Far side of the Moon - AbsoluteAstronomy.com

The far side of the Moon is the lunar

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

hemisphere that is permanently turned away, and is not visible from the surface of the Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

. The far hemisphere was first photographed by the Soviet Luna 3

Luna 3

The Soviet space probe Luna 3 of 1959 was the third space probe to be sent to the neighborhood of the Moon, and this mission was an early feat in the spaceborne exploration of outer space...

probe in 1959, and was first directly observed by human eyes when the Apollo 8

Apollo 8

Apollo 8, the second manned mission in the American Apollo space program, was the first human spaceflight to leave Earth orbit; the first to be captured by and escape from the gravitational field of another celestial body; and the first crewed voyage to return to Earth from another celestial...

mission orbited the Moon in 1968. The rugged terrain is distinguished by a multitude of crater impacts, as well as relatively few lunar maria. It includes the second largest known impact feature in the Solar System, the South Pole-Aitken basin

South Pole-Aitken basin

The South Pole-Aitken basin is an impact crater on Earth's Moon. Roughly in diameter and 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. This moon basin was named for two features on opposing sides; the...

. The far side has been suggested as a potential location for a large radio telescope

Radio telescope

A radio telescope is a form of directional radio antenna used in radio astronomy. The same types of antennas are also used in tracking and collecting data from satellites and space probes...

, as it would be shielded from possible radio interference from Earth. To date, there has been no ground exploration of the far side of the Moon.

Definition

Tidal forces from Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

have slowed down the moon's rotation so that the same side is always facing the Earth, a phenomenon called tidal locking

Tidal locking

Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...

. The other face, most of which is never visible from the Earth (18% of it can be observed over time due to libration

Libration

In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar libration:...

), is therefore called the "far side of the Moon". The far side should not be confused with the "dark side" (the hemisphere that is not illuminated by the Sun at a given point in time), as the two are the same only during a full moon

Full moon

Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...

and, during a new moon

New moon

In astronomical terminology, the new moon is the lunar phase that occurs when the Moon, in its monthly orbital motion around Earth, lies between Earth and the Sun, and is therefore in conjunction with the Sun as seen from Earth...

, the near side is the dark side. Both the near and far sides receive (on average) almost equal amounts of light from the Sun. However, the term "dark side of the moon" is commonly used poetically to refer to the far side.

Differences

The two hemispheres have distinctly different appearances, with the near side covered in multiple, large maria

Lunar mare

The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...

(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

Lunar Prospector

The Lunar Prospector mission was the third selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, including mapping of surface composition and possible...

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 farside South Pole-Aitken basin

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

Oceanus Procellarum

Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...

on the near side.

Another factor in the large difference between the two hemispheres is that the near side has been shielded from impacts by the Earth via the synchronous rotation that keeps the far side exposed to impactors coming from outer space.

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

Giant impact hypothesis

The giant impact hypothesis states that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. The colliding body is sometimes called Theia for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.The giant impact...

. 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 farside highlands.

Exploration

Until the late 1950s little was known about properties of the far side of the Moon. Libration

Libration

s of the Moon periodically allowed limited glimpses of features that are located near the lunar limb on the far side. 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 viewed through libration is the Mare Orientale

Mare Orientale

Mare Orientale is one of the most striking large scale lunar features, resembling a target ring bull's-eye. Located on the extreme western edge of the lunar nearside, this impact basin is difficult to see from an Earthbound perspective.Material from this basin was not sampled by the Apollo program...

, which is a prominent impact basin spanning almost 1000 kilometres (621.4 mi), yet this was not even named as a feature until 1906, by Julius Franz

Julius Heinrich Franz

Julius Heinrich Franz was a German astronomer.Franz was educated in Berlin, after which he was the principal astronomer at the Royal Observatory in Königsberg. In 1882 he was a member of a team sent to the town of Aiken, South Carolina, to observe the transit of Venus...

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

Lunar Orbiter 4

Lunar Orbiter 4 was designed to take advantage of the fact that the three previous Lunar Orbiters had completed the required needs for Apollo mapping and site selection...

in 1967.

On October 7, 1959 the Soviet

Soviet Union

The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

probe Luna 3

Luna 3

The Soviet space probe Luna 3 of 1959 was the third space probe to be sent to the neighborhood of the Moon, and this mission was an early feat in the spaceborne exploration of outer space...

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 (1:13 600 000 scale

Scale (map)

The scale of a map is defined as the ratio of a distance on the map to the corresponding distance on the ground.If the region of the map is small enough for the curvature of the Earth to be neglected, then the scale may be taken as a constant ratio over the whole map....

) 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

Zond 3

Zond 3 was a member of the Soviet Zond program sharing designation Zond, while being part of Mars 3MV project. It was unrelated to Zond spacecraft designed for manned circumlunar mission . Zond 3 completed a successful Lunar flyby, taking a number of good quality photographs for its time...

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. In 1967 the second part of the "Atlas of the Far Side of the Moon" was published in Moscow

Moscow

Moscow is the capital, the most populous city, and the most populous federal subject of Russia. The city is a major political, economic, cultural, scientific, religious, financial, educational, and transportation centre of Russia and the continent...

, based on data from Zond 3, with the catalog now including 4,000 newly discovered features of lunar far side landscape. In the same year the first "Complete Map of the Moon" (1:5 000 000 scale) and updated complete globe (1:10 000 000 scale), featuring 95 percent of the lunar surface were released in the Soviet Union.

As a lot of prominent landscape features of the far side were discovered by Soviet

Soviet Union

The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

space probes, Soviet scientists selected names for them. This caused some controversy, and the International Astronomical Union

International Astronomical Union

The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

, leaving many of those names intact, later assumed the role of naming lunar features on this hemisphere.

