Salyut 4

Salyut 4, also designated DOS 4, was a Soviet orbital space station launched on 26 December 1974 into an orbit of about 343 by 355 kilometres inclined at 51.6 degrees. It was essentially a copy of the DOS 3 station, which had been lost as Kosmos 557, and unlike its sibling it was a complete success. The station kept the basic design of Salyut 1 but switched to three large solar panels mounted on the forward module, carried about 2,000 kilograms of scientific equipment and used the new Delta autonomous navigation system. Three crews tried to stay aboard, with Soyuz 17 and Soyuz 18 docking and Soyuz 18a suffering a launch abort, and the second stay lasted 63 days. The uncrewed Soyuz 20 remained docked for three months to prove long-term durability. Salyut 4 was deorbited on 2 February 1977.