“Did you hear that whistling sound too?”

“Sounds like – you know, outer-space type music.”

“I wonder what it is.”

This conversation, between Apollo 10 astronauts Eugene Cernan and John Young, as their craft flew around the far side of the moon, remained under wraps for over four decades.

While transcripts were released in 2008, audio of the discussion, and the sounds that the astronauts were referencing, is only just being made public.

The Apollo 10 crew discuss strange noises heard on the far side of the moon

‘Weird music’

Out of radio contact with Earth and all alone on the far side of the moon, the astronauts were clearly not expecting to hear anything on their instruments.

“You hear that? That whistling sound? Whoooooo,” says Cernan on the recording.

“That sure is weird music.”

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According to a new Science Channel series “NASA’s Unexplained Files,” the sounds were so weird that the team debated whether or not to mention it to their superiors at NASA, out of fear that it could cast doubt on their suitability for future spaceflight.

However, in a statement released this week by NASA, Cernan himself cast doubt on this claim.

“I don’t remember that incident exciting me enough to take it seriously. It was probably just radio interference. Had we thought it was something other than that we would have briefed everyone after the flight. We never gave it another thought,” he said.

Unexplained? Not quite

While the trailers for the Science Channel series (and accompanying media coverage) make great store of the “unexplained” nature of the sounds, the truth is likely more scientific than sci-fi.

A NASA technician on the TV show supports Cernan’s assessment that the “radios in the two spacecraft [the lunar module and the command module] were interfering with each other.”

This explanation is disputed by the ponderous TV voiceover and astronaut Al Worden, who says on the show that “logic tells me that if there was something recorded on there, then there’s something there.”

Worden’s assertion that the sounds are unexplained is not one shared by his fellow astronauts, however.

The Apollo program included 12 manned missions that launched in the late 1960s and early '70s. Three missions orbited the Earth: Apollo 7, Apollo 9 and Apollo-Soyuz. Two missions orbited the moon: Apollos 8 and 10. Apollo 13 made a lunar swingby -- a malfunction forced NASA to cancel the landing. Six missions actually landed on the moon: Apollos 11, 12, 14, 15, 16 and 17.

The Apollo 11 lunar module Eagle prepares to land men on the moon for the first time. During each of the six Apollo missions that landed on the moon, two astronauts walked on the lunar surface.

Apollo 8 was the first manned spacecraft to reach the moon. The mission was designed to test the spacecraft and crew, but it did not include a lunar landing. From left are crew members James Lovell, William Anders and Frank Borman.

Apollo 8 launched on December 21, 1968, and entered lunar orbit on Christmas Eve. That night, the crew held a live broadcast and showed pictures of the Earth from their spacecraft. "The vast loneliness is awe-inspiring and it makes you realize just what you have back there on Earth," Lovell said. They ended the broadcast taking turns reading from the Book of Genesis.

The Apollo 10 mission was just like a lunar landing mission -- but without the landing. Crew members, from left, are Gene Cernan, John Young and Thomas Stafford. They launched on May 18, 1969, made 31 orbits of the moon and splashed down in the Pacific on May 26.

Apollo 11 was the first manned mission to land on the moon. The crew members, from left, were Neil Armstrong, Michael Collins and Edwin "Buzz" Aldrin. The mission launched on July 16, 1969.

On July 20, 1969, Armstrong became the first human to set foot on another world. NASA says more than half a billion people watched on television as Armstrong climbed down the ladder to the moon and proclaimed: "That's one small step for a man, one giant leap for mankind."

Armstrong took most of the photographs during his historic moonwalk, so you don't see many pictures of him -- this was before the age of the selfie. This rare shot from Aldrin shows Armstrong near the lunar module Eagle.

Aldrin was lunar module pilot and the second man to walk on the moon. On each lunar landing mission, one crew member stayed in orbit in the command module. On this mission that was Collins.

