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Posted by pompos 03/04/2009 @ 18:14

Tags : moon, solar system, astronomy and space, sciences

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'Twilight Saga: New Moon' script found in trash...and returned - Entertainment Weekly
Casey Ray, a beauty salon owner, found the scripts for two Robert Pattinson films -- The Twilight Saga: New Moon and the romantic drama Memoirs, another Summit Entertainment production expected to film this summer -- in a trash bin in St. Louis....
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Gotz Otto playing Moon Nazi in 'Iron Sky' - Hollywood Reporter
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Robotic spiders may be headed to the moon - DVICE
The next visitors to the moon we send to earth may not be humans. No, instead they may be these robotic spiders, designed to quickly swarm across the lunar surface, collecting data through sensors and cameras. The concept is an entry in the Google X...
New Moon movie soundtrack hopefuls all around -
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JOSH MOON: Is Favre really a better bet than Jackson? - Montgomery Advertiser
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Dakota Fanning Finishes Up “New Moon” Duties - The Gossip Girls
The 15-year-old “I Am Sam” sweetheart spent only a week up north in Canada, busily shooting scenes for the “Twilight” sequel “New Moon.” Meanwhile, Miss Fanning is lined up to reunite with “Twilight” star Kristen Stewart, as both will be joining forces...
Ban Ki-Moon hails release of US-Iranian reporter from jail - Times
UN Secretary-General Ban Ki-moon on Wednesday (May 13) welcomed the release of US-Iranian journalist Roxana Saberi, who was sentenced to jail on charges of spying for the United States. "He (Ban) had discussed the matter on several occasions with the...
UN to expand relief efforts in Pakistan - CNN
UNITED NATIONS (CNN) -- UN Secretary-General Ban Ki-moon agreed on the need for additional humanitarian aid after meeting with Pakistani President Asif Zardari on Tuesday. Pakistani president Asif Zardari (left) met UN Secretary-General Ban Ki-moon in...
Uni students invent 'radiation-proof' cloth for Moon tents - Register
By Lewis Page • Get more from this author In news which could be a boon for the fledging inflatable Moon habitat tent podule industry, a group of textiles students from North Carolina say they have invented a radiation-proof, solar power generating...

Moon landing

In the Ocean of Storms, a widely reprinted 1967 Soviet painting by Aleksei Leonov and Andrei Sokolov, depicts a future traveler examining the Luna 9 braking rocket and landing capsule which had performed the first unmanned moon landing in 1966.  Leonov, who had previously made the first spacewalk, was at this time generally viewed as the Soviet cosmonaut most likely to become the first human on the Moon.

A moon landing is the arrival of an intact manned or unmanned spacecraft on the surface of a planet's natural satellite. The concept has been a goal of humankind since it was first appreciated that the Moon is Earth's closest large celestial body. One of the clearest early examples of the concept in fiction was Jules Verne's novel From the Earth to the Moon, written in 1865, although landing was made as the sequel, Around the Moon, reveals.

Since the Soviet Union first succeeded in implementing the concept in 1966, this term referred to 18 spacecraft landings on the Moon through 1976. Nine of these missions returned to Earth bearing samples of moon rocks. United States and India are the other countries to make unmanned moon landings.

The Soviet Union later achieved sample returns via the unmanned Luna 16, Luna 20 and Luna 24 moon landings. Since this was during the time of the Cold War, the contest to be the first on the Moon was one of the most visible facets of the Space Race.

Progress in space exploration has since broadened the phrase to include other moons in the solar system as well. The Huygens probe of the Cassini mission to Saturn performed a successful unmanned moon landing on Titan in 2005. Similarly, the Soviet probe Phobos 2 came within 120 miles (190 km) of performing an unmanned moon landing on Mars' moon Phobos in 1989 before radio contact with that lander was suddenly lost. A similar Russian sample return mission called Phobos-Grunt ("grunt" means "soil" in Russian) is scheduled for launch in October 2009. There is widespread interest in performing a future moon landing on Jupiter's moon Europa to drill down and explore the possible liquid water ocean beneath its icy surface.

The United States space agency NASA achieved the first manned landing on Earth's Moon as part of the Apollo 11 mission commanded by Neil Armstrong. On July 20, 1969, Armstrong landed the lunar module Eagle on the surface of the Moon with a companion, while the third astronaut orbited above. Armstrong and Buzz Aldrin spent a day on the surface of the Moon before returning to Earth. NASA carried out six manned moon landings between 1969 and 1972.

The primary concern of any moon landing is the high velocity involved that arises from the effects of gravity. In order to go to any moon, a spacecraft must first leave the gravity well of the Earth. The only practical way of accomplishing this currently is with a rocket. Unlike other airborne vehicles such as balloons or jets, only a rocket can continue to increase its speed at high altitudes in the vacuum outside the Earth's atmosphere.

Upon approach of the target moon, the spacecraft must decelerate enough to land safely. The velocity to be shed from the target moon's gravitational attraction is roughly equal to the escape velocity of the target moon. For Earth's Moon, this figure is 2.4 kilometers per second or around 6,000 miles per hour. This change in velocity (referred to as the delta-v) is usually provided by a landing rocket, which must be carried into space by the original launch vehicle as part of the overall spacecraft. An exception is a moon landing on Titan such as that carried out by the Huygens probe. As the only moon with an atmosphere, landings on Titan may be accomplished by using atmospheric entry techniques that are generally lighter in weight than a rocket with equivalent capability.

