The far side of the Moon is the hemisphere of the Moon that always faces away from Earth. The far side's terrain is rugged with a multitude of impact craters and relatively few flat lunar maria. It has one of the largest craters in the Solar System, the South Pole–Aitken basin. Both sides of the Moon experience two weeks of sunlight followed by two weeks of night; the far side is sometimes called the "dark side of the Moon", meaning unseen rather than lacking light.
About 18 percent of the far side is occasionally visible from Earth due to libration. The remaining 82 percent remained unobserved until 1959, when it was photographed by the Soviet Luna 3 space probe. The Soviet Academy of Sciences published the first atlas of the far side in 1960. The Apollo 8 astronauts were the first humans to see the far side with the naked eye when they orbited the Moon in 1968. All manned and unmanned soft landings had taken place on the near side of the Moon, until 3 January 2019 when the Chang'e 4 spacecraft made the first landing on the far side.
Tidal forces from Earth have slowed down the Moon's rotation to the point where the same side is always facing the Earth—a phenomenon called tidal locking. The other face, most of which is never visible from the Earth, is therefore called the "far side of the Moon". Over time, some parts of the far side can be seen due to libration. In total, 59 percent of the Moon's surface is visible from Earth at one time or another. Useful observation of the parts of the far side of the Moon occasionally visible from Earth is difficult because of the low viewing angle from Earth (they cannot be observed "full on").
The phrase "dark side of the Moon" does not refer to "dark" as in the absence of light, but rather "dark" as in unknown: until humans were able to send spacecraft around the Moon, this area had never been seen. While many misconstrue this to think that the "dark side" receives little to no sunlight, in reality, both the near and far sides receive (on average) almost equal amounts of light directly from the Sun. However, the near side also receives sunlight reflected from the Earth, known as earthshine. Earthshine does not reach the area of the far side that cannot be seen from Earth. Only during a full Moon (as viewed from Earth) is the whole far side of the Moon dark. The word "dark" has expanded to refer also to the fact that communication with spacecraft can be blocked while the spacecraft is on the far side of the Moon, during Apollo space missions for example.
The two hemispheres of the Moon have distinctly different appearances, with the near side covered in multiple, large maria (Latin for 'seas,' since the earliest astronomers incorrectly thought that these plains were seas of lunar water). The far side has a battered, densely cratered appearance with few maria. Only 1% of the surface of the far side is covered by maria, compared to 31.2% on the near side. One commonly accepted explanation for this difference is related to a higher concentration of heat-producing elements on the near-side hemisphere, as has been demonstrated by geochemical maps obtained from the Lunar Prospector gamma-ray spectrometer. While other factors, such as surface elevation and crustal thickness, could also affect where basalts erupt, these do not explain why the far side South Pole–Aitken basin (which contains the lowest elevations of the Moon and possesses a thin crust) was not as volcanically active as Oceanus Procellarum on the near side.
It has also been proposed that the differences between the two hemispheres may have been caused by a collision with a smaller companion moon that also originated from the Theia collision. In this model, the impact led to an accretionary pile rather than a crater, contributing a hemispheric layer of extent and thickness that may be consistent with the dimensions of the far side highlands.
The far side has more visible craters. This was thought to be a result of the effects of lunar lava flows, which cover and obscure craters, rather than a shielding effect from the Earth. NASA calculates that the Earth obscures only about 4 square degrees out of 41,000 square degrees of the sky as seen from the Moon. "This makes the Earth negligible as a shield for the Moon [and] it is likely that each side of the Moon has received equal numbers of impacts, but the resurfacing by lava results in fewer craters visible on the near side than the far side, even though both sides have received the same number of impacts."
Newer research suggests that heat from Earth at the time when the Moon was formed is the reason the near side has fewer impact craters. The lunar crust consists primarily of plagioclases formed when aluminium and calcium condensed and combined with silicates in the mantle. The cooler, far side experienced condensation of these elements sooner and so formed a thicker crust; meteoroid impacts on the near side would sometimes penetrate the thinner crust here and release basaltic lava that created the maria, but would rarely do so on the far side.
