While their capsule was preparing to re-enter the Earth’s atmosphere after a 23 day mission aboard the Salyut 1 (the first space station), the Soyuz 11 capsule suddenly depressurized, asphyxiating the three man crew. When the capsule landed as planned, the ground crew attempted resuscitation, but the men had been long dead.
Commander Georgi Dobrovolski, Flight Engineer Vladislav Volkov and Test Engineer Viktor Patsayev are the only three persons to have died in space. They were posthumously awarded the honor Hero of the Soviet Union (as did all Cosmonauts) and three lunar craters now carry their names.
Soyuz 11 pinback-buttons and magnet sets are available at An Enormous Door.