crew, provided routine orientation of the outspost.

During the same period, the Soyuz-18 crew also performed tests of the thermal-control and life-support systems on Salyut-4.

On July 9, the cosmonauts themselves proposed focusing on aurora and high-altitude cloud photography, this time over the Northern Hemisphere.

The observations of two additional active areas on the eastern and western sections of the solar disk were completed on July 13, 1975.

Returning to Earth

As their mission was drawing to a close, the Soyuz-18 cosmonauts returned to medical checks on July 15. On the same day, they got the news about the successful launch of the Soyuz and Apollo spacecraft for the joint ASTP mission and they got in touch with the orbiting Soyuz-19 crew via radio in the evening of July 16.

There was another round of scientific experiments, this time using the Freon fluid-behavior equipment, conducted on July 18, 1975, intended to benefit the development of hydraulic systems for spacecraft.

On July 20, 1975, the Soyuz-18 crew began mothballing operations aboard Salyut-4, preparing for the return to Earth. They were reported bringing film, samples from their scientific work and other materials into the Descent Module.

Before departing Salyut-4, the crew fired the engines of the Soyuz-18 spacecraft docked at the station, raising the orbital altitude of the joint stack. It was the first time, the transport ship had been used to boost the station's orbit, instead of reliance of the outpost's own engines, which was an important experience for future missions.

On July 26, 1975, the cosmonauts undocked from the Salyut-4 aboard their Soyuz-18 transport and successfully landed in Kazakhstan, some 56 kilometers northeast of Arkalyk at 17:18:18 Moscow Time.

They had spent nearly 63 days in orbit, which was a new record for the Soviet space program, though a couple of weeks short of the third US expedition aboard Skylab, two years earlier.

Upon emerging from their capsule, the cosmonauts walked unassisted to a medical tent deployed by the rescue personnel in a clear demonstration that Soviet methods for combating effects of weightlessness in long-duration missions were working. (50)

Soviet sources emphasized the significance of the Soyuz-17 and -18 missions for validating the latest strategy of using Earth-orbiting stations for multiple applications in space. The progressively longer expeditions were offering an opportunity for the crews to adapt to weightlessness and set up effective operations in orbit within the first week, after which, the cosmonauts could provide flexible and effective control of multiple activities in orbit during a prolonged periods of time. (152, 2)

Next page: Soyuz 7K-S

The article by Anatoly Zak; last update: July 28, 2025

Page editor: Alain Chabot; last edit: July 26, 2025

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Members of the Soyuz-18 crew: Vitaly Sevastyanov (left) and Petr Klimuk during training at Star City.

Rollout of Soyuz-18 from the vehicle assembly building to the launch pad.

Soyuz-18 arrives at launch pad.

Soyuz-18 on the launch pad in Tyuratam.

Sevastyanov (left) and Klimuk during a traditional meeting with launch personnel at Site 1 shortly after installation of their vehicle on the pad.

Soyuz-18 crew board the spacecraft on May 24, 1975.

Soyuz-18 at liftoff on May 24, 1975.

Members of the Soyuz-18 crew, Vitaly Sevastyanov (left) and Petr Klimuk, work inside Salyut.