Far side of the Moon

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Photograph of the far side of the Moon, with Mare Orientale (center left) and the mare of the crater Apollo (top left) being visible, taken by Orion spacecraft during the Artemis 1 mission Far side of the Moon.png
Photograph of the far side of the Moon, with Mare Orientale (center left) and the mare of the crater Apollo (top left) being visible, taken by Orion spacecraft during the Artemis 1 mission

The far side of the Moon is the lunar hemisphere that always faces away from Earth, opposite to the near side, because of synchronous rotation in the Moon's orbit. Compared to the near side, the far side's terrain is rugged, with a multitude of impact craters and relatively few flat and dark lunar maria ("seas"), giving it an appearance closer to other barren places in the Solar System such as Mercury and Callisto. It has one of the largest craters in the Solar System, the South Pole–Aitken basin. The hemisphere has sometimes been called the "dark side of the Moon", where "dark" means "unknown" instead of "lacking sunlight" – each location on the Moon experiences two weeks of sunlight while the opposite location experiences night. [1] [2] [3] [4]

Contents

About 18 percent of the far side is occasionally visible from Earth due to oscillation and 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 in person when they orbited the Moon in 1968. All crewed and uncrewed soft landings had taken place on the near side of the Moon, until January 3, 2019 when the Chang'e 4 spacecraft made the first landing on the far side. [5] [6]

Astronomers have suggested installing a large radio telescope on the far side, where the Moon would shield it from possible radio interference from Earth. [7]

Definition

Due to tidal locking, the inhabitants of the central body (Earth) will never be able to see the satellite's (Moon) green area Synchronous rotation.svg
Due to tidal locking, the inhabitants of the central body (Earth) will never be able to see the satellite's (Moon) green area

Tidal forces from Earth have slowed 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 crescent-shaped edges of the far side can be seen due to libration. [8] 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").

A common misconception is that the Moon does not rotate on its axis. If that were so, the whole of the Moon would be visible to Earth over the course of its orbit. Instead, its rotation period matches its orbital period, meaning it turns around once for every orbit it makes: in Earth terms, it could be said that its day and its year have the same length (i.e., ~29.5 earth days).

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. [1] [2] [3] In reality, both the near and far sides receive (on average) almost equal amounts of light directly from the Sun. This symmetry is complicated by sunlight reflected from the Earth onto the near side (earthshine), [9] and by lunar eclipses, which occur only when the far side is already dark. Lunar eclipses mean that the side facing earth receives fractionally less sunlight than the far side when considered over a long period of time.

At night under a "full Earth" the near side of the Moon receives on the order of 10 lux of illumination (about what a city sidewalk under streetlights gets; this is 34 times more light than is received on Earth under a full Moon) whereas the far side of the Moon during the lunar night receives only about 0.001 lux of starlight. [9] 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. [10]

Differences

Detailed view with Mare Moscoviense visible, by the Lunar Reconnaissance Orbiter (LRO) Moon Farside LRO.jpg
Detailed view with Mare Moscoviense visible, by the Lunar Reconnaissance Orbiter (LRO)

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, [11] 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. [12] 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. However, the chemical composition of the far side is inconsistent with this model.[ citation needed ]

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." [13]

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. [14]

Exploration

Early exploration

The 7 October 1959 image by Luna 3, which revealed for the first time the far side of the Moon. Clearly visible is Mare Moscoviense (top right) and a mare triplet of Mare Crisium, Mare Marginis and Mare Smythii (left center). Luna 3 moon.jpg
The 7 October 1959 image by Luna 3, which revealed for the first time the far side of the Moon. Clearly visible is Mare Moscoviense (top right) and a mare triplet of Mare Crisium, Mare Marginis and Mare Smythii (left center).
When the first ever image of the far side of the Moon (A) is restored using advanced noise removal techniques (B) and compared to later LRO mission from NASA (C) the important feature points are distinctly visible and a clear, one to one mapping of the visible feature points are noticeable. Luna 3 grainy photo restoration and comparison with LRO.png
When the first ever image of the far side of the Moon (A) is restored using advanced noise removal techniques (B) and compared to later LRO mission from NASA (C) the important feature points are distinctly visible and a clear, one to one mapping of the visible feature points are noticeable.

