According to some information released by NASA, astronauts on the Apollo 10 heard what they described as "outer space type music" - and no one said anything about it for almost 40 years.
In 1969, two months before Apollo 11 made history and landed on the moon, Apollo 10 traveled to the far side of the moon - the side that faces away from Earth. The astronauts were out of radio contact with Houston for about an hour, and according to an episode of "NASA's Unexplained Files" on the Science Channel, they picked up some unusual noises.
The recordings and transcripts were made classified until 2008. NASA scientists say interference from magnetic fields caused the sounds, but others say no because the moon doesn't have a magnetic field or enough of an atmosphere to even cause an interference.
Other experts say it's as simple as static between the different radio frequencies. But some are questioning why NASA chose to keep it a secret for forty years.
See the most iconic photos in space travel history:
Most iconic photos in space travel history
After 40 years, NASA declassifies 'outer space music' heard by astronauts on Apollo 10
Photograph of the far side of the moon taken by the luna 3 space probe on october 28, 1959. (Photo by: Sovfoto/UIG via Getty Images)
Earth Rise Viewed From The Moon, The First Photograph Of Earth Taken From The Vicinity Of The Moon, Captured By Lunar Orbiter 1, Aug, 23, 1966. (Photo By Encyclopaedia Britannica/UIG Via Getty Images)
UNITED STATES - FEBRUARY 03: The American Scientists William Pickering, James Van Allen And Werner Von Braum (From Left To Right) Raising A Replica Of The Explorer Satellite During A Press Conference In Washington, In Which They Announced Its Being Put Into Orbit, On February 3, 1958. (Photo by Keystone-France/Gamma-Keystone via Getty Images)
Soviet cosmonaut yuri gagarin, first man in space, in the capsule of vostok 1, april 12, 1961. (Photo by: Sovfoto/UIG via Getty Images)
Astronaut Edward White in Extravehicular Activity, during the Gemini 4 mission, He spent 21 minutes, outside the capsule (June 3, 1965). (Photo by Photo12/UIG/Getty Images)
NEW YORK, USA - UNDATED: Large color photograph, 20 by 16 inches, of a nearly nose-on view of the Gemini 7 spacecraft as seen and photographed by Tom Stafford onboard Gemini 6. Part of Gemini 6 is seen in the foreground. INSCRIBED AND SIGNED: 'Gemini 6 & 7, Tom Stafford, Plt, 15 Dec 1965' and additionally signed by WALLY SCHIRRA with 'CDR.' Estimate: $1,000 - 1,500. When Bonhams had their first space sale last year it became the highest-grossing American space history auction ever. On 13th April 2010 Bonhams will be selling more incredible space lots. Timed to coincide with the anniversary of Apollo 13, the sale comprises almost 300 lots including flight plan sheets, emblems, medallions, hardware, models, lunar surface equipment, charts and photographs. Many items come directly from astronauts' own collections. (Photo by Bonhams / Barcroft Media / Getty Images)
(GERMANY OUT) Apollo 1 disaster: a tragedy struck the Apollo 1 mission when a fire inside the space capsule caused the death of all three astronauts (3 weeks before its planned launch) - the exterior of the burned space capsule (Photo by Astro-Graphs/ullstein bild via Getty Images)
376713 15: (FILE PHOTO) The Apollo 11 Saturn V space vehicle lifts off July 16, 1969 from Kennedy Space Center's Launch Complex in Florida. The space craft was injected into lunar orbit on July 19 with Astronauts Neil A. Armstrong, Michael Collins and Edwin E. Aldrin Jr. on board. The 30th anniversary of the Apollo 11 Moon landing mission is celebrated July 20, 1999. (Photo by NASA/Newsmakers)
NBC NEWS -- Apollo 11 Moon Landing -- Pictured: (l-r) Kinescope images of astronaut Commander Neil Armstrong taking the first steps on the moon during the Apollo 11 Space Mission's moon landing for the first time in history on July 21, 1969 (Photo by NBC NewsWire/NBC/NBCU Photo Bank via Getty Images)
UNSPECIFIED - CIRCA 1754: US Astronaut Buzz Aldrin, walking on the Moon July 20 1969. Taken during the first Lunar landing of the Apollo 11 space mission by NASA. (Photo by Universal History Archive/Getty Images)
Earth Day, first held April 22, 1970, is now celebrated every year by more than a billion people in 180 nations around the world. All work together for the common goal of preserving the Earth and leaving it a better place for the future. This photo of Earth is from 1972. (Photo by NASA/MCT/MCT via Getty Images)
