Mr. Titov was considered Russia's greatest living space legend. Police were treating his death as accidental, although no reason was given for the presence of the gas.
Mr. Titov was part of the 20-member team of test pilots who trained at Star City near Moscow in preparation for the first manned spaceflight in April 1961. Mr. Titov and Yuri Gagarin were the front-runners, cosmonaut instructor Cesar Solovyov said in a 1996 interview. Mr. Titov was brighter, Mr. Solovyov said, but he became backup cosmonaut for the pioneering flight, in part because Mr. Gagarin had a winning smile.
Missing out on the first manned flight was a lifelong regret for Mr. Titov, he later said in interviews, but it did not mar a distinguished career in the Soviet space program.
During his own flight in August 1961, the 25-year-old Mr. Titov orbited the Earth 17 times in the Vostok 2 and was in space for 25 hours, 18 minutes. He was the first man to sleep in space.
Mr. Titov was awarded the highest Soviet award, Hero of the Soviet Union. He wrote several books on space flight and was e