Whatever method is used to slow a spacecraft as it nears a moon, the key requirement for a "true" moon landing is to be traveling at a survivable speed upon reaching the moon's surface that allows continued operation after touchdown. Such landings may be characterized as "soft" if a human could survive them, and "hard" if only a ruggedized machine would do so. Initial American attempts at performing the first hard moon landing in 1962 failed; the Soviets succeeded in making the first successful hard landing on the Moon in 1966. Generally a hard landing is categorized as one occurring at 100 miles per hour or slower.

Above these speeds, the space mission ends not in a landing but a so-called crash impact where the vehicle and its instruments do not survive touchdown, which without braking rockets generally occurs at speeds of 3000-5000 miles per hour. Such impacts can occur because of malfunctions in a spacecraft, or they can be deliberately arranged for vehicles that do not have an on board landing rocket such as the 2008 Indian MIP. There have been many such moon crashes. For example, during the Apollo program the S-IVB third stage of the Saturn V moon rocket as well as the spent ascent stage of the lunar module were deliberately crashed on the moon several times to provide impacts registering as a moonquake on seismometers that had been left on the lunar surface. Such crashes were instrumental in mapping the internal structure of the Moon.

If a return to Earth is desired after a moon landing is accomplished, the escape velocities of the moon and Earth must again be overcome for the spacecraft to come to rest on the surface of the Earth. Rockets must be used to leave the moon and return to space. Upon reaching Earth, atmospheric entry techniques are used to absorb the kinetic energy of a returning spacecraft and reduce its speed for safe landing. These functions greatly complicate a moon landing mission and lead to many additional operational considerations. Any moon departure rocket must first be carried to the moon's surface by a moon landing rocket, increasing the latter's required size. The moon departure rocket, larger moon landing rocket and any Earth atmosphere entry equipment such as heat shields and parachutes must in turn be lifted by the original launch vehicle, greatly increasing its size by a significant and almost prohibitive degree. This necessitates optimizing the sizing of stages in the launch vehicle as well as consideration of using space rendezvous between multiple spacecraft and reaching intermediate orbits prior to landing; in particular, lunar orbit rendezvous. Thus systems engineering and logistics become major factors in the design of any moon landing mission.

The intense and expensive effort devoted in the 1960s to achieving first an unmanned and then ultimately a manned moon landing can only be understood in the political context of its historical era. World War II with its 60 million dead, half Soviet, was fresh in the memory of all adults. In the 1940s, the war had introduced many new and deadly innovations including blitzkrieg-style surprise attacks used in the invasion of Poland and in the attack on Pearl Harbor; the V-2 rocket, a ballistic missile which killed thousands in attacks on London; and the atom bomb, which killed tens of thousands in the atomic bombings of Hiroshima and Nagasaki. In the 1950s, tensions mounted between the two ideologically opposed superpowers of the United States and the Soviet Union that had emerged as victors in the conflict, particularly after the development by both countries of the hydrogen bomb.

On October 4, 1957, the Soviet Union launched Sputnik 1 as the first artificial satellite to orbit the Earth and so initiated the Space Age. This unexpected event was a source of pride to the Soviets and shock to the Americans. This dramatic and successful demonstration of the new R-7 Semyorka rocket on only its third test flight meant that the Soviets could use ballistic missiles carrying hydrogen bombs in a surprise attack against any target on Earth, a frightening new capability the Americans did not have. Further, the steady beeping of the radio beacon aboard Sputnik 1 as it passed overhead every 96 minutes was widely viewed on both sides as effective propaganda to Third World countries demonstrating the technological superiority of the Soviet political system compared to the American one. This perception was reinforced by a string of subsequent rapid-fire Soviet space achievements. In 1959, the R-7 rocket was used to launch the first escape from Earth's gravity into a solar orbit, the first crash impact onto the surface of the Moon and the first photography of the never-before-seen far side of the Moon. These were the Luna 1, Luna 2 and Luna 3 spacecraft, respectively.

The American response to these Soviet achievements was to greatly accelerate previously languishing space and missile projects. Military efforts were initiated to develop and produce mass quantities of intercontinental ballistic missiles (ICBMs) that would bridge the so-called missile gap and enable a policy of deterrence to nuclear war with the Soviets known as Mutually Assured Destruction or MAD. These newly-developed missiles were made available to civilians of the newly formed NASA space agency for various projects which would demonstrate the payload, guidance accuracy and reliabilities of American ICBMs to the Soviets. While NASA stressed peaceful and scientific uses for these rockets, their use in various lunar exploration efforts also had secondary goal of realistic, goal-oriented testing of the missiles themselves and development of associated infrastructure just as the Soviets were doing with their R-7. The tight schedules and lofty goals selected by NASA for lunar exploration also had an undeniable element of generating counter-propaganda to show to other countries that American technological prowess was the equal and even superior to that of the Soviets.

