The Salyut (Russian: Салют, Salute or Firework) program was a series of space stations launched by the Soviet Union in the 1970s. The Salyuts were all relatively simple structures consisting of a single main module placed into orbit in a single launch. The program was originally designated the DOS 7-K program, with each Salyut station receiving a designation.
Salyut 1 (DOS 1) was launched April 19, 1971. It was a first human-made space station in the world. Its first crew launched in Soyuz 10 but was unable to board it due to a failure in the docking mechanism; its second crew launched in Soyuz 11 and remained on board for 23 productive days. Unfortunately, a pressure-equallization valve in the Soyuz 11 reentry capsule opened prematurely when the crew returned to Earth, killing all three. Salyut 1 reentered Earth's atmosphere October 11, 1971.
Salyut 2 was launched April 3, 1973. It was not really a part of the same program as the other Salyut stations, instead being the highly classified prototype military space station Almaz. It was given the designation Salyut 2 to conceal its true nature. Despite its successful launch, within two days the as-yet-unmanned Salyut 2 began losing pressure and its flight control failed; the cause of the failure was likely due to shrapnel piercing the station when the discarded Proton rocket upper stage that had placed it in orbit later exploded nearby.
The Salyut space station that Almaz had substituted for, designated DOS 3, was launched on May 11, 1973, three days before the launch of Skylab. Due to errors in the flight control system while out of the range of ground control, the station fired its orbit-correction engines until it consumed all of its fuel. Since the spacecraft was already in orbit and had been registered by Western radar, the Soviets disguised the launch as "Cosmos 557" and quietly allowed it to reenter Earth's atmosphere and burn up a week later. It was revealed to have been a Salyut station only much later.
Salyut 3 was launched on June 25, 1974. It was another Almaz military space station, this one launched successfully. It tested a wide variety of reconnaisance sensors, returning a canister of film for analysis. On January 24, 1975 trials of the on-board 23mm Nudelmann aircraft cannon (other sources say it was a Nudelmann NR-30 30mm gun) were conducted with positive results at ranges from 3000 m to 500 m. Cosmonauts have confirmed that a target satellite was destroyed in the test. The next day, the station was ordered to deorbit. Only one of the three intended crews successfully boarded and manned the sation, but Salyut 3 was an overall success.
Salyut 4 (DOS 4) was launched on December 26, 1974. It was essentially a copy of the DOS 3, and unlike its ill-fated sibling it was a complete success. Three crews made stays aboard Salyut 4, including one of 63 days duration, and an unmanned Soyuz capsule remained docked to the station for three months, proving the systems' long-term durability. Salyut 4 was deorbited February 3, 1977.
Salyut 5 was launched on June 22, 1976. It was the third and last Almaz military space station. Its launch and subsequent mission were both completed successfully, with three crews launching and two successfully boarding the craft for lengthy stays (the second crew was unable to dock and had to abort). Salyut 5 reentered on August 8, 1977.
Salyut 6 was launched on September 29, 1977. Although it resembled the previous Salyut stations in overall design, it featured several revolutionary advances including a second docking port where a unmanned Progress cargo spacecraft could dock and refuel the station. From 1977 until 1982 Salyut 6 was visited by five long-duration crews and 11 short-term crews, including cosmonauts from Warsaw Pact countries. The very first long-duration crew on Salyut 6 broke a record set onboard Skylab, staying 96 days in orbit. The longest flight onboard Salyut 6 lasted 185 days. The fourth Salyut 6 expedition deployed a 10-meter radio-telescope antenna delivered by a cargo ship. After Salyut 6 manned operations were discontinued in 1981, a heavy unmanned spacecraft called TKS and developed using hardware left from the canceled Almaz program was docked to the station as a hardware test. Salyut 6 was deorbited July 29, 1982.
Salyut 7 was launched on April 19, 1982. It was the back-up vehicle for Salyut 6 and very similar in equipment and capabilities, though several more advanced features were included. It was aloft for four years and two months, during which time it was visited by 10 crews constituting 6 main expeditions and 4 secondary flights (including French and Indian cosmonauts). Aside from the many experiments and observations made on Salyut 7, the station also tested the docking and use of large modules with an orbiting space station. The modules were called "Heavy Cosmos modules." They helped engineers develop technology necessary to build Mir. Salyut 7 deorbited on February 7, 1991.