THE MOON information

• THE MOON, EARTH’S COMPANION IN SPACE, IS OUR PLANET’S LONE SATELLITE. IT IS THE LARGEST AND BRIGHTEST OBJECT IN THE NIGHT SKY AND THE ONLY ONE WHOSE SURFACE FEATURES CAN BE EASILY SEEN WITH THE NAKED EYE.

With a diameter one-quarter of Earth’s, the  Moon is the largest satellite in the solar system compared to its parent planet. Earth and the Moon exert a powerful influence on each other through their gravity.

Tidal forces have slowed the Moon’s rotation so that it spins once on its axis in the same time it takes to orbit Earth (27.32 days), and consequently keeps one face permanently toward our planet.

The Moon is a barren ball of rock that lacks sufficient gravity to hold on to a substantial atmosphere. Exposed alternately to the heat of the Sun and the emptiness of space, the lunar surface experiences wild temperature swings, from 248°F (120°C) at local noon to –274°F (–170°C) in the middle of the long lunar night. The floors of permanently shadowed craters get even colder.

With no weather or tectonic activity to erase craters, much of the Moon’s battered landscape preserves a barely altered record of conditions in our part of the solar system over the last 4 billion years.Earth and the Moon have been close partners for about 4.5 billion years.

Although the Moon is much smallerthan Earth, it influences our planet in many ways and has fascinated humans for thousands of years.

MOON STRUCTURE

• AS A RELATIVELY SMALL BODY, THE MOON HAS COOLED CONSIDERABLY IN THE 4.5 BILLION YEARS SINCE ITS FORMATION. ITS ROCKY INTERIOR HAS LARGELY SOLIDIFIED AROUND A CORE OF RED-HOT OR PARTIALLY MOLTEN IRON.

The Moon’s proximity to Earth has permitted scientists to investigate its inner structure in detail. Using seismometers placed on the surface by astronauts during the Apollo Moon landings, geologists can map the lunar interior by measuring the properties of moonquakes—seismic tremors triggered when tidal forces distort the shape of the Moon, or when meteorite impacts send shock waves through its interior.

More recently, spacecraft, including NASA’s twin GRAIL (Gravity Recovery and Interior Laboratory) satellites,have mapped the Moon’s structure by measuring slight variations in its gravitational field.

The Moon’s inner core is remarkably small,only around 150 miles (240 km) across.

EARTH’S COMPANION

• THE MOON IS SO LARGE COMPARED TO EARTH, AND ORBITS SO CLOSE TO ITS PARENT PLANET, THAT THE TWO WORLDS EXERT A CONSIDERABLE INFLUENCE ON EACH OTHER THROUGH THE FORCE OF GRAVITY.

The Moon’s large size relative to its parent planet is due to the unique way in which it formed. Most natural satellites in the solar system either formed from leftover debris after the new planet took shape or are small, captured objects such as asteroids.

Consequently,moons are usually dwarfed by their parent planet. Earth’s moon, in contrast, formed after a collision between Earth and another planet created a huge cloud of debris. Today, separated by an average distance of 238,900 miles (384,400 km), Earth and its moon exert a strong gravitational pull on one another that generates tidal forces in both worlds. These forces have slowed the Moon’s period of rotation and raise substantial tides in Earth’s oceans.

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DESTINATION HADLEY RILLE

• A STEEP-SIDED VALLEY RUNNING FOR 60 MILES (100 KM) ACROSS THE LUNAR LANDSCAPE,HADLEY RILLE IS THE REMNANT OF AN ANCIENT LAVA STREAM THAT FLOWED ACROSS THE MOON’S SURFACE AROUND 3 BILLION YEARS AGO.

Hadley Rille lies on the edge of the Mare Imbrium impact basin at the foot of the Montes Apenninus mountain range. The valley originates at an elongated crater called Bela, from where it winds across a plain known as the Palus Putridinus. The Rille is thought to be a lava channel that formed when lava flowed across the lunar surface like a river.

This image was taken in 1971, when Hadley Rille was observed during the Apollo 15 mission. Astronaut David Scott left a memorial sculpture and plaque at the site to commemorate astronauts who died in training and on missions.

EARTHRISE

This awe-inspiring Earthrise—the rising of Earth over the Moon’s horizon—was filmed by the Japanese spacecraft Kaguya on April 6, 2008, from an altitude of about 60 miles (100 km). For Kaguya to capture this spectacle, the orbits of the Moon, Earth, Sun, and spacecraft had to line up. Since Earth is almost stationary when viewed from the Moon, an Earthrise is most easily observed from orbiting spacecraft.

To see one from the Moon’s surface, an astronaut would need to be standing near one of the Moon’s poles.

CRATERS

• THE SURFACE OF THE MOON IS COVERED IN COUNTLESS CRATERS OF ALL SIZES. THEIR FORMATION HAS BEEN THE DRIVING FORCE SHAPING THE LUNAR LANDSCAPE FOR MORE THAN 4.5 BILLION YEARS.