The crew of Apollo 12, from left, were Charles Conrad, Richard Gordon and Alan Bean. Conrad and Bean walked on the moon. Gordon stayed on the command module. The mission launched November 14, 1969, landed on the moon November 19 and returned to Earth on November 24.

Bean carries equipment on the moon during the Apollo 12 mission.

Conrad examines the Surveyor 3, a spacecraft that landed on the moon in 1967. The Apollo 12 crew touched down about 600 feet from the older spacecraft. They collected its television camera and several other pieces and brought them back to Earth.

The crew members of Apollo 13 -- from left, Fred Haise, James Lovell and John Swigert -- are seen after splashdown in April 1970. Apollo 13 was scheduled to be the third lunar landing mission. The crew launched on April 11, 1970, but two days later and about 205,000 miles from Earth, the service module oxygen tank ruptured, crippling the spacecraft. "Houston, we've had a problem," Lovell said. Instead of landing, the crew did a flyby and came home, safely splashing down on April 17. Lovell's book "Lost Moon" became the basis for the motion picture "Apollo 13."

The crew of Apollo 14: from left, Stuart Roosa, Alan Shepard and Edgar Mitchell. The mission launched January 31, 1971, landed on the moon February 5 and returned to Earth on February 9. Shepard and Mitchell conducted moonwalks while Roosa orbited in the command module.

Shepard stands beside a portable workbench set up on the moon.

Mitchell stands by the U.S. flag during his mission's first spacewalk.

Apollo 15 was the first mission capable of a longer stay on the moon, and the crew had its own rover. From left are Jim Irwin, David Scott and Alfred Worden. Irwin and Scott walked on the moon while Worden kept watch in the command module. The mission launched July 26, 1971, landed on the moon July 30 and returned to Earth on August 7.

Scott salutes the U.S. flag during an Apollo 15 moonwalk.

Irwin works near the lunar rover.

The Apollo 16 crew, from left: Thomas Mattingly, John Young and Charles Duke. Young and Duke walked on the moon while Mattingly stayed in the command module. The mission launched on April 16, 1972, landed on the moon April 20 and returned to Earth on April 27.

Young leaps from the surface of the moon as he salutes the U.S. flag.

Duke collects lunar samples.

Apollo 17 was NASA's last manned mission to the moon. It launched on December 7, 1972, landed on the moon December 11 and splashed down on Earth on December 19. From left are Harrison Schmitt, Gene Cernan and Ronald Evans. Schmitt and Cernan walked on the moon while Evans orbited in the command module. The crew stayed on the surface 75 hours and collected 243 pounds of lunar material.

Schmitt stands next to a huge lunar boulder during an Apollo 17 moonwalk. The lunar rover is in the background. Schmitt was the 12th man to set foot on the moon, but Cernan was the last to leave.

Cernan holds a corner of the American flag during the first Apollo 17 moonwalk. Cernan, the last man on the moon, had these parting words as he left: "We leave as we came and, God willing, as we shall return -- with peace, and hope for all mankind."

The American men who went to the moon

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Michael Collins, the pilot of Apollo 11 and the first person to fly around the far side of the moon by himself (while teammates Buzz Aldrin and Neil Armstrong were exploring the Lunar surface), also recalled hearing strange sounds, but did not think too much of it.

“There is a strange noise in my headset now, an eerie woo-woo sound,” he wrote in his book “Carrying the Fire: An Astronaut’s Journeys.”

“Had I not been warned about it, it would have scared the hell out of me (…) fortunately the radio technicians (rather than the UFO fans) had a ready explanation for it: it was interference between the LM’s and Command Module’s VHF radios.”

Cosmonaut Yuri Gagarin, the first human to fly into space, was launched in a Vostok 1 space capsule on April 12, 1961. He spent 108 minutes orbiting the Earth before parachuting back to firm ground.

Less than a month after Gagarin's trip, astronaut Alan Shepard became the first American to travel into space. On May 5, 1961, Shepard piloted Freedom 7, the first manned mission in the Mercury program. His suborbital flight lasted a little more than 15 minutes.

