Far side of the Moon

This article concerns the far side of the Moon. For other uses of the expression see Far side (disambiguation).

The Far side of the Moon is the lunar hemisphere that is permanently turned away from the Earth. This face is not visible because the rotation of the moon about its axis is synchronized with its orbital period. This lock-step synchronization was achieved by the tidal forces between the Earth and the Moon.

Until the far side of the Moon was photographed by the Soviet probe Luna 3 in 1959, little was known about its properties. Librations of the Moon periodically allowed limited glimses of features that are located near the lunar limb on the far side. But these features were seen from a low angle, hindering useful observation. (It proved difficult to distinguish a crater from a mountain range.) The remaining 41% of the surface on the far side remained unknown, and its properties were subject to much speculation.

An example of a far side feature that can be viewed through libration is the Mare Orientale, which is a prominent impact basin spanning almost 1,000 kilometers. Yet this wasn't even named as a feature until 1906, by Julius Franz in Der Mond. The true nature of the basin was discovered in the 1960s when rectified images were projected onto a globe. It was photographed in fine detail by Lunar Orbiter 4 in 1967.

As the far side was first viewed by Soviet space probes, the Russians selected many of the names for the prominent features. This action provoked some controversy, and so the International Astronomical Union later assumed the role of naming lunar features on this hemisphere. However many of the names selected by the Soviets are still recognized.

The far side was first observed directly by human eyes during the Apollo 8 mission in 1968. It has been seen by all crew members of the Apollo 11 through Apollo 17 missions since that time, and photographed by multiple lunar probes. Spacecraft passing behind the Moon were out of direct radio communication with the Earth, and had to wait until the orbit allowed transmission. During the Apollo missions, the main engine of the Service Module was fired when the vessel was behind the Moon, producing some tense moments in Mission Control before the craft reappeared.

"The backside looks like a sand pile my kids have played in for some time. It's all beat up, no definition, just a lot of bumps and holes."
—Astronaut William Anders, describing the view during the Apollo 8 mission.

The two hemispheres have a distinctly different appearance, with the near side covered in multiple, large maria. The far side has a battered, densely-cratered appearance with few mares. Only 2.5% of the surface of the far side is covered by maria, compared to 31.2% on the near side. The most likely explanation for this difference is that the crust of the Moon is 40 km thicker on the far side. Thus it was more difficult for molten materials to penetrate to the surface.

Because the far side of the Moon is shielded from radio transmissions from the Earth, it is considered a good location for placing radio telescopes for use by Astronomers. Small, bowl-shaped craters provide a natural formation for a stationary telescope similar to Arecibo in Puerto Rico. For much larger-scale telescopes, the 100-kilometer diameter crater Daedalus is sitated near the center of the far side, and the 3-km-high rim would help to block stray communications from orbiting satellites. Other potential candidates for a radio telescope include the Saha crater and the South Pole-Aitken Basin.

Before deploying radio telescopes to the far side, several problems must be overcome. The fine lunar dust can serve to contaminate equipment, vehicles, and space suits. The conducting materials used for the radio dishes must also be carefully shielded against the effects of solar flares. Finally the area about the telescopes must be protected against contamination by other radio sources.

The L2 Lagrange point of the Earth-Moon system is located about 62,800 km above the far side. This has also been proposed as the location of a future radio telescope, performing a Lissajous ("Halo") orbit about the Lagrangian point.

One of the NASA missions to the Moon under study would send a sample-return lander to the South Pole-Aitken basin, the location of a major impact event that created a formation nearly 2,400 kilometers across. The size of this impact has created a deep penetration into the lunar surface, and a sample returned from this site could be analyzed for information concerning the interior of the Moon.

The far side was sometimes referred to as the "Dark Side", due to the lack of human knowledge concerning that hemisphere. The word "Dark" in a cultural context was meant to express a lack of information, rather than the actual lighting conditions. It is similar to the use of the expression "darkest continent", which portrayed the lack of knowledge concerning the conditions in the interior of the African continent by most of Europe in the 19th century.

Fictional References

The novel "Space" by James Michener tells the fictional story of an Apollo 18 mission to the far side of the Moon. The novel was the source for a 1985 TV mini-series of the same name.
The scientifically-questionable premise for the TV program "Space: 1999" was the explosion of a nuclear waste dump on the far side of the Moon. This propelled the Moon out of Earth's orbit and deep into space.