Until the late 1950s, little was known about the far side of the Moon. Librations of the Moon periodically allowed limited glimpses of features near the lunar limb on the far side, but only up to 59% of the total surface of the moon. These features, however, were seen from a low angle, hindering useful observation. (It proved difficult to distinguish a crater from a mountain range.) The remaining 82% 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 seen through libration is the Mare Orientale, which is a prominent impact basin spanning almost 1,000 km (600 miles), yet this was not 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. The basin was photographed in fine detail by Lunar Orbiter 4 in 1967.
Before space exploration began, astronomers did not expect that the far side would be different from the side visible to Earth. On October 7, 1959, the Soviet probe Luna 3 took the first photographs of the lunar far side, eighteen of them resolvable, covering one-third of the surface invisible from the Earth. The images were analysed, and the first atlas of the far side of the Moon was published by the USSR Academy of Sciences on November 6, 1960. It included a catalog of 500 distinguished features of the landscape. A year later, the first globe (1:13600000 scale) containing lunar features invisible from the Earth was released in the USSR, based on images from Luna 3. On July 20, 1965, another Soviet probe, Zond 3, transmitted 25 pictures of very good quality of the lunar far side, with much better resolution than those from Luna 3. In particular, they revealed chains of craters, hundreds of kilometers in length, but, unexpectedly, no mare plains like those visible from Earth with the naked eye. In 1967, the second part of the Atlas of the Far Side of the Moon was published in Moscow, based on data from Zond 3, with the catalog now including 4,000 newly discovered features of the lunar far side landscape. In the same year, the first Complete Map of the Moon (1:5000000 scale) and updated complete globe (1:10000000 scale), featuring 95 percent of the lunar surface, were released in the Soviet Union.
As many prominent landscape features of the far side were discovered by Soviet space probes, Soviet scientists selected names for them. This caused some controversy, and the International Astronomical Union, leaving many of those names intact, later assumed the role of naming lunar features on this hemisphere.
The first truly comprehensive and detailed mapping survey of the far side was undertaken by the American unmanned Lunar Orbiter program launched by NASA from 1966 to 1967. Most of the coverage of the far side was provided by the final probe in the series, Lunar Orbiter 5.
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.
It has been seen by all crew members of the Apollo 8 and Apollo 10 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.
Geologist-astronaut Harrison Schmitt, who became the last to step onto the Moon, had aggressively lobbied for his landing site to be on the far side of the Moon, targeting the lava-filled crater Tsiolkovskiy. Schmitt's ambitious proposal included a special communications satellite based on the existing TIROS satellites to be launched into a Farquhar–Lissajous halo orbit around the L2 point so as to maintain line-of-sight contact with the astronauts during their powered descent and lunar surface operations. NASA administrators rejected these plans on the grounds of added risk and lack of funding.
The China National Space Administration's Chang'e 4 made the first soft landing on the lunar far side on 3 January 2019. The craft included a lander equipped with a low-frequency radio spectrograph and geological research tools.
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 (60 mi) crater Daedalus is situated near the center of the far side, and the 3-kilometer-high (2 mi) rim would help to block stray communications from orbiting satellites. Another potential candidate for a radio telescope is the Saha crater.
Before deploying radio telescopes to the far side, several problems must be overcome. The fine lunar dust can 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 around 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 (39,000 mi) above the far side, which has also been proposed as a location for a future radio telescope which would perform a Lissajous 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 km (1,500 mi) across. The force 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.
Because the near side is partly shielded from the solar wind by the Earth, the far side maria are expected to have the highest concentration of helium-3 on the surface of the Moon. This isotope is relatively rare on the Earth, but has good potential for use as a fuel in fusion reactors. Proponents of lunar settlement have cited the presence of this material as a reason for developing a Moon base.