In contrast to Soviet lunar exploration triumphs in 1959, success eluded initial American efforts to reach the Moon with the Pioneer and Ranger programs. Fifteen consecutive U.S. unmanned lunar missions over a six year period from 1958 to 1964 all failed their primary photographic missions; however Rangers 4 and 6 successfully repeated the Soviet lunar impacts as part of their secondary missions. Failures included three American attempts in 1962 to hard land small seismometer packages released by the main Ranger spacecraft. These surface packages were to use retrorockets to survive landing, unlike the parent vehicle, which was designed to deliberately crash onto the surface. The final three Ranger probes performed successful high altitude lunar reconnaissance photography missions during intentional crash impacts at around 6,000 miles per hour as planned.

Three different designs of Pioneer lunar probes were flown on three different modified ICBMs. Those flown on the Thor booster modified with an Able upper stage carried an infrared image scanning television system with a resolution of 1 milliradian to study the Moon's surface, an ionization chamber to measure radiation in space, a diaphragm/microphone assembly to detect micrometeorites, a magnetometer, and temperature-variable resistors to monitor spacecraft internal thermal conditions. The first, a mission managed by the United States Air Force, exploded during launch; all subsequent Pioneer lunar flights had NASA as the lead management organization. The next two returned to Earth and burned up upon reentry into the atmosphere after achieved maximum altitudes of around 70,000 and 900 miles (1,400 km), far short of the roughly 250,000 miles (400,000 km) required to reach the vicinity of the Moon.

NASA then collaborated with the United States Army's Ballistic Missile Agency to fly two extremely small cone-shaped probes on the Juno ICBM, carrying only photocells which would be triggered by the light of the Moon and a lunar radiation environment experiment using a Geiger-Müller tube detector. The first of these reached an altitude of only around 64,000 miles (103,000 km), serendipitously gathering data that established the presence of the Van Allen radiation belts before reentering Earth's atmosphere. The second passed by the moon at a distance of over 37,000 miles (60,000 km), twice as far away as planned and too far away to trigger either of the on board scientific instruments, yet still becoming the first American spacecraft to reach a solar orbit.

The final Pioneer lunar probe design consisted of four "paddlewheel" solar panels extending from a one-meter diameter spherical spin-stabilized spacecraft body that was equipped to take images of the lunar surface with a television-like system, estimate the Moon's mass and topography of the poles, record the distribution and velocity of micrometeorites, study radiation, measure magnetic fields, detect low frequency electromagnetic waves in space and use a sophisticated integrated propulsion system for maneuvering and orbit insertion as well. None of the four spacecraft built in this series of probes survived launch on its Atlas ICBM outfitted with an Able upper stage.

Following the unsuccessful Atlas-Able Pioneer probes, NASA's Jet Propulsion Laboratory embarked upon an unmanned spacecraft development program whose modular design could be used to support both lunar and interplanetary exploration missions. The interplanetary versions were known as Mariners; lunar versions were Rangers. JPL envisioned three versions of the Ranger lunar probes: Block I prototypes, which would carry various radiation detectors in test flights to a very high Earth orbit that came nowhere near the Moon; Block II, which would try to accomplish the first Moon landing by hard landing a seismometer package; and Block III, which would crash onto the lunar surface without any braking rockets while taking very high resolution wide-area photographs of the Moon during their descent.

The Ranger 1 and 2 Block I missions were virtually identical. Spacecraft experiments included a Lyman-alpha telescope, a rubidium-vapor magnetometer, electrostatic analyzers, medium-energy-range particle detectors, two triple coincidence telescopes, a cosmic-ray integrating ionization chamber, cosmic dust detectors, and scintillation counters. The goal was to place these Block I spacecraft in a very high Earth orbit with an apogee of 670,000 miles (1,080,000 km). From that vantage point, scientists could make direct measurements of the magnetosphere over a period of many months while engineers perfected new methods to routinely track and communicate with spacecraft over such large distances. Such practice was deemed vital to be assured of capturing high-bandwidth television transmissions from the Moon during a one-shot fifteen minute time window in subsequent Block II and Block III lunar descents. Both Block I missions suffered failures of the new Agena upper stage and never left low earth parking orbit after launch; both burned up upon reentry after only a few days.

The first attempts to perform a Moon landing took place in 1962 during the Rangers 3, 4 and 5 missions flown by the United States. All three Block II missions carried a 94 pound, two-foot diameter landing sphere (made of balsa wood) designed to withstand a 150 mile per hour impact. This lander (code-named Tonto) was designed to provide impact cushioning using an exterior blanket of crushable balsa wood and an interior filled with incompressible liquid freon. A 56 pound, one-foot diameter metal payload sphere floated and was free to rotate in a liquid freon reservoir contained in the landing sphere. This payload sphere contained six silver-cadmium batteries to power a fifty milliwatt radio transmitter, a temperature sensitive voltage controlled oscillator to measure lunar surface temperatures, and a seismometer that was designed with sensitivity high enough to detect the impact of a five pound meteorite on the opposite side of the Moon. Weight was distributed in the payload sphere so it would rotate in its liquid blanket to place the seismometer into an upright and operational position no matter what the final resting orientation of the external landing sphere. After landing plugs were to be opened allowing the freon to evaporate and the payload sphere to settle into upright contact with the landing sphere. Four pounds of water were also included to provide thermal control for the lander, absorbing heat and boiling off as low-pressure steam during the hot lunar daytime and retaining sufficient heat to allow the lander electronics to avoid freezing temperatures during the cold lunar nighttime. The batteries and water supply were sized to allow up to three months of operation for the payload sphere. Various mission constraints limited the landing site to Oceanus Procellarum on the lunar equator, which the lander ideally would reach 66 hours after launch.

