 Its first crew launched in Soyuz 10 but were unable to board due to a failure in the docking mechanism; its second crew launched in Soyuz 11 and remained on board for 23 productive days.The world's first successful crewed flight to a space station was however overshadowed when the crew was killed before the re-entry of Soyuz 11 on 30 June 1971.A pressure equalisation valve in the descent module of the Soyuz opened prematurely when the three modules of the spacecraft separated, suffocating all three.