On 20 February 2001, the bureau announced that a special agent, Robert Hanssen (born 1944) had been arrested for spying for the Soviet Union and then Russia from 1979 to 2001. He is serving 15 consecutive life sentences without the possibility of parole at ADX Florence, a federal supermax prison near Florence, Colorado. Hanssen was arrested on February 18, 2001 at Foxstone Park near his home in Vienna, Virginia, and was charged with selling US secrets to the USSR and subsequently Russia for more than US$1.4 million in cash and diamonds over a 22-year period. On July 6, 2001, he pleaded guilty to 15 counts of espionage in the United States District Court for the Eastern District of Virginia. His spying activities have been described by the US Department of Justice's Commission for the Review of FBI Security Programs as "possibly the worst intelligence disaster in U.S. history".

Answer this question, if possible (if impossible, reply "unanswerable"): When was Hanssen a spy for Russia/the SU?
1979 to 2001