No cameras were carried by the Ranger landers, and no pictures were to be captured from the lunar surface during the mission. Instead, the ten-foot-high, 730 pound Ranger Block II mother ship carried a 200 scan line television camera which was to capture images from 2,400 miles (3,900 km) down to 37 miles (60 km) during the free-fall descent to the lunar surface. The 13 pound camera was designed to transmit a picture every 10 seconds. Other instruments gathering data before the mother ship crashed onto the Moon at 6,500 miles per hour were a gamma ray spectrometer to measure overall lunar chemical composition and a radar altimeter. At eight seconds before impact and 13 miles (21 km) above the lunar surface, the radar altimeter was to give a signal ejecting the landing capsule and its 236 pound solid-fueled braking rocket overboard from the Block II mother ship. The braking rocket was to slow the landing sphere to a dead stop at 1,100 feet (340 m) above the surface and separate, allowing the landing sphere to free fall once more and hit the surface at a survivable speed of 100 miles per hour.

On Ranger 3, failure of the Atlas guidance system and a software error aboard the Agena upper stage combined to put the spacecraft on a course that would miss the Moon. Attempts to salvage lunar photography during a flyby of the Moon were thwarted by in-flight failure of the onboard flight computer. This was probably because of prior heat sterilization of the spacecraft by keeping it above the boiling point of water for 24 hours on the ground, to protect the Moon from being contaminated by Earth organisms. Heat sterilization was also blamed for subsequent in-flight failures of the spacecraft computer on Ranger 4 and the power subsystem on Ranger 5. Only Ranger 4 reached the Moon in an uncontrolled crash impact on the far side of the Moon.

Heat sterilization was discontinued for the final four Block III Ranger probes. These replaced the Block II landing capsule and its retrorocket with a heavier, more capable television system to support landing site selection for upcoming Apollo manned moon landing missions. Six cameras weighing a total of 350 pounds were designed to take thousands of high-altitude photographs in the final twenty minute period before crashing on the lunar surface. Camera resolution was 1,132 scan lines, far higher than the 525 lines found in a typical American 1964 home television. The final pictures taken were expected to have a resolution of around two feet. While Ranger 6 suffered a failure of this camera system and returned no photographs despite an otherwise successful flight, the subsequent Ranger 7 mission to Mare Cognitum was a complete success. Breaking the six year string of failure in American attempts to photograph the moon at close range, the Ranger 7 mission was viewed as a national turning point and instrumental in allowing the key 1965 NASA budget appropriation to pass through the United States Congress intact without a reduction in funds for the Apollo manned moon landing program. Subsequent successes with Ranger 8 and Ranger 9 further buoyed American hopes.

While American lunar exploration missions were undertaken in full view of public scrutiny, Soviet moonshots of the 1960s and 1970s were conducted under a policy of extreme governmental secrecy. Only with the coming of glasnost in the late 1980s and the fall of the Soviet Union in 1991 did historical records come to light allowing a true accounting of Soviet lunar efforts. Unlike the American tradition of assigning a particular mission name in advance of launch, the Soviets assigned a public "Luna" mission number only if a launch resulted in a spacecraft going beyond Earth orbit. If the attempt failed in Earth orbit before departing for the Moon, it was frequently (but not always) given a "Sputnik" or "Cosmos" earth-orbit mission number to hide its failure in reaching the Moon. Launch explosions were not acknowledged at all. This policy had the effect of hiding Soviet moonshot failures from public view, making their successes seem even more impressive.

The Luna 9 spacecraft, launched by the Soviet Union, performed the first successful Moon landing on February 3, 1966 using the "hard landing" technique. Airbags protected its 200 pound ejectable capsule which survived an impact speed of over 30 miles per hour—the speed of many automobile accidents causing fatalities on Earth. Luna 13 duplicated this feat with a similar moon landing on December 24, 1966. Both returned panoramic photographs that were the first views from the lunar surface.

The American robotic Surveyor program was part of an effort to locate a safe site on the Moon for a human landing and test under actual lunar conditions the radar and landing systems required to make a true controlled touchdown. Five of Surveyor's seven missions made successful unmanned moon landings.

Within four months of each other in early 1966 the Soviet Union and the United States had accomplished successful moon landings with unmanned spacecraft. To the general public both countries had demonstrated roughly equal technical capabilities by returning photographic images from the surface of the Moon. These pictures provided a key affirmative answer to the crucial question of whether or not lunar soil would support upcoming manned landers with their much greater weight.

However, the Luna 9 hard landing of a ruggedized sphere using airbags at a 30-mile (48 km)-per-hour ballistic impact speed had much more in common with the failed 1962 Ranger landing attempts and their planned 100-mile (160 km)-per-hour impacts than with the Surveyor 1 soft landing on three footpads using its radar-controlled, adjustable-thrust retrorocket. While Luna 9 and Surveyor 1 were both major national accomplishments, only Surveyor 1 had reached its landing site employing key technologies that would be needed for a crewed flight. Thus as of mid-1966, the United States had begun to pull ahead of the Soviet Union in the so-called Space Race to land a man on the Moon.

