Russian cosmonaut Valery Polyakov, who held the record for the .longest single stay in space, died at the age of 80.

Valery Polyakov spent 437 days orbiting the Earth on the Mir space station between 1994 and 1995.

He conducted experiments to see if people could maintain their mental health if they were to make a long journey to Mars.

Tests revealed no impairment of his cognitive function as a result of the 14-month expedition.

Polyakov's death was announced by the Russian space agency, which used his honorary titles, including the hero of the Soviet Union and pilot-cosmonaut of the USSR.

Polyakov was born in 1942 in the city of Tula, south of the capital Moscow, and qualified first as a doctor and later as an astronaut.

He left for his first mission in August 1988. He spent eight months in orbit on that mission.

His space mission six years later gave Poliakov the record for the longest journey into space, a record that has yet to be surpassed.

Polyakov lived and worked on the Mir space station from 8 January 1994 to 22 March 1995, reportedly orbiting the Earth more than 7,000 times.

He later said that the duration of the trip was the equivalent of travelling to Mars and back.

On Polyakov's return, he is said to have declined an offer to be carried out of his capsule, which is standard practice as astronauts reacclimatise to gravity on Earth.

Instead, he climbed out with the assistance of others.

The Mir space station was sent into orbit in 1986, first under the control of the Soviet Union and later Russia.

Deployed during the Cold War, the 135-tonne (135,000kg) satellite was used with the cooperation of the Soviet Union and the US, despite longstanding political tensions.

It proved instrumental in developing an understanding of how humans might be able to live and work in space, before being retired in 2001.