Some conspiracy theorists, notably Milton William Cooper, have alleged that some Apollo astronauts had seen UFOs on the far side of the Moon but were told to keep quiet about them. Some have allegedly reported seeing an alien base (code named "Luna") and even encountered aliens who told them to stay off the Moon. Some photographs circulated on the Internet purport to show a large "castle" on the Moon. The Apollo 20 hoax claims that a manned mission landed on the far side. NASA states that these claims are hoaxes.
Former United States Secretary of Defense Robert McNamara stated that several unspecified officials ("Chiefs") within The Pentagon were opposed to a Nuclear Test Ban Treaty between the United States and the Soviet Union, on the premise that the Soviets would continue nuclear weapons testing on the far side of the Moon, far from the observations of American observers. McNamara considered this premise "absurd" and that "[they were] out of [their] minds," but he believed that it was an example of the state of mind of some Pentagon officials during the Cold War. Ironically, it was later revealed that the Pentagon had their own plan to detonate a nuclear weapon on the Moon as part of the experiment Project A119. The project was created not only to help in answering some of the mysteries in planetary astronomy and astrogeology, but also as a show of force intended to boost domestic confidence in the astro-capabilities of the United States, a boost that was thought to be needed after the Soviet Union took an early lead in the Space Race and who were thought by some to be working on a similar project.
Belyaev is a lunar impact crater that is attached to the outer edge of the Mare Moscoviense, on the far side of the Moon. It is a worn formation with a small crater pair overlaying the southern rim, and several smaller craters across the relatively irregular interior.Champollion (crater)
Champollion is a lunar impact crater that lies on the far side of the Moon. It is located north-northeast of the larger crater Shayn, and south-southeast of Chandler.
This crater formation has been heavily damaged by subsequent impacts, and is now a battered depression in the surface. An unnamed crater overlays the eastern rim, and the remaining inner wall is incised along much of its circumference by lesser impacts. One of these forms a trough in the northern inner wall, with side walls that extend almost to the center of the floor.Chang'e 4
Chang'e 4 (; Chinese: 嫦娥四号; pinyin: Cháng'é Sìhào; literally: 'Chang'e No. 4') is a Chinese lunar exploration mission that achieved the first soft landing on the far side of the Moon, on 3 January 2019. A communication relay satellite, Queqiao, was first launched to a halo orbit near the Earth-Moon L2 point in May 2018. The robotic lander and Yutu 2 (Chinese: 玉兔二号; literally :"Jade Rabbit No. 2") rover were launched on 7 December 2018 and entered orbit around the Moon on 12 December 2018.
The mission is the follow-up to Chang'e 3, the first Chinese landing on the Moon. The spacecraft was originally built as a backup for Chang'e 3 and became available after Chang'e 3 landed successfully in 2013. The configuration of Chang'e 4 was adjusted to meet new scientific objectives. Like its predecessors, the mission is named after Chang'e, the Chinese Moon goddess.Chappell (crater)
Chappell is a lunar impact crater that is located on the far side of the Moon, in the northern hemisphere just to the north of the crater Debye. This feature is located in a heavily bombarded section of the surface, and much of the outer rim of the crater is overlain by many smaller craters. The northern rim in particular has been almost completely disintegrated, while small craters also overlie the rim to the northwest and southeast. What remains of the rim forms a rounded, somewhat irregular edge to the crater depression.
In contrast the interior floor is not notably marked by impacts except for a few tiny craterlets. The interior surface is more level and featureless compared to the rugged terrain that surrounds the crater. Near the midpoint of the floor is a low, rounded central ridge.Coblentz (lunar crater)
Coblentz is a small lunar impact crater that is located on the far side of the Moon, to the south of the much larger crater Bolyai. This crater retains a circular rim, but it has been worn by impact erosion. This is particularly so at the southern end where an irregular gap exists in the rim. The interior floor is relatively featureless save for several tiny craterlets.
A ridge arcs from the northwest rim of Coblentz to join the southern rim of Bolyai. There are several patches of dark (low albedo) material just to the south and southwest of Coblentz.Cooper (crater)
Cooper is a lunar impact crater that is located in the northern hemisphere on the far side of the Moon. It lies to the east of the large walled plain D'Alembert, and west-southwest of the crater Chappell.
