Salyut 6, DOS-5, was a Soviet orbital space station, the eighth flown as part of the Salyut programme. Launched on 29 September 1977 by a Proton rocket, the station was the first of the 'second-generation' type of space station. Salyut 6 possessed several revolutionary advances over the earlier Soviet space stations, which it nevertheless resembled in overall design. These included the addition of a second docking port, a new main propulsion system and the station's primary scientific instrument, the BST-1M multispectral telescope. The addition of the second docking port made crew handovers and station resupply by unmanned Progress freighters possible for the first time, which in turn allowed the programme to evolve from short-duration station visits to long-duration expeditions, marking the beginning of the transition to multi-modular, long-term research stations in space. From 1977 until 1982, Salyut 6 was visited by five long- and eleven short-duration crews, including cosmonauts from Warsaw Pact countries as part of the Intercosmos programme. These crews were responsible for carrying out the primary missions of Salyut 6, including astronomy, Earth-resources observations and the study of the effect of spaceflight on the human body. Following the completion of these missions and the launch of its successor, Salyut 7, Salyut 6 was deorbited on 29 July 1982, almost five years after its launch.