Salyut 4

Mission Profile

Salyut 4 (DOS-4) was a Soviet civilian space station launched on 26 December 1974 aboard a Proton-K, hosting two long-duration crews in 1975 as part of the progressive development of Soviet space station operations. The station was equipped with three solar telescopes for studying the Sun in X-ray, ultraviolet, and optical wavelengths, and hosted crews of two who set a Soviet duration record of 63 days during the second expedition. Salyut 4 also demonstrated an automated resupply capability with the uncrewed Soyuz 20 docking for 90 days to test life support system longevity. The station was deorbited on 3 February 1977 after 770 days in orbit, leaving behind a legacy of solar astronomy data and long-duration spaceflight experience that fed directly into the more ambitious Salyut 6 and 7 programs.