Salyut 4 (DOS 4) (Russian: Салют-4; English translation: Salute 4) was a Salyut space station launched on December 26, 1974 into an orbit with an opogee of 355 km, a perigee of 343 km and an orbital inclanation of 51.6 degrees. It was essentially a copy of theDOS 3, and unlike its ill-fated sibling it was a complete success. Three crews attempted to make stays aboard Salyut 4 (Soyuz 17 and Soyuz 18 docked; Soyuz 18a suffered a launch abort). The second stay was for 63 days duration, and an unmanned Soyuz capsule remained docked to the station for three months, proving the system's long-term durability. Salyut 4 was deorbited February 2, 1977, and re-entered the Earth's atmosphere on February 3.
The crew worked between 15 and 20 hours a day, including their 21⁄2 hour exercise period. One of their activities included testing communication equipment for tracking ships and contacting mission control via a Molniya satellite.
Astrophysics was a major component of the mission, with the station's solar telescope activated on 16 January.The crew later discovered that the main mirror of the telescope had been ruined by direct exposure to sunlight when the pointing system failed. They resurfaced the mirror on 3 February and worked out a way of pointing the telescope using a stethoscope, stopwatch, and the noises the moving mirror made in its casing.
On 14 January, a ventilation hose was set up from Salyut 4 to keep the Soyuz ventilated while its systems were shut down. On 19 January it was announced that ion sensors were being used to orient the station, a system described as being more efficient.A new teleprinter was used for communications from the ground crew, freeing the Salyut crew from constant interruptions during their work.
The cosmonauts began powering down the station on 7 February and they returned to Earth in the Soyuz capsule two days later. They safely landed near Tselinograd in a snowstorm with winds of 72 km/h and wore gravity suits to ease the effects of re-adaptation.