The far side was first observed directly by human

Human

Humans are the only living species in the Homo genus...

eyes during the Apollo 8

Apollo 8

mission in 1968. Astronaut

Astronaut

An astronaut or cosmonaut is a person trained by a human spaceflight program to command, pilot, or serve as a crew member of a spacecraft....

William Anders

William Alison Anders is a former United States Air Force officer, NASA astronaut, businessman, and engineer. He is, along with Apollo 8 crewmates Frank Borman and Jim Lovell, one of the first three persons to have left Earth orbit and traveled to the Moon .-Biography:Anders was born to Arthur...

described the view:
It has been seen by all crew members of the Apollo 8

Apollo 8

and Apollo 10

Apollo 10

Apollo 10 was the fourth manned mission in the American Apollo space program. It was an F type mission—its purpose was to be a "dry run" for the Apollo 11 mission, testing all of the procedures and components of a Moon landing without actually landing on the Moon itself. The mission included the...

through Apollo 17

Apollo 17

Apollo 17 was the eleventh and final manned mission in the American Apollo space program. Launched at 12:33 a.m. EST on December 7, 1972, with a three-member crew consisting of Commander Eugene Cernan, Command Module Pilot Ronald Evans, and Lunar Module Pilot Harrison Schmitt, Apollo 17 remains the...

missions since that time, and photographed by multiple lunar probes. Spacecraft passing behind the Moon were out of direct radio

Radio

Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

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

Lyndon B. Johnson Space Center

The Lyndon B. Johnson Space Center is the National Aeronautics and Space Administration's center for human spaceflight training, research and flight control. The center consists of a complex of 100 buildings constructed on 1,620 acres in Houston, Texas, USA...

before the craft reappeared.

Geologist-astronaut Harrison Schmitt

Harrison Schmitt

Harrison Hagan "Jack" Schmitt is an American geologist, a retired NASA astronaut, university professor, and a former U.S. senator from New Mexico....

, 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

Tsiolkovskiy (crater)

Tsiolkovskiy is a large lunar impact crater that is located on the far side of the Moon. It lies in the southern hemisphere, to the west of the crater Gagarin, and northwest of Milne. Just to the south is Waterman, with Neujmin to the south-southwest. This feature was discovered on photographs sent...

. Schmitt's ambitious proposal included a special communications satellites based on the existing TIROS satellites to be launched into a Farquhar-Lissajous halo orbit

Lissajous orbit

In orbital mechanics, a Lissajous orbit, , named after Jules Antoine Lissajous, is a quasi-periodic orbital trajectory that an object can follow around a Lagrangian point of a three-body system without requiring any propulsion. Lyapunov orbits around a libration point are curved paths that lie...

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 based on added risk and lack of funding.

Potential

Because the far side of the Moon is shielded from radio transmissions from the Earth, it is considered a good location for placing radio telescope

Radio telescope

A radio telescope is a form of directional radio antenna used in radio astronomy. The same types of antennas are also used in tracking and collecting data from satellites and space probes...

s for use by astronomer

Astronomer

An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

s. Small, bowl-shaped craters provide a natural formation for a stationary telescope

Telescope

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

similar to Arecibo

Arecibo Observatory

The Arecibo Observatory is a radio telescope near the city of Arecibo in Puerto Rico. It is operated by SRI International under cooperative agreement with the National Science Foundation...

in Puerto Rico

Puerto Rico

Puerto Rico , officially the Commonwealth of Puerto Rico , is an unincorporated territory of the United States, located in the northeastern Caribbean, east of the Dominican Republic and west of both the United States Virgin Islands and the British Virgin Islands.Puerto Rico comprises an...

. For much larger-scale telescopes, the 100 kilometres (62 mi) diameter crater Daedalus

Daedalus (crater)

Daedalus is a prominent crater located near the center of the far side of the Moon. The inner wall is terraced, and there is a cluster of central peaks on the relatively flat floor...

is situated near the center of the far side, and the 3 km (2 mi)-high rim would help to block stray communications from orbiting satellites. Another potential candidate for a radio telescope is the Saha crater

Saha (crater)

Saha is a lunar impact crater on the Moon's far side, behind the eastern limb as seen from the Earth. It lies less than one crater diameter due east of the similar-sized Wyld, and to the north-northwest of the large walled plain Pasteur....

.

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 flare

Solar flare

A solar flare is a sudden brightening observed over the Sun surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy . The flare ejects clouds of electrons, ions, and atoms through the corona into space. These clouds typically reach Earth a day...

s. Finally the area about the telescopes must be protected against contamination by other radio sources.

The Lagrange point of the Earth-Moon system is located about 62800 km (39,022.2 mi) above the far side, which has also been proposed as a location for a future radio telescope which would perform a Lissajous orbit

Lissajous orbit

about the Lagrangian point.

One of the NASA

NASA

The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

missions to the Moon under study would send a sample-return lander to the South Pole-Aitken basin

South Pole-Aitken basin

, the location of a major impact event that created a formation nearly 2400 kilometres (1,491 mi) 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.

Because the near side is partly shielded from the solar wind

Solar wind

The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

by the Earth, the far side maria

Lunar mare

are expected to have the highest concentration of helium-3

Helium-3

Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...

on the surface of the Moon. This isotope

Isotope

Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

is relatively rare on the Earth, but has good potential for use as a fuel in fusion

Nuclear fusion

Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

reactors. Proponents of lunar settlement have cited presence of this material as a reason for development of a Moon base.

External links

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Definition

Differences

Exploration

Potential

See also

External links