Kosmos 4 was the first Soviet reconnaissance satellite to successfully reach orbit, it was a low resolution reconnaissance satellite derived from the Vostok spacecraft used for crewed flights. Kosmos 4 conducted a four-day mission, to measure radiation before and after the U.S. nuclear tests conducted during project Starfish Prime - a high-altitude nuclear test conducted by the US on July 9, 1962, and was the largest nuclear test conducted in outer space.
However, leaks from the oxygen system tanks used for the orientation system in Kosmos 4 resulted in the premature return of the spacecraft after three days of flight. During most of the flight the spacecraft was uncontrollable, before being deorbited and landing by parachute on April 29, 1962, and recovered by the Soviet forces in the steppe in Kazakhstan.
