Page "Boris Volynov" Paragraph 11

Because Soyuz 21 was returning early, it was outside the normal recovery window, and encountered strong winds as it descended, which caused uneven firing of the retrorockets.

It made a hard landing around midnight 200 km southwest of Kokchetav, Kazakhstan.

Zholobov's illness was apparently caused by nitric acid fumes leaking from the Salyut's propellant tanks.

However, other reports indicate that the crew failed to properly follow their physical exercise program and suffered from lack of sleep.