1975-05-24 USSR Soyuz 18

A Soyuz 18 First Day Cover from "KNIGA", postmarked in Moscow with a hand cancel on September 12, 1975, signed by Pyotr Klimuk and Vitaly Sevastyanov.

Soyuz 18 was launched from Baikonur on May 24, 1975, carrying cosmonauts Pyotr I. Klimuk (commander) and Vitaly I. Sevastyanov (flight engineer). Their mission was to continue the scientific and engineering work begun by the first crew of Salyut 4, Soyuz 17. NASA records the mission as the second crewed expedition to Salyut 4, lasting more than 62 days.

Only six weeks earlier, on April 5, 1975, another Soyuz spacecraft carrying Vasili Lazarev and Oleg Makarov had suffered a launch-vehicle failure and was forced to make an emergency abort. Klimuk and Sevastyanov, who had been the backup crew, were subsequently assigned to fly Soyuz 18.

A Soyuz 18 docking with the Salyut 4 space station cover, postmarked in Moscow on May 25, 1975, signed by Pyotr Klimuk and Vitaly Sevastyanov.

Salyut 4 was one of the early Soviet long-duration orbital space stations. Soyuz 17 had occupied it earlier in 1975, and Klimuk and Sevastyanov became its second and final crew. Soyuz 18 crew conducted approximately 90 scientific and engineering experiments during their stay. Their mission lasted approximately 63 days, establishing a new Soviet endurance record at the time.

In fact, while Soyuz 18 was operating aboard Salyut 4, the Apollo-Soyuz crews were preparing for their historic rendezvous. During the Apollo-Soyuz mission in July, the five ASTP astronauts and cosmonauts communicated with Klimuk and Sevastyanov aboard Salyut 4. This temporarily brought the number of people in space to seven, tying the record for the largest number of humans simultaneously in orbit at that time. NASA notes that Klimuk jokingly referred to them as the "magnificent seve".