Russian cosmonaut Valery Polyakov, the holder of the world record for the longest single stay in space, has died at age 80.

Polyakov spent a record 437 days and 18 hours aboard the Mir space station from January 1994 to March 1995, a feat that earned him the Hero of Russia title.

Russia’s state space agency Roscosmos announced Polyakov’s death on Monday. It did not indicate the time or cause of his death.

Polyakov was born in the Soviet industrial heartland of Tula on April 24, 1942. He specialized in astronautics medicine and dedicated himself to space medicine after earning a doctoral degree at a Moscow medical school.

“His research has helped prove that the human body is ready to travel not only to Earth’s orbit, but also into deep space,” Roscosmos said in its statement expressing condolences over Polyakov’s death.

In an interview shared by Roscosmos, Polyakov claimed the 280 million kilometers he had amassed in his 437 consecutive days in space “is enough to reach Mars and come back.”

Polyakov made a total of two space expeditions, during which he spent 678 days and 16 hours in orbit. 

He was awarded the title Hero of the Soviet Union for his first space flight from August 1988 to April 1989, where he spent 240 days and 22 hours in orbit.