Advances in other areas were necessary before manned spacecraft could follow unmanned ones to the surface of the Moon. Of particular importance was developing the expertise to perform flight operations in lunar orbit. Ranger, Surveyor and initial Luna moon landing attempts all utilized flight paths from Earth that traveled directly to the lunar surface without first placing the spacecraft in a lunar orbit. Such direct ascents use a minimum amount of fuel for unmanned spacecraft on a one-way trip.

In contrast, manned vehicles need additional fuel after a lunar landing to enable a return trip back to Earth for the crew. Leaving this massive amount of required Earth-return fuel in lunar orbit until it is actually used later in the mission is far more efficient than taking such fuel down to the lunar surface in a Moon landing and then hauling it all back into space yet again, working against lunar gravity both ways. Such considerations lead logically to a lunar orbit rendezvous mission profile for a manned Moon landing.

Accordingly, beginning in mid-1966 both the U.S. and U.S.S.R. naturally progressed into missions which featured lunar orbit operations as a necessary prerequisite to a manned Moon landing. The primary goals of these initial unmanned orbiters were extensive photographic mapping of the entire lunar surface for the selection of manned landing sites and, for the Soviets, the checkout of radio communications gear that would be used in future soft landings.

An unexpected major discovery from initial lunar orbiters were vast volumes of dense materials beneath the surface of the moon's maria. Such mascons can send a manned mission dangerously off course in the final minutes of a moon landing when aiming for a relatively small landing zone that is smooth and safe. Mascons were also found over a longer period of time to greatly disturb the orbits of low-altitude satellites around the Moon, making their orbits unstable and forcing an inevitable crash on the lunar surface in the relatively short period of months to a few years. Thus all lunar orbiter satellites eventually become unintentional "lunar landers" at the end of their missions.

Controlling the location of impact for spent lunar orbiters can have scientific value. For example, in 1999 the NASA Lunar Prospector orbiter was deliberately targeted to impact a permanently shadowed area of Shoemaker Crater near the lunar south pole. It was hoped that energy from the impact would vaporize suspected shadowed ice deposits in the crater and liberate a water vapor plume that would be detectable from Earth. No such plume was observed. However, a small vial of ashes from the body of pioneer lunar scientist Eugene Shoemaker was delivered by the Lunar Prospector to the crater named in his honor - currently the only human remains on the Moon today.

Luna 10 became the first spacecraft to orbit the Moon on 3 April 1966.

It was possible to aim a spacecraft from Earth so that it will loop around the Moon and return to Earth without actually entering lunar orbit, following the so-called free return trajectory. Such circumlunar loop missions are simpler than actual lunar orbit missions because rockets for lunar orbit braking and Earth return are not required. However, a manned circumlunar loop trip poses significant challenges above and beyond those found in a manned low-Earth-orbit mission, offering valuable lessons in preparation for a manned moon landing. Foremost among these are mastering the demands of re-entering the Earth's atmosphere upon returning from the Moon. Manned Earth-orbiting vehicles such as the Space Shuttle return to Earth from speeds of around 17,000 miles per hour. Due to the effects of gravity, a vehicle returning from the Moon hits Earth's atmosphere at a much higher speed of around 25,000 miles per hour. The g-loading on astronauts during the resulting deceleration can be at the limits of human endurance even during a nominal reentry. Slight variations in the vehicle flight path and reentry angle during a return from the Moon can easily result in fatal levels of deceleration force.

Achieving a manned circumlunar loop flight prior to a manned lunar landing became a primary goal of the Soviets with their Zond spacecraft program. The first three Zonds were unmanned planetary probes; after that, the Zond name was transferred to a completely separate manned program. The initial focus of these later Zonds was extensive testing of required high-speed reentry techniques. This focus was not shared by the Americans, who chose instead to bypass the stepping stone of a manned circumlunar loop mission and never developed a separate spacecraft for this purpose.

Initial manned spaceflights in the early 1960s placed a single person in low Earth orbit during the Soviet Vostok and American Mercury programs. A two-flight extension of the Vostok program known as Voskhod effectively used Vostok capsules with their ejection seats removed to achieve Soviet space firsts of multiple person crews in 1964 and spacewalks in early 1965. These capabilities were later demonstrated by the Americans in ten Gemini low Earth orbit missions throughout 1965 and 1966, using a totally new second-generation spacecraft design that had little in common with the earlier Mercury. These Gemini missions went on to prove critical techniques for orbital rendezvous and docking that were crucial to a manned lunar landing mission profile.

After the end of the Gemini program, the Soviets Union began flying their second-generation Zond manned spacecraft in 1967 with the ultimate goal of looping a cosmonaut around the moon and returning him immediately to Earth. The Zond spacecraft was launched with the simpler and already operational Proton launch rocket, unlike the parallel Soviet manned moon landing effort also underway at the time based on third-generation Soyuz spacecraft requiring development of the advanced N-1 booster. The Soviets thus believed they could achieve a manned Zond circumlunar flight years before an American manned lunar landing and so score a propaganda victory. However, significant development problems delayed the Zond program and the success of the American Apollo lunar landing program led to the eventual termination of the Zond effort.