Damaged Apollo 13 Service Module, The Severely Damaged Apollo 13 Service Module (Sm) As Photographed From The Lunar Module/Command Module, An Entire Panel On The Sm Was Blown Away By The Explosion Of An Oxygen Tank. (Photo By Encyclopaedia Britannica/UIG
UNITED STATES - MAY 13: Pioneer 11, launched by NASA on 6th April 1973, returned the first close-up pictures of the ringed planet Saturn. The results, although visually spectacular, were rather disappointing from a scientific point of view. The second largest planet in the Solar System, Saturn was first observed through a telescope by Galileo in 1610, but its rings were not identified until 1659, by Christiaan Huygens. It is a gas giant similar in atmospheric composition to Jupiter, and rotates very quickly, causing it to appear oblate (flattened at the poles). The rings are composed of ice and ice-coated dust and rock. Their origin and formation are not precisely understood, but it seems that tidal effects caused by some of Saturnï¿½s moons play a role in maintaining their structure. (Photo by SSPL/Getty Images)
UNITED STATES - APRIL 24: This spectacular view of Skylab, clearly showing the sun shield, was taken by the crew of Skylab 4, the last manned mission to the space station, as they returned home. Skylab 4 Astronauts Gerald Carr, Edward Gibson and William Pogue lived aboard Skylab from 16th November 1973 to 8th February 1974 setting what was then a world spaceflight endurance record of 84 days. Skylab was intended to have two solar panels to supply electrical power to the station, but when the station arrived in orbit in 1973, one was found to be missing, while the other had not deployed. The first crew to visit the station made a spacewalk and were able to deploy the panel, restoring power to Skylab. The absence of the missing panel can clearly be seen in this picture. (Photo by SSPL/Getty Images)
21st July 1976: The first colour photograph taken on the surface of the planet Mars, by the Viking 1 probe. (Photo by MPI/Getty Images)
This dramatic view of Jupiter's Great Red Spot and its surroundings was obtained by Voyager 1 on Feb. 25, 1979. (Photo by: Universal History Archive/UIG via Getty Images)
UNITED STATES - JANUARY 13: The age of the Space Shuttle begins with the launch of Columbia on the STS-1 mission. Commander John Young and Pilot Robert Crippen were at the controls. (Photo by SSPL/Getty Images)
UNITED STATES - OCTOBER 29: Mission Specialist Bruce McCandless II is seen further away from the confines and safety of his ship than any previous astronaut has ever been. This space first was made possible by the Manned Manuevering Unit or MMU, a nitrogen jet propelled backpack. After a series of test maneuvers inside and above Challengerï¿½s payload bay, McCandless went ï¿½free-flyingï¿½ to a distance of 320 feet away from the Orbiter. This stunning orbital panorama view shows McCandless out there amongst the black and blue of Earth and space. (Photo by SSPL/Getty Images)
IN SPACE: In this NASA handout, a view of nearly 10,000 galaxies are seen in a Hubble Telescope composite photograph released March 9, 2004. The Hubble Ultra Deep Field (HUDF) photograph is a composite of a million one-second exposures and reveals galaxies from the time shortly after the big bang. (Photo by NASA/Getty Images)
Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this sharper global view of Pluto. (The lower right edge of Pluto in this view currently lacks high-resolution color coverage.) The images, taken when the spacecraft was 280,000 miles (450,000 kilometers) away from Pluto, show features as small as 1.4 miles (2.2 kilometers). That’s twice the resolution of the single-image view captured on July 13 and revealed at the approximate time of New Horizons’ July 14 closest approach. (Photo via NASA/JHUAPL/SwRI)
These dark, narrow, 100 meter-long streaks called recurring slope lineae flowing downhill on Mars are inferred to have been formed by contemporary flowing water. Recently, planetary scientists detected hydrated salts on these slopes at Hale crater, corroborating their original hypothesis that the streaks are indeed formed by liquid water. The blue color seen upslope of the dark streaks are thought not to be related to their formation, but instead are from the presence of the mineral pyroxene. The image is produced by draping an orthorectified (Infrared-Red-Blue/Green(IRB)) false color image (ESP_030570_1440) on a Digital Terrain Model (DTM) of the same site produced by High Resolution Imaging Science Experiment (University of Arizona). Vertical exaggeration is 1.5. (Photo by NASA/JPL/University of Arizona)