Sputnik I, the world's first satellite, was launched by the Soviet Union on October 4, 1957. It orbited the Earth every 98 minutes.

Laika the dog is pictured aboard Sputnik II on November 13, 1957. She was the first animal to orbit the Earth. She did not survive her trip, but the mission provided valuable data that paved the way for the first human in space.

John Glenn, aboard the Friendship 7, became the first American to orbit the Earth on February 20, 1962. He also set a record as the oldest astronaut in space when, at the age of 77, he went on a mission aboard the Space Shuttle Discovery in 1996.

Valentina Tereshkova, seen here with Yuri Gagarin, is the first woman to fly into space. She piloted the Vostok 6 on June 16, 1963.

Soviet cosmonaut Vladimir Komarov was the first human to die during a space mission. He died when the Soyuz 1 spacecraft crashed during its return to Earth on April 23, 1967. It was his second spaceflight.

On July 20, 1969, the Apollo 11 mission put the first humans on the moon. Neil Armstrong famously commemorated his first steps on the moon by saying, "That's one small step for man, one giant leap for mankind."

Edwin "Buzz" Aldrin, who was with Armstrong on Apollo 11, salutes the US flag on the lunar surface. Aldrin followed Armstrong and became the second man to walk on the moon.

This is the first photograph of Earth's south polar ice cap. It was taken by the crew of Apollo 17 as the astronauts traveled to the moon in December 1972.

Skylab, the United States' first space station, orbited Earth from 1973 to 1979. The Soviet program had launched their first space station, Salyut, in 1971, and it stayed in space for 15 years.

On July 15, 1975, Cold War adversaries temporarily broke the thaw when the United States and the Soviet Union embarked on their first joint space mission. Russia's Soyuz craft launched seven hours before the US Apollo craft, and the two vehicles linked up 52 hours after Soyuz lifted off. Here, the two crews pose for a portrait.

Soyuz 19 commander Alexei Leonov, left, shakes hands with Thomas Stafford, commander of the Apollo 18, on that first joint mission in 1975.

Gen. Arnaldo Tamayo Mendez, center, looks at a homemade rocket in Havana, Cuba, in 2009. Mendez became the first Latin American, the first person of African descent and the first Cuban to fly in space when he flew aboard the Soviet Soyuz 38 on September 18, 1980.

Columbia made the first orbital flight of NASA's space shuttle program on April 12, 1981. Here, crew members John Watts Young, left, and Robert Laurel Crippen hold a model of the orbiter in 1979.

Sally Ride became the first American woman to go into space when she was part of a crew aboard the space shuttle Challenger in June 1983.

Guion "Guy" Bluford was the first African-American to go into space. He was a mission specialist on the space shuttle Challenger in 1983.

In February 1984, astronaut Bruce McCandless became the first astronaut to float in space untethered, thanks to a jetpack-like device called the Manned Maneuvering Unit. The units are no longer used, but astronauts now wear a similar backpack device in case of emergency.

Jan Davis and Mark Lee were the first couple to go into space together when the husband and wife were astronauts on the space shuttle Endeavour in 1992.

The private company SpaceX sent an unmanned capsule with supplies to the International Space Station on October 7, 2012. It was the first commercial space mission and the first of a dozen commercial cargo flights under a contract with NASA.

New close-up images of a region near Pluto's equator were released by NASA in 2015. The New Horizons spacecraft was launched in 2006. It's the first spacecraft to explore Pluto and its moons. The mission completed the reconnaissance of the classical solar system, and it made the United States the first nation to send a space probe to every planet from Mercury to Pluto.

Famous firsts in space

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Collins explained that the noise began when the radios in the two vehicles were both turned on and in close proximity to each other.

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Unlike Apollo 10, the Apollo 11 lunar module did land on the moon’s service, after which the “woo-woo” noises stopped.

So much for “outer-space music.”