This crater formation has been heavily worn and eroded by impact erosion. Little remains of the original rim, although its form can still be traced across the surface. Multiple small craters lie across the rim and inner wall, leaving a ring-shaped formation of ridges in the lunar terrain. The interior floor is slightly less rough than the surrounding surface, with a cluster of small craterlets near the northeast inner wall.Ctesibius (crater)
Ctesibius is a small lunar impact crater that is located near the equator, on the far side of the Moon. It is named after the ancient Greek-Egyptian inventor Ctesibius. It lies between the larger crater Abul Wáfa to the west and the slightly smaller Heron to the east.
The outer wall of Ctesibius is wide and sharp-edged, with little erosion. A low ridge is attached to the southern rim, and curves to the south-southeast. At the center of the relatively flat interior is a low ridge. Faint traces of ray material lies across the western floor and rim from Necho to the southeast.Danjon (crater)
Danjon, is a lunar impact crater on the far side of the Moon. It lies less than a crater diameter to the east-southeast of the larger crater Langemak. To the east-northeast of Danjon is the crater Perepelkin, and due south lies the walled plain Fermi.
The northeastern rim of Danjon is overlain by the smaller crater D'Arsonval. Danjon overlies the southeastern corner of the slightly smaller satellite crater Danjon X. The outer rim of this crater is worn and eroded, particularly at the southern end, and the interior floor is irregular and marked by several small craterlets.Dante (crater)
Dante is a lunar impact crater that is located on the far side of the Moon. It lies in the northern hemisphere exactly opposite the prime meridian facing the Earth. The nearest craters of note are Larmor to the north and Morse to the southeast. To the southwest is the oddly shaped Buys-Ballot.
This crater is overlain by part of the ray system radiating from Larmor Q to the northwest. The rim of Dante is circular but somewhat eroded. The fresh crater Dante G is attached to the exterior along the east-southeastern rim. The interior floor of this crater is uneven and marked by several small impacts.
The crater lies within the Freundlich-Sharonov Basin.Deutsch (crater)
Deutsch is a lunar impact crater on the far side of the Moon. It lies to the southwest of the larger crater Seyfert. About one crater to the east-northeast is Polzunov.
Its name comes from Armin Joseph Deutsch, an American astronomer and science-fiction author.
This crater has a relatively low, eroded rim that is heavily damaged along the southeastern section. This portion is overlain by Deutsch F along the east and Deutsch L to the south, with an irregular region between these two formations. The interior floor of Deutsch is relatively level, but is marked by a number of small impacts.
A ray from Giordano Bruno to the north-northwest passes along the western edge of Deutsch.Dewar (crater)
Dewar is a lunar impact crater that lies on the Moon's far side. Less than one crater diameter to the south-southwest is the crater Stratton. Vening Meinesz is a little over one crater diameter to the northwest. The slightly worn rim of this crater is roughly circular, with a small outward protrusion along the southern edge. The interior floor is marked by several small impacts along the eastern side.
Dewar lies on the south side of an anomalously low albedo area of terrain (dark patch) on the far side of the moon. The low-albedo area is also a geochemical anomaly, and is high in iron oxide and titanium dioxide. It has been interpreted as a cryptomare.Dunér (crater)
Dunér is an old lunar impact crater that is located in the northern hemisphere on the far side of the Moon. It lies to the southeast of the crater Chernyshev, and west-southwest of the Perkin–Debye crater pair.
This crater has been heavily battered by multiple small impacts, leaving a barely surviving outer rim that forms little more than a circular rise in the surface. The interior is in similar condition, and small craters cover many parts of the crater. The most notable impacts consist of a chain of overlapping small craters that run from near the midpoint and cross the southeastern rim. This chain is greater in length than the diameter of Dunér.Dziewulski (crater)
Dziewulski is a lunar impact crater on the far side of the Moon. It lies between the craters Edison to the north and Popov to the south. The outer rim of this crater has been considerably worn by impacts, particularly along the southwest quadrant where the satellite crater Dziewulski Q overlies the rim and the interior floor. The northern rim is also heavily disrupted, and several small crater lie along the southeast rim. The interior floor and surrounding terrain has been resurfaced.
