Cosmos 638 was an uncrewed test of the 1975 Apollo-Soyuz Test Project Soyuz. It carried an APAS-75 androgynous docking system. This was followed by another uncrewed test of this spacecraft type, Cosmos 672. It was a Soyuz 7K-TM spacecraft.

When the air was released from the orbital module (which is ejected before re-entry of the capsule) it caused unexpected motions with the spacecraft. This led to the next test also being uncrewed.

This is Soyuz's 26th flight.

The Soyuz is a Soviet crewed spaceship, developed to made manned lunar missions. This version called 7K will fly 4 times on the giant launcher N1, and several tens of times on Proton to fly over the Moon, which will be successful during the mission Zond 4. Soyuz will become the first spacecraft to transport living beings to the Moon during the flight of Zond 5, with two turtles. Subsequently, it is adapted to low orbit and will fly on the Soyuz launcher to serve the Salyut and Mir stations and the ISS.