General Muhammed Ahmed Faris, a military aviator who became the first Syrian in space, fled to Turkey on Sunday after defecting from President Bashar al-Assad's regime, the Anatolia news agency reported.
Before crossing into Turkey, Faris visited the headquarters of the Free Syrian Army in Aleppo in a show of solidarity with rebel forces battling Assad's troops in Syria's biggest city, it said.
Anatolia said it was the fourth attempt to defect by Faris, who joined a Soviet crew on the orbiting space station Mir in 1987.
Faris was born in 1951 in Aleppo, where the Syrian army was pounding rebel positions on Sunday ahead of a threatened ground assault on the city.
Scores of senior Syrian military officers have been crossing into Turkey to link up with the FSA in recent months as the fighting in Syria escalates, often accompanied by rank-and-file troops.
Turkey has taken in more than 45,000 refugees fleeing the conflict across the border, and is also sheltering the military defectors in a separate camp where security is higher.