Apollo 10 astronauts heard what they described as "outer space type music" during their historic mission around the dark side of the moon in 1969, according to a NASA audio tape.Crew members Eugene Cernan and John Young discuss the odd whistling sound in the tape that was confidential in 1969 at the height of the space race with the Soviet Union.

NASA said the tape, which was featured on the U.S. Science cable network earlier this week, was made publicly available since 1973. A NASA engineer said the unusual sound was interference between the module and the mission command center's radios.