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.

Engineering experiments included propellant transfer system tests with implications for future operation of the freight-carrying Progress spacecraft.

As he fired the jets to move the spacecraft away, the docking mechanism jammed, resulting in the Soyuz being undocked but still linked to Salyut.

[1] Zholobov's illness was apparently caused by nitric acid fumes leaking from the Salyut's propellant tanks; other reports, however, indicate that the crew failed to follow their physical exercise program and suffered from lack of sleep.

[7] Sources at NASA have reported that psychologists with the Russian Aviation and Space Agency cited Soyuz 21 as ending prematurely due to unspecified "interpersonal issues" with the crew.