Soyuz 20 (Russian: Союз 20, Union 20) was an unmanned spacecraft launched by the Soviet Union. It was a long-duration test of the Soyuz spacecraft that docked with the Salyut 4 space station. Soyuz 20 performed comprehensive checking of improved on-board systems of the spacecraft under various flight conditions. It also carried a biological payload. Living organisms were exposed to three months in space. It was recovered on February 16, 1976.
First spacecraft to dock unmanned with a Salyut station. Carried life sciences experiments, qualified Soyuz for long-duration flights attached to a station, and served as proof-of-concept mission for Progress development.
Mir Hardware Heritage/Part 2 - Almaz, Salyut, and Mir