Like Zond, Apollo moon flights were generally launched on a free return trajectory that would return them to Earth via a circumlunar loop in the event that a Service Module malfunction failed to place them in lunar orbit as planned. This option was implemented after an explosion aboard the Apollo 13 mission in 1970, which is the only manned circumlunar loop mission flown to date.

Zond 5 was the first spacecraft to carry life from Earth to the vicinity of the Moon and return, initiating the final lap of the Space Race with its payload of turtles, insects, plants and bacteria. Despite the failure suffered in its final moments, the Zond 6 mission was reported by Soviet media as being a success as well. Although hailed worldwide as remarkable achievements, both of these Zond missions actually flew off-nominal reentry trajectories resulting in deceleration forces that would have been fatal to human crewmembers had they been aboard. As a result, the Soviets secretly planned to continue unmanned Zond tests until their reliability to support manned flight had been demonstrated. However, believing from faulty CIA intelligence that a Soviet manned lunar flight was imminent in late 1968, NASA fatefully changed the flight plan of Apollo 8 from an Earth-orbit to a riskier lunar orbit mission scheduled for late December 1968.

In early December 1968 the launch window to the Moon opened for the Soviet launch site in Baikonur, giving the USSR their final chance to beat the US to the Moon. Cosmonauts went on alert and asked to fly the Zond spacecraft then in final countdown at Baikonour on the first manned trip to the Moon. Ultimately, however, the Soviet Politburo decided the risk of crew death was unacceptable given the combined poor performance to that point of Zond/Proton and so scrubbed the launch of a manned Soviet lunar mission. Their decision proved to be a wise one, since this unnumbered Zond mission was destroyed in another unmanned test when it was finally launched several weeks later.

By this time flights of the third generation American Apollo spacecraft had begun. Far more capable than the Zond, the Apollo spacecraft had the necessary rocket power to slip into and out of lunar orbit and to make course adjustments required for a safe reentry during the return to Earth. The Apollo 8 mission carried out the first manned trip to the Moon on 24 December 1968, certifying the Saturn V booster for manned use and flying not a circumlunar loop but instead a full ten orbits around the Moon before returning safely to Earth. Apollo 10 then performed a full dress rehearsal of a manned moon landing in May 1969. This mission stopped short at ten miles (16 km) altitude above the lunar surface, performing necessary low-altitude mapping of trajectory-altering mascons using a factory prototype lunar module that was too overweight to allow a successful landing. With the failure of the unmanned Soviet sample return moon landing attempt Luna 15 in July 1969, the stage was set for Apollo 11.

The U.S. Moon exploration program originated during the Eisenhower administration. In a series of mid-1950s articles in Collier's magazine, Wernher von Braun had popularized the idea of a manned expedition to the Moon to establish a lunar base. A manned Moon landing posed several daunting technical challenges to the U.S. and USSR. Besides guidance and weight management, atmospheric re-entry without ablative overheating was a major hurdle. After the Soviet Union's launch of Sputnik, von Braun promoted a plan for the United States Army to establish a military lunar outpost by 1965.

After the early Soviet successes, especially Yuri Gagarin's flight, U.S. President John F. Kennedy looked for an American project that would capture the public imagination. He asked Vice President Lyndon Johnson to make recommendations on a scientific endeavor that would prove U.S. world leadership. The proposals included non-space options such as massive irrigation projects to benefit the Third World. The Soviets, at the time, had more powerful rockets than the United States, which gave them an advantage in some kinds of space missions. Advances in U.S. nuclear weapons technology had led to smaller, lighter warheads, and consequently, rockets with smaller payload capacities. By comparison, Soviet nuclear weapons were much heavier, and the powerful R-7 rocket was developed to carry them. More modest potential missions such as flying around the Moon without landing or establishing a space lab in orbit (both were proposed by Kennedy to von Braun) were determined to offer too much advantage to the Soviets, since the U.S. would have to develop a heavy rocket to match the Soviets. A Moon landing, however, would capture world imagination while functioning as propaganda.

Mindful that the Apollo Program would economically benefit most of the key states in the next election—particularly his home state of Texas because NASA's base was in Houston—Johnson championed the Apollo program. This superficially indicated action to alleviate the fictional "missile gap" between the U.S. and USSR, a campaign promise of Kennedy's in the 1960 election. The Apollo project allowed continued development of dual-use technology. Johnson also advised that for anything less than a lunar landing the USSR had a good chance of beating the U.S. For these reasons, Kennedy seized on Apollo as the ideal focus for American efforts in space. He ensured continuing funding, shielding space spending from the 1963 tax cut and diverting money from other NASA projects. This dismayed NASA's leader, James E. Webb, who urged support for other scientific work.

The Saturn V booster was the key to U.S. moon landings. It used more efficient liquid hydrogen fuel instead of kerosene in its upper stages in order to lift heavier payloads beyond Earth orbit. The Saturn had a perfect record of zero failures in thirteen launches. By contrast, the Soviet N-1 exploded in flight during four secret test launches and never achieved operational status.