Until the late 1950s, little was known about the far side of the Moon. Librations 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. [15] 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 expected that the far side would be similar to the side visible to Earth. [16] On 7 October 1959, the Soviet probe Luna 3 took the first photographs of the lunar far side, eighteen of them resolvable, [17] [16] covering one-third of the surface invisible from the Earth. [18] The images were analysed, and the first atlas of the far side of the Moon was published by the USSR Academy of Sciences on 6 November 1960. [19] [20] It included a catalog of 500 distinguished features of the landscape. [21]

In 1961, the first globe (1:13600000 scale) [22] containing lunar features invisible from the Earth was released in the USSR, based on images from Luna 3. [23] On 20 July 1965, another Soviet probe, Zond 3, transmitted 25 pictures of very good quality of the lunar far side, [24] with much better resolution than those from Luna 3. In particular, they revealed chains of craters, hundreds of kilometers in length, [18] but, unexpectedly, no mare plains like those visible from Earth with the naked eye. [16]

In 1967, the second part of the Atlas of the Far Side of the Moon was published in Moscow, [25] [26] based on data from Zond 3, with the catalog now including 4,000 newly discovered features of the lunar far side landscape. [18] In the same year, the first Complete Map of the Moon (1:5000000 scale [22] ) and updated complete globe (1:10000000 scale), featuring 95 percent of the lunar surface, [22] were released in the Soviet Union. [27] [28]

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, though the Soviet Academy of Sciences selected many non-Soviet names, including Jules Verne, Marie Curie and Thomas Edison. [29] The International Astronomical Union later accepted many of the names.

Further survey mission

On 26 April 1962, NASA's Ranger 4 space probe became the first spacecraft to impact the far side of the Moon, although it failed to return any scientific data before impact. [30]

The first truly comprehensive and detailed mapping survey of the far side was undertaken by the American uncrewed 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 far side was first seen directly by human eyes during the Apollo 8 mission in December, 1968. Astronaut William Anders described the view:

“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.”

The far side of the Moon, with Mare Marginis and Mare Smythii visible, photographed by Apollo 16 in 1972. It is much more cratered than the near side of the Moon. Back side of the Moon AS16-3021.jpg
The far side of the Moon, with Mare Marginis and Mare Smythii visible, photographed by Apollo 16 in 1972. It is much more cratered than the near side of the Moon.

It has been seen by all 24 men who flew on Apollo 8 and Apollo 10 through Apollo 17, 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 Apollo 17's 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 idea of utilizing Earth–Moon L2 for communications satellite covering the Moon's far side has been realized, as China National Space Administration launched Queqiao and Queqiao-2 relay satellite in 2018. [31] It has since been used for communications between the Chang'e 4 lander and Yutu 2 rover that have successfully landed in early 2019 on the lunar far side and ground stations on the Earth. L2 is proposed to be "an ideal location" for a propellant depot as part of the proposed depot-based space transportation architecture. [32]

Soft landing

The Chang'e-4 lander imaged by the Yutu-2 rover on the lunar far side. ChangE-4 - PCAM.png
The Chang'e-4 lander imaged by the Yutu-2 rover on the lunar far side.

The China National Space Administration's Chang'e 4 achieved humanity's first ever soft landing on the lunar far side on 3 January 2019 and deployed Yutu-2 lunar rover onto far side lunar surface. [33]

The craft included a lander equipped with a low-frequency radio spectrograph and geological research tools. [34] The far side of the Moon provides a good environment for radio astronomy as interferences from the Earth are blocked by the Moon.

In February 2020, Chinese astronomers reported, for the first time, a high-resolution image of a lunar ejecta sequence, and, as well, direct analysis of its internal architecture. These were based on observations made by the Lunar Penetrating Radar (LPR) on board the Yutu-2 rover. [35] [36]

The Lunar Surface Electromagnetics Experiment (LuSEE-Night) lander, a mission to soft land as early as 2026 a robotic observatory on the far side designed to measure electromagnetic waves from the early history of the universe is being developed by NASA and the United States Department of Energy. [37]

The first panorama from the far side of the moon.jpg
The first panorama from the far side of the Moon taken by Chang'e 4

Potential uses and missions

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. [38]

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. [39]

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. [40] 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. [41]

Named features

Some of the features of the geography of the far side of the Moon are labeled in this image Craters of the Far Side of the Moon.jpg
Some of the features of the geography of the far side of the Moon are labeled in this image

See also

Related Research Articles

<span class="mw-page-title-main">Moon</span> Natural satellite orbiting Earth

The Moon is Earth's only natural satellite. It orbits at an average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth. Over time Earth's gravity has caused tidal locking, causing the same side of the Moon to always face Earth. Because of this, the lunar day and the lunar month are the same length, at 29.5 Earth days. The Moon's gravitational pull – and to a lesser extent, the Sun's – are the main drivers of Earth's tides.

