Soyuz 33 marked the first time the Soyuz's propulsion system failed in orbit. While 1 km from Salyut 6, Soyuz 33's engine shutdown early. Another attempt to start the engine also caused a shutdown. The Salyut 6 crew saw an unusual glow from Soyuz 33 and mission control aborted the flight. The backup engine fired to bring the craft and crew home. This called into question the reliability of Soyuz 32's engine for the Salyut 6 crew. After modifications, an uncrewed Soyuz 34 was launched to give the station crew a good return craft.