February 16, 1937 - March 23, 1961: Age 24
Valentin Bondarenko was a Ukrainian cosmonaut. On March 23, 1961, in the oxygen-rich atmosphere of a training simulator, he removed some biosensors from his body and used a cotton ball moistened with alcohol to wash the sticky stuff off his skin. He tossed the cotton ball aside and it landed on an electric hot plate, where it caught fire. The fire ignited Bondarenko's suit.
Because of the pressure difference between the simulator and outside, it took several minutes for an attending doctor to get in and help him. He suffered third-degree burns over nearly the whole of his body: only the soles of his feet were relatively unharmed, as his flight boots had protected them. He died of shock eight hours after the accident.
Because of the Soviet media style of the time, news of his death was hushed up; however he had already appeared in group photographs and his disappearance from subsequent photos fueled speculation about cosmonauts dying in failed launches. The real story was not released publicly for 20 years.