There's no doubt space is an extremely dangerous place, but only a handful of space travelers have died there. Space is brutally inhospitable to human life, so it's a small wonder that out of the 561 people who have ventured beyond the safety of Earth, only three have died there.
The three brave spacefarers who lost their lives in space were cosmonauts Georgy Dobrovolsky, Vladislav Volkov, and Viktor Patsayev. All three died on the Soyuz 11 mission of Jun 1971.
The first space station to park itself above Earth's atmosphere was the USSR's Salyut 1, which launched on April 19, 1971. Just a few days later, a crew of three Soviets blasted off aboard Soyuz 10 intending to enter the space station and stay in orbit for a full month. Though the Soyuz 10 crew docked safely with the Salyut 1, issues with the entry hatch prevented them from entering the space station. During their premature return trip back to Earth, toxic chemicals leaked into the air supply of Soyuz 10, causing one cosmonaut to pass out. However, all three members of the crew ultimately made it home safe with no long-lasting effects.
Just a few months later, on June 6, the Soyuz 11 mission took another crack at accessing the space station. Unlike the previous crew, the three Soyuz 11 cosmonauts Georgi Dobrovolski, Vladislav Volkov, and Viktor Patsayev successfully entered Salyut 1. Once aboard, they spent the next three weeks not only setting a new record for the longest time spent in space but also carrying out loads of experiments focused on how the human body deals with extended periods of weightlessness.
On June 29, the cosmonauts loaded back into the Soyuz 11 spacecraft and began their descent to Earth. And that's when tragedy struck. To those on the ground, everything about Soyuz 11's reentry seemed to go off without a hitch. The spacecraft appeared to make it through the atmosphere just fine, ultimately landing in Kazakhstan as planned. It wasn't until recovery crews opened the hatch that they discovered all three crew members inside were dead.
The mission ended in disaster when the crew capsule depressurized during preparations for re-entry, killing the three-man crew. The three crew members of Soyuz 11 are the only humans known to have died in space.
As a direct result of the decompression deaths of the Soyuz 11 crew, the USSR quickly made the shift to requiring all cosmonauts to wear pressurized space suits during reentry a practice that's still in place today.