 During the Voskhod 2 mission in March 1965 , cosmonauts Pavel Belyayev and Alexey Leonov could not completely seal the spacecraft hatch after Leonov 's historic first walk in space . The spacecraft 's environmental control system responded to the leaking air by adding more oxygen to the cabin , causing the concentration level to rise as high as 45 % . The crew and ground controllers worried about the possibility of fire , remembering Bondarenko 's death four years earlier . 