Beginning at the southern edge, a chain of craters forms a linear formation running to the southeast past Popov. This feature is named Catena Dziewulski.Ellerman (crater)
Ellerman is a lunar impact crater on the far side of the Moon. It lies within the outer blanket of ejecta that surrounds the Mare Orientale impact basin, and is located to the west of the Montes Cordillera mountain range. To the northwest of Ellerman is the larger crater Gerasimovich.
Probably due to its location amidst rugged surroundings, the otherwise circular rim of this crater is somewhat irregular and polygonal in shape. The loose material along the inner walls has slid down to form a ring of talus around the base, leaving walls that slope straight down with no terraces. There is a small crater along the northern rim top.Espin (crater)
Espin is a lunar impact crater that lies on the far side of the Moon, just beyond the northeastern limb. It lies to the west-southwest of the larger crater Seyfert, and northwest of Deutsch.
This is a worn formation with heavy damage along the northern rim. Several small craters lie along the northern edge, and a crater lies across the southern rim. The northern part of the interior floor is somewhat irregular, but it is more level to the south. A ray from the crater Giordano Bruno to the north-northwest reaches the western interior of Espin.Evershed (crater)
Evershed is a lunar impact crater on the far side of the Moon, named after the English solar astronomer John Evershed. It is located to the northeast of the larger crater Cockcroft, and to the north of the smaller Van den Bergh.
This crater has a worn outer rim that is somewhat indented and narrower along the eastern side where the formation overlays an older crater. The satellite crater Evershed R is attached to the outer southwest rim. There are small craters along the southern and southeast rim. The interior floor contains an irregular ridge near the midpoint and some rugged terrain in the south, with various tiny craterlets marking the remaining relatively level surface.Fesenkov (lunar crater)
Fesenkov is a lunar impact crater on the far side of the Moon. It is located to the east-southeast of the prominent crater Tsiolkovskiy, and less than a crater diameter to the north of Stark.
This is an eroded feature with an outer rim that has been irregular and somewhat rugged due to a history of lesser bombardments in the vicinity. The interior floor is somewhat uneven, particularly in the eastern half, and there is a central rise at the midpoint. From the outer northeastern rim is a chain of tiny secondary impacts leading to the east, radial to the Tsiolkovsky impact.Freundlich (crater)
Freundlich is a lunar impact crater that is located on the far side of the Moon. It lies midway between the craters Trumpler to the north-northwest and the irregular Buys-Ballot to the south-southeast. This crater has a circular rim that is more heavily eroded at the northern and southern ends. Groups of craters lie across the floor to the southeast and the north, and individual small craters lie elsewhere within the interior. The crater is named after Erwin Freundlich.
The crater lies within the Freundlich-Sharonov Basin.Luna 3
Luna 3, or E-2A No.1 (Russian: Луна 3) was a Soviet spacecraft launched in 1959 as part of the Luna programme. It was the first-ever mission to photograph the far side of the Moon and the third Soviet space probe to be sent to the neighborhood of the Moon. Though it returned rather poor pictures by later standards, the historic, never-before-seen views of the far side of the Moon caused excitement and interest when they were published around the world, and a tentative Atlas of the Far Side of the Moon was created after image processing improved the pictures.
These views showed mountainous terrain, very different from the near side, and only two dark, low-lying regions which were named Mare Moscoviense (Sea of Moscow) and Mare Desiderii (Sea of Desire). Mare Desiderii was later found to be composed of a smaller mare, Mare Ingenii (Sea of Ingenuity), and several other dark craters. The reason for this difference between the two sides of the Moon is still not fully understood, but it seems that most of the dark lavas that flowed out to produce the maria formed under the Earth-facing half.Luna 3 was followed by the United States with Ranger 7, Ranger 8, and Ranger 9.