Salyut 6 EO-4

Salyut 6 EO-4 was the fourth long-duration resident expedition aboard Salyut 6. Leonid Popov commanded the flight with Valeri Ryumin as flight engineer, launching on Soyuz 35 from Baikonur Site 31/6 on April 9, 1980 and docking with Salyut 6 the following day. The crew maintained station systems, replaced attitude-control and life-support components, synchronized station clocks, handled Progress resupply vehicles, and supported biological, Earth-observation, materials-science, and technology experiments. They also hosted multiple visiting crews during a busy Intercosmos period, including the Soyuz 36 and Soyuz 37 exchanges. Popov and Ryumin returned on Soyuz 37 on October 11, 1980 after nearly 185 days in orbit, setting a new human spaceflight endurance record for the time.