<span class="mw-page-title-main">Oceanus Procellarum</span> Vast lunar mare on the western edge of the near side of Earths Moon

Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Moon. It is the only one of the lunar maria to be called an "Oceanus" (ocean), due to its size: Oceanus Procellarum is the largest of the maria ("seas"), stretching more than 2,500 km (1,600 mi) across its north–south axis and covering roughly 4,000,000 km2 (1,500,000 sq mi), accounting for 10.5% of the total lunar surface area.

<span class="mw-page-title-main">Mare Imbrium</span> Vast lunar mare filling a basin on Earths Moon

Mare Imbrium is a vast lava plain within the Imbrium Basin on the Moon and is one of the larger craters in the Solar System. The Imbrium Basin formed from the collision of a proto-planet during the Late Heavy Bombardment. Basaltic lava later flooded the giant crater to form the flat volcanic plain seen today. The basin's age has been estimated using uranium–lead dating methods to approximately 3.9 billion years ago, and the diameter of the impactor has been estimated to be 250 ± 25 km. The Moon's maria have fewer features than other areas of the Moon because molten lava pooled in the craters and formed a relatively smooth surface. Mare Imbrium is not as flat as it would have originally been when it first formed as a result of later events that have altered its surface.

<span class="mw-page-title-main">South Pole–Aitken basin</span> Large impact crater on the Moon

The South Pole–Aitken basin is an immense impact crater on the far side of the Moon. At roughly 2,500 km (1,600 mi) in diameter and between 6.2 and 8.2 km (3.9–5.1 mi) deep, it is one of the largest known impact craters in the Solar System. It is the largest, oldest, and deepest basin recognized on the Moon. It is estimated that it was formed 4.2 to 4.3 billion years ago, during the Pre-Nectarian epoch. It was named for two features on opposite sides of the basin: the lunar South Pole at one end and the crater Aitken on the northern end. The outer rim of this basin can be seen from Earth as a huge mountain chain located on the Moon's southern limb, sometimes informally called "Leibnitz mountains".

<span class="mw-page-title-main">Lander (spacecraft)</span> Type of spacecraft

A lander is a spacecraft that descends towards, then comes to rest on the surface of an astronomical body other than Earth. In contrast to an impact probe, which makes a hard landing that damages or destroys the probe upon reaching the surface, a lander makes a soft landing after which the probe remains functional.

<span class="mw-page-title-main">Shackleton (crater)</span> Lunar impact crater

Shackleton is an impact crater that lies at the lunar south pole. The peaks along the crater's rim are exposed to almost continual sunlight, while the interior is perpetually in shadow. The low-temperature interior of this crater functions as a cold trap that may capture and freeze volatiles shed during comet impacts on the Moon. Measurements by the Lunar Prospector spacecraft showed higher than normal amounts of hydrogen within the crater, which may indicate the presence of water ice. The crater is named after Antarctic explorer Ernest Shackleton.

<span class="mw-page-title-main">Von Kármán (lunar crater)</span> Lunar impact crater

Von Kármán is a large lunar impact crater that is located in the southern hemisphere on the far side of the Moon. The crater is about 186 km (116 mi) in diameter and lies within an immense impact crater known as the South Pole–Aitken basin of roughly 2,500 km (1,600 mi) in diameter and 13 km (8.1 mi) deep. Von Kármán is the site of the first soft-landing on the lunar far side by the Chinese Chang'e 4 spacecraft on 3 January 2019.

<span class="mw-page-title-main">Moon landing</span> Arrival of a spacecraft on the Moons surface

A Moon landing or lunar landing is the arrival of a spacecraft on the surface of the Moon, including both crewed and robotic missions. The first human-made object to touch the Moon was Luna 2 in 1959.

<span class="mw-page-title-main">Geology of the Moon</span> Structure and composition of the Moon

The geology of the Moon is quite different from that of Earth. The Moon lacks a true atmosphere, and the absence of free oxygen and water eliminates erosion due to weather. Instead, the surface is eroded much more slowly through the bombardment of the lunar surface by micrometeorites. It does not have any known form of plate tectonics, it has a lower gravity, and because of its small size, it cooled faster. In addition to impacts, the geomorphology of the lunar surface has been shaped by volcanism, which is now thought to have ended less than 50 million years ago. The Moon is a differentiated body, with a crust, mantle, and core.

