Soyuz 18 was the second mission to have the designation Soyuz 18. The first, Soyuz 18a, ended with an in-flight abort during launch. The Soyuz 18 mission number was then given to the next flight. The two-person crew was the second to reach the Salyut 4 space station and performed a two month biology, astrophysics, medical, and environmental science mission. The flight overlapped with the Soyuz-Apollo Test Program and marked the first time two unrelated Soviet missions were in space together.