Whatever he said in private, Kennedy needed a different message to gain public support to uphold what he was saying and his views. Later in 1963, Kennedy asked Vice President Johnson to investigate the possible technological and scientific benefits of a Moon mission. Johnson concluded that the benefits were limited, but, with the help of scientists at NASA, he put together a powerful case, citing possible medical breakthroughs and interesting pictures of Earth from space. For the program to succeed, its proponents would have to defeat criticism from politicians on the left, who wanted more money spent on social programs, and on those on the right, who favored a more military project. By emphasizing the scientific payoff and playing on fears of Soviet space dominance, Kennedy and Johnson managed to swing public opinion: by 1965, 58 percent of Americans favored Apollo, up from 33 percent two years earlier. After Johnson became President in 1963, his continuing defense of the program allowed it to succeed in 1969, as Kennedy had originally hoped.

Soviet leader Nikita Khrushchev did not relish "defeat" by any other power, but equally did not relish funding such an expensive project. In October 1963 he said that the USSR was "not at present planning flight by cosmonauts to the Moon", while insisting that the Soviets had not dropped out of the race. Only after another year would the USSR fully commit itself to a Moon-landing attempt, which ultimately failed.

At the same time, Kennedy had suggested various joint programs, including a possible Moon landing by Soviet and American astronauts and the development of better weather-monitoring satellites. Khrushchev, sensing an attempt by Kennedy to steal Russian space technology, rejected the idea: if the USSR went to the Moon, it would go alone. Korolyov, the RSA's chief designer, had started promoting his Soyuz craft and the N-1 launcher rocket that would have the capability of carrying out a manned Moon landing. Khrushchev directed Korolyov's design bureau to arrange further space firsts by modifying the existing Vostok technology, while a second team started building a completely new launcher and craft, the Proton booster and the Zond, for a manned cislunar flight in 1966. In 1964 the new Soviet leadership gave Korolyov the backing for a Moon landing effort and brought all manned projects under his direction. With Korolyov's death and the failure of the first Soyuz flight in 1967, the co-ordination of the Soviet moon landing program quickly unraveled. The Soviets built a landing craft and selected cosmonauts for the mission that would have placed Aleksei Leonov on the Moon's surface, but with the successive launch failures of the N1 booster in 1969, plans for a manned landing suffered first delay and then cancellation.

In total twenty-four American astronauts have traveled to the Moon, with twelve walking on its surface and three making the trip twice. Apollo 8 was a lunar-orbit-only mission, Apollo 10 included powered descent and then an abort-mode ascent of the LM, while Apollo 13, originally scheduled as a landing, ended up as a lunar fly-by, by means of free return trajectory; thus, none of these missions made landings. Apollo 7 and Apollo 9 never left Earth orbit. Apart from the inherent dangers of manned moon expeditions as seen with Apollo 13, one reason for their cessation according to astronaut Alan Bean is the cost it imposes in government subsidies.

Unlike other international rivalries, the Space Race has remained unaffected in a direct way regarding the desire for territorial expansion. After the successful landings on the Moon, the U.S. explicitly disclaimed the right to ownership of any part of the Moon.

President Richard Nixon had speechwriter William Safire prepare a condolence speech for delivery in the event that Armstrong and Aldrin became marooned on the Moon's surface and could not be rescued.

In the 1940s writer Arthur C Clarke forecast that man would reach the Moon by 2000.

On August 16, 2006, the Associated Press reported that NASA is currently missing the original Slow-scan television tapes (which were made before the scan conversion for conventional TV) of the Apollo 11 Moon walk. Some news outlets have mistakenly reported that the SSTV tapes were found in Western Australia, but those tapes were only recordings of data from the Apollo 11 Early Apollo Surface Experiments Package.

India became the third country to reach the surface of the moon with a dedicated scientific probe when its lunar orbiting spacecraft Chandrayaan 1 released the Moon Impact Probe. MIP reached the surface of the Moon at 2034 UT(0804 IST) on Nov 14 2008. This date was choosen to commemorate the birthday of Jawaharlal Nehru, the first Indian Prime Minister who initiated India's space program. Developed in India by the Indian Space Research Organisation (ISRO), the MIP had the Indian flag painted on its exterior. Athough Japan and ESA had previously commanded their orbiters Hiten and SMART-1 to crash in selected zones on the Moon's surface at the end of their respective lifetimes, India's MIP was the first probe designed specifically for a trip to the lunar surface since the Soviet lander Luna 24 in 1976.

Weighing 34 kilograms, the box shaped MIP carried three instruments – a video imaging system, a mass spectrometer and a radar altimeter. The video imaging system took pictures of the moon’s surface from high altitudes as MIP approached it, relaying those pictures back to Earth during the MIP's descent. The mass spectrometer made measurements of the extremely thin lunar atmosphere. The radar altimeter measured the rate of descent of the MIP probe to the lunar surface, testing that technology for future Indian soft landing missions. Such a soft landing is planned for 2010 or 2011 during the upcoming Chandrayaan 2 mission.

The Indian MIP-1 probe did not include braking rockets and was destroyed upon impacting the lunar surface at its planned speed of 3,100 miles per hour. Its achievement is roughly equivalent to the American Ranger 7 mission flown in 1964.

