He retired from the Air Force in 1972, after 21 years of service. His autobiographies Return to Earth (1973) and Magnificent Desolation (2009) recount his struggles with clinical depression and alcoholism in the years after leaving NASA. Aldrin continues to advocate for space exploration, particularly a human mission to Mars, and developed the Aldrin cycler, a special spacecraft trajectory that makes travel to Mars more efficient in terms of time and propellant. He has been accorded numerous honors, including the Presidential Medal of Freedom in 1969.