How the Moon’s craters formed was not fully understood until the 1960s, when the first robot lunar landers showed that craters of all sizes existed, including tiny ones. This discovery confirmed that the craters must have been caused by impacts from space,rather than by volcanic eruptions.

It is now clear that craters cover all areas of the lunar terrain,although in some places the oldest craters have been covered over by later events, including volcanic eruptions and further impacts. The Moon is not the only heavily cratered body in the solar system, but it is the one we can study in greatest detail.

How craters form

The Moon’s well-preserved craters have given astronomers a detailed understanding of the crater formation process. The size and shape of a crater are determined mainly by the kinetic energy of the incoming object (a combination of its speed and mass).

Far side of the Moon

First revealed by Soviet spacecraft in the late 1950s, the lunar far side looks very different from the side we usually see. It appears more heavily cratered, principally because of the lack of lunar seas, or maria, formed by lava flows.

One theory is that the Moon’s crust is thicker   on the far side, which makes it harder for magma to rise to the surface and create maria.

Another possibility is that the far side of the moon cooled and solidified more quickly than the near side, forming robust rocks that  limited the depth of the far side’s impact basins.

Sea of Moscow

(Mare Moscoviense)

1 Incoming space rock Meteoroids approach the lunar surface at a variety of speeds, depending on whether they are catching up with the Moon or meeting it head-on.

2 Initial impact

The impact creates a shock wave thatvaporizes the meteoroid and ripples outinto the crust, compressing and heating it also bowl-shaped shock front.

3 Ejecta blanket

As the shock wave passes, material from thelanding site is thrown out, forming a layer of  debris on the surrounding landscape known as an ejecta blanket.

4 Crater

The result is a surface depression. In a large the crust may rebound to form a central peak; the sides may slump under their own weight to form terraces.

HIGHLANDS AND PLAINS

• THE LUNAR LANDSCAPE CAN BE BROADLY DIVIDED IN TWO DISTINCT TYPES OF TERRAIN: BRIGHT, HEAVILY CRATERED HIGHLANDS AND RELATIVELY SMOOTH, DARK PLAINS KNOWN AS LUNAR SEAS, OR MARIA.

The highlands represent the original ancient crust of the Moon,formed as its surface began to solidify from a molten magma ocean 4.5 billion years ago. They are dominated by bright silicate minerals similar to those of Earth’s crust, and they feature countless craters laid one on top of another over billions of years. The maria, meanwhile,are flat and sparsely cratered plains consisting of dark basaltic lavas.

Studies of the boundaries between the two regions show that the lunar maria are later surfaces that have erased all traces of earlier craters.

STORY OF THE MOON

• THE BIGGEST AND BRIGHTEST OBJECT IN THE NIGHT SKY HAS ALWAYS BEEN AN INVITING SUBJECT TO STUDY, AND PEOPLE HAVE TRACKED ITS MONTHLY CYCLE OF PHASES SINCE PREHISTORIC TIMES.

Moon-watching was important to the first agricultural societies of the Stone Age because the Moon’s phases served as a calendar, telling farmers when to sow and harvest crops. By Babylonian times, astronomers not only understood the phases

but could predict lunar eclipses, and by Greek times they knew the Moon was spherical and caused tides. Over the following centuries, our understanding of the Moon progressed in small steps as more details came to light: the nature of its rugged surface, its elliptical orbit, and its lack of air. But the giant leap in understanding came in the 20th century when the Moon became the first alien world people have set foot on.

20,000 BCE

Prehistoric calendar In the Ishango region of central Africa,people mark a bone with a series ofnotches that appear to track the monthly cycle and phases of the Moon. Modern researchers believe the Ishango bone is an early lunar calendar.

500 BCE

Predicting eclipses Babylonian astronomers (based in what is now Iraq) keep detailed records of lunar eclipses. They discover that eclipses occur in a repeating cycle and gain the ability to predict when eclipses will occur.

1969–72

Crewed missions

During the Apollo series of lunar missions,US astronauts land on the Moon, place measuring apparatus on its surface, and collect rock samples. Analysis of the samples greatly increases knowledge of the Moon’s surface composition, formation, and history.

1980S

Origins understood 

There is now agreement among scientists on the origins of the Moon. The favored hypothesis is that the Moon formed from a ring of debris around Earth—the aftermath of a collision between our planet and a planet the size of Mars.

1966

First soft landing

Another Soviet spacecraft, Luna 9, is the first to make a soft landing on the Moon.It confirms that lunar soil is firm enough to support the weight of a landing craft and that people will be able to walk on the Moon’s surface without sinking.

Impact theory

English astronomer Richard Proctor proposes that the Moon’s craters are caused by meteorite impacts and not, as generally believed, by volcanic activity. Proctor’s view is not fully accepted by astronomers until well into the 20th century.

1757

Moon mass measured

The French astronomer Alexis Clairaut, one of the leading mathematicians of the time,makes the first accurate measurement of the mass of the Moon, using the results of his observations to hone Isaac Newton’s early calculations.

