Soyuz 20 mission is the designation for unmanned flight of a Soviet Soyuz spacecraft to the space station Salyut 4 It was the third visit by a Soyuz spacecraft in this space station and the 37th flight in the Russian Sojusprogramm.
Soyuz 20 was a modified variant ( GRAU index 11F615A9 ) of the Soyuz spacecraft and carried out the complete review of the improved on-board systems of the spacecraft under various flight conditions. The entire flight took place automatically. The spacecraft carried a biological payload. Living organisms were subjected to three months of weightlessness.