Soyuz 21

Soyuz 21 was a 1976 Soviet crewed mission to the Salyut 5 space station, the first of three flights to the station. The mission's objectives were mainly military in scope, but included other scientific work. The mission ended abruptly with cosmonauts Boris Volynov and Vitaly Zholobov returning to Earth after 49 days in orbit. The precise reason for the early end of the mission was the subject of much speculation, but was reported to be an emergency evacuation after the Salyut atmosphere developed an acrid odor, possibly from "the station's atmosphere becoming toxic as a result of the chemicals being used to expose surveillance photographs. By some accounts, Zholobov, in particular, fell ill."

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