Valery Polyakov, the Russian cosmonaut who set the record for the longest single stay in space, has died at age 80, Russia’s space agency announced Monday.

Polyakov’s record of 437 days in space began on Jan. 8, 1994, when he and two others blasted off on a two-day flight to the Mir space station Mir, operated by the Soviet Union and later by Russia. While aboard Mir, he orbited the Earth more than 7,000 times, before returning on March 22, 1995.

Upon landing, Polyakov declined to be carried out of the Soyuz capsule, as is common practice to allow readjustment to the pull of gravity. He was helped to climb out but was able to walk to a nearby chair and then a transport vehicle.

Polyakov had trained as a physician and wanted to demonstrate that the human body could endure extended periods in space.

Polyakov previously had spent 288 days in space on a mission in 1988 and 1989.

The announcement by space agency Roscosmos did not state a cause of death.