Vladimir Aleksandrovich Dzhanibekov (Russian: Владимир Александрович Джанибеков, born 13 May 1942) is a former cosmonaut who made five flights. Dzhanibekov made five flights: Soyuz 27, Soyuz 39, Soyuz T-6, Soyuz T-12 and Soyuz T-13. In all he had spent 145 days, 15 hours and 56 minutes in space over these five missions. He had also performed two EVAs with a total time of 8 hours and 35 minutes. In 1985 he noted the effects of the tennis racket theorem, subsequently also called the Dzhanibekov effect, by showing that an object's second principal axis is unstable while in free-fall rotation.