<span class="mw-page-title-main">Exploration of the Moon</span> Missions to the Moon

The physical exploration of the Moon began when Luna 2, a space probe launched by the Soviet Union, made an impact on the surface of the Moon on September 14, 1959. Prior to that the only available means of exploration had been observation from Earth. The invention of the optical telescope brought about the first leap in the quality of lunar observations. Galileo Galilei is generally credited as the first person to use a telescope for astronomical purposes; having made his own telescope in 1609, the mountains and craters on the lunar surface were among his first observations using it.

<span class="mw-page-title-main">Near side of the Moon</span> Hemisphere of the Moon facing the Earth

The near side of the Moon is the lunar hemisphere that always faces towards Earth, opposite to the far side. Only one side of the Moon is visible from Earth because the Moon rotates on its axis at the same rate that the Moon orbits the Earth—a situation known as tidal locking.

<span class="mw-page-title-main">Halo orbit</span> Periodic, three-dimensional orbit

A halo orbit is a periodic, three-dimensional orbit associated with one of the L1, L2 or L3 Lagrange points in the three-body problem of orbital mechanics. Although a Lagrange point is just a point in empty space, its peculiar characteristic is that it can be orbited by a Lissajous orbit or by a halo orbit. These can be thought of as resulting from an interaction between the gravitational pull of the two planetary bodies and the Coriolis and centrifugal force on a spacecraft. Halo orbits exist in any three-body system, e.g., a Sun–Earth–orbiting satellite system or an Earth–Moon–orbiting satellite system. Continuous "families" of both northern and southern halo orbits exist at each Lagrange point. Because halo orbits tend to be unstable, station-keeping using thrusters may be required to keep a satellite on the orbit.

<span class="mw-page-title-main">Lunar Reconnaissance Orbiter</span> NASA robotic spacecraft orbiting the Moon

The Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting the Moon in an eccentric polar mapping orbit. Data collected by LRO have been described as essential for planning NASA's future human and robotic missions to the Moon. Its detailed mapping program is identifying safe landing sites, locating potential resources on the Moon, characterizing the radiation environment, and demonstrating new technologies.

<span class="mw-page-title-main">Lunar south pole</span> Southernmost point on the Moon

The lunar south pole is the southernmost point on the Moon. It is of interest to scientists because of the occurrence of water ice in permanently shadowed areas around it. The lunar south pole region features craters that are unique in that the near-constant sunlight does not reach their interior. Such craters are cold traps that contain fossil record of hydrogen, water ice, and other volatiles dating from the early Solar System. In contrast, the lunar north pole region exhibits a much lower quantity of similarly sheltered craters.

<span class="mw-page-title-main">Chang'e 4</span> Chinese lunar lander & rover

Chang'e 4 is a robotic spacecraft mission in the Chinese Lunar Exploration Program of the CNSA. China achieved humanity's first soft landing on the far side of the Moon with its touchdown on 3 January 2019.

<span class="mw-page-title-main">Chang'e 6</span> Planned Chinese lunar sample-return mission

Chang'e 6 is a planned robotic Chinese lunar exploration mission that has been officially announced for the first half of 2024 and is expected to launch around May 2024. It will attempt China's second sample return mission. The mission will attempt to obtain the first-ever soil and rock samples from the lunar far side and return it to the Earth; the primary phase of the mission is expected to last about 53 days. Like its predecessors, the spacecraft is named after the Chinese moon goddess Chang'e.

The following outline is provided as an overview of and topical guide to the Moon:

<span class="mw-page-title-main">Queqiao relay satellite</span> Chinese satellite

Queqiao relay satellite (Chinese: 鹊桥号中继卫星; pinyin: Quèqiáo hào zhōngjì wèixīng; lit. 'Magpie Bridge relay satellite'), is the first of the pair of communications relay and radio astronomy satellites for the Chinese Lunar Exploration Program. The China National Space Administration (CNSA) launched the Queqiao relay satellite on 20 May 2018 to a halo orbit around the Earth–Moon L2 Lagrangian point Queqiao is the first ever communication relay and radio astronomy satellite at this location.

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