Soyuz 21 was the first of three missions to the Salyut 5 space station. It ended suddenly after 49 days because the Salyut station's air began causing severe discomfort to the crew. Zholobov was reported to be ill upon landing from nitric acid fumes. This was the first of 21 back-to-back human spaceflights where the Soviet Union was the only nation capable of launching humans into space as the U.S. developed the Space Shuttle.