30 June 1971: Soyuz 11, with Cosmonauts Georgiy Timofeyevich Dobrovolsky, Vladislav Nikolayevich Volkov and Viktor Ivanovich Patsayev ended their 22 days aboard the Salyut 1 space station in orbit and began their return to Earth. At 2128 hours on the 29th, they undocked and completed three more orbits while they prepared for reentry.
At 0135 hours, the Soyuz spacecraft retrorockets fired to decelerate the ship so it would drop back into the atmosphere. 12 minutes, 15 seconds later, at an altitude of 104 miles (168 kilometers), a series of explosive bolts which connected the descent module to the service module detonated. They were intended to fire individually to limit the force on the capsule. Instead, they all fired simultaneously. The impact cause a seal in a pressure-equalization valve to fail and the capsule depressurized. Within 3 minutes, 32 seconds, the capsule’s atmospheric pressure had dropped to zero.
The cosmonauts were not wearing pressure suits. They quickly died.
Georgiy Dobrovolsky, Vladislav Volkov and Viktor Patsayev are the only people from Earth to have died in space since manned space flight began, 12 April 1961, with the flight of Yuri Gagarin.