Salyut 6, also known as 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.