1959

The far side

The Soviet spacecraft Luna 3 returns the first photographs of the far side of the Moon, which has never been seen before. These images reveal a heavily cratered surface that has fewer dark, flat regions, or maria, than the near side.

MISSIONS TO THE MOON

• OUR NEAREST NEIGHBOR HAS BEEN A TARGET FOR SPACECRAFT FOR OVER 50 YEARS. IT REMAINS THE ONLY DESTINATION BEYOND LOW  EARTH ORBIT THAT CREWED CRAFT HAVE VISITED, AND THE ONLY  SOLAR SYSTEM BODY BESIDES EARTH THAT PEOPLE HAVE WALKED ON. 

After a string of failures in the 1950s, the first craft to reach the Moon’s surface was  the Soviet probe Luna 2, which deliberately crash-landed in 1959. Three weeks later,  Luna 3 returned the first photos of the far side, causing great excitement. Dozens  of missions followed as the US and USSR raced to conquer the new frontier of space.  

More recent missions have aimed to undertake scientific research, but the Moon remains a compelling target for nations eager to demonstrate technological prowess.

Man on the Moon

• On July 20, 1969, 500 million people watched on TV as Neil Armstrong became the first   person to set foot on the lunar surface,  announcing, “That’s one small step for man, one  giant leap for mankind.” Altogether, 12 people  walked on the Moon between 1969 and 1972  in six successful missions. u TRANQUILITY BASE Aldrin and Armstrong’s  spacewalk was televised around the world.  

Walking on the Moon

The astronauts weighed only one-sixth of their normal weight  on the Moon—and so did their  life-support backpacks. Too  heavy to wear for long on Earth,  they were easily carried on the  Moon. Walking normally was  out of the question. Some used  a “kangaroo hop,” others a loping  walk. Some even enjoyed “skiing”  or gliding over the Moon dust,  by pushing off with their toes.

Return to the Moon

After the Apollo program ended in 1972, and the last Luna probe visited the Moon in 1976, there were no missions until Japan’s Hiten in 1990.  

Today, space agencies around the world are planning future missions to the Moon and beyond. 

, Japan achieved its first ever lunar  flyby, lunar  orbiter, and  lunar surface  impact with  Hiten: only the  third nation ever to  achieve this. 

MAPPING MISSIONS

• The launch of the Clementine spacecraft in 1994 heralded NASA’s return to the Moon.  Over the course of its 71-day orbit,  Clementine mapped all of the  15 million square miles  (38 million square kilometers)  of the Moon. NASA followed up  this successful mission with the Lunar  Prospector in 1998 and the LRO in 2009. u CLEMENTINE bounced radio  waves off the Moon’s surface and  found the first evidence of water ice. 

Multitalented

Clementine not only carried equipment into outer space to  test how it coped with a space  environment, but it also mapped  the topography (height) of the  Moon’s surface and the thickness  of its crust, taking over a million  pictures in total. Data provided by  Clementine suggested that there  may be frozen water in the deep  craters near the south pole. 

Lunar Prospector

The Lunar Prospector orbited for one year,  looking for ice beneath  the Moon’s poles. It also  searched for minerals and gases that  could be used on future manned  lunar bases and made into fuel for  launching spacecraft from the  Moon into outer space. 

Lunar Reconnaissance Orbiter (LRO)

The unmanned LRO was launched in 2009 to investigate possible sites for setting up a manned  base on the Moon. The Lunar Crater Observation  and Sensing Satellite (LCROSS) was sent up at  the same time. It was crashed into the surface in  a search for water ice. 

THE MOON CLEMENTINE’S data  guided the Lunar Prospector  orbiter to study places that  might contain water ice. 

THE LCROSS

mission confirmed that there was a little  water ice in at least one of the  Moon’s craters. LRO’s pictures also disproved claims that the  Apollo missions were a hoax.

The space nations have several plans for future lunar exploration. 

■ Chang’e-II (2010) A Chinese lunar orbiter. 

■ Luna-Glob 1 (2012) A Russian unmanned orbiter. 

■ ESA’s ESMO project (2013/2014) The first ever European Student Moon Orbiter.

■ Chandrayaan-2 (2013) India plans to land a rover on the Moon.

■ Luna-Glob 2 (2013) A joint Russian orbiter-rover mission with Chandrayaan-2. 

■ Chang’e III (2013) A lunar lander and rover are planned.

■ Luna-Grunt (2014 and 2015) Two separate lunar orbiters and landers planned. 

ESA (2017–2020) A lunar lander, capable of delivering cargo and exploration equipment to the Moon. 

■ India (2020) India’s first manned Moon mission.

And on to Mars? 

Several nations are eager to be the first to set up a manned base on the Moon, powered by the Sun and using water ice at the poles. Valuable minerals could be mined and sent back to Earth and the base could provide a stepping-stone for missions to Mars and other planets. China has already booked a ride for one of its satellites on Russia’s Phobos-Grunt robot mission to Mars in 2011 in a joint exploration venture.