The moon landings are often referenced by Americans to criticize the failure of a government project or service. A typical example of this appears in the aftermath of a natural disaster: "We can land man on the moon and bring him back safely, but we can't stop a bit of floodwater".

The next lunar orbiter currently scheduled for launch is NASA's Lunar Reconnaissance Orbiter mission. The Lunar Precursor Robotic Program (LPRP) is a program of robotic spacecraft missions which NASA will use to prepare for future human spaceflight missions to the Moon. Two LPRP missions, the Lunar Reconnaissance Orbiter (LRO) and the Lunar CRater Observation and Sensing Satellite (LCROSS), are scheduled to launch early in 2009.

Russia plans to send cosmonauts to the Moon by 2025 and establish a permanent manned base there in 2027-2032.

ISRO, the Indian National Space agency, has announced the Chandrayaan program for Lunar exploration. The second mission Chandrayaan II plans to land a motorised rover by 2010/2011.

Other nations, including China, have expressed interest in pursuing human landings on the Moon, but none have currently announced formal plans.

The Google Lunar X Prize competition offers a $20 million award for the first privately-funded team to land a robotic probe on the Moon. Like the Ansari X Prize before it, the competition aims to advance the state of the art in private space exploration.

Some conspiracy theorists have insisted that the Apollo moon landings were a hoax. These accusations flourish in part because predictions by enthusiasts that Moon landings would become commonplace have not yet come to pass. Some claims can be empirically discredited by three retroreflector arrays left on the Moon by Apollo 11, 14 and 15. Today, anyone on Earth with an appropriate laser and telescope system may bounce laser beams off these devices, verifying deployment of the Lunar Laser Ranging Experiment at historically documented Apollo moon landing sites. This evidence gives strong standing to Man-Made devices having made successful landings. The Mythbusters have tried to determine if the Moon Landing really was a hoax which included some of these experiments, why the footprint was so clear, why the flag was waving and why Neil Armstrong was viewed so clearly while the section was in a shadow (see MythBusters (2008 season)#Episode 104 – "NASA Moon Landing").

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Colonization of the Moon

A full moon

The colonization of the Moon is the proposed establishment of permanent human communities on the Moon. Science fiction writers and advocates of space exploration have seen settlement of the Moon as a logical step in the expansion of humanity beyond the Earth. National claims to the best locations on the moon will eventually lead to another space race. Polar colonies would be ideal for avoiding long cold nights and to take full advantage of the sun. Nations first to arrive at the poles might lay claim to them, similar to claims made at the Earth's north pole.

Permanent human habitation on a planetary body other than the Earth is one of science fiction's most prevelant themes. As technology has advanced, and concerns about the future of humanity on Earth have increased, the argument that space colonization is an achievable and worthwhile goal has gained momentum. Because of its proximity to Earth, the Moon has been seen as a prime candidate for the location of humanity's first permanently occupied extraterrestrial base.

Should attempts at colonization go ahead, economic concerns are likely to lead to settlements being created near mines and processing centers, or near the poles where a continuous source of solar energy can be harnessed. While it would be relatively easy to resupply a lunar base from Earth, in comparison to a Martian base, the Moon is likely to play a large role in the development of long-duration closed-loop life support systems. Duplicating the ecology of Earth so that wastes can be recycled is essential to any long term effort of space exploration. The wealth of knowledge gained by extracting and refining resources on the Moon would positively affect efforts to build colonies elsewhere in the Solar System.

The notion of siting a colony on the Moon originated before the space age; Konstantin Tsiolkovsky (1857-1935), among others, suggested such a step. From the 1950s onwards, a number of concepts and designs have been suggested by scientists, engineers and others.

Noted science fiction author Arthur C. Clarke proposed a lunar base of inflatable modules covered in lunar dust for insulation in 1954. A spaceship, assembled in low Earth orbit, would be launched towards the Moon, and astronauts would set up the igloo-like modules and an inflatable radio mast. Subsequent steps would include the establishment of a larger, permanent dome; an algae-based air purifier; a nuclear reactor for the provision of power; and electromagnetic cannons to launch cargo and fuel to interplanetary vessels in space.

In 1959, John S. Rinehart suggested that the safest design would be a structure that could " in a stationary ocean of dust," since there were, at the time this concept was outlined, theories that there could be mile-deep dust oceans on the Moon. The design proposed consisted of a half-cylinder with half-domes at both ends, with a micrometeoroid shield placed above the base.

The Project Horizon was a 1959 study regarding the U.S. Army's plan to establish a fort on the Moon by 1967. H. H. Koelle, a German rocket engineer of the Army Ballistic Missile Agency (ABMA) was leading the Project Horizon study. The first landing would be carried out by two "soldier-astronauts" in 1965 and more construction workers would soon follow. Through numerous launches (61 Saturn I and 88 Saturn V), 245 tons of cargo would be transported to the outpost by 1966.

Exploration of the lunar surface by spacecraft began in 1959 when the Soviet Luna 2 mission crash-landed into the surface. The same year, the Luna 3 mission radioed photographs to Earth of the Moon's hitherto unseen far side, marking the beginning of a decade-long series of unmanned lunar explorations.

Responding to the Soviet program of space exploration, US President John F. Kennedy in 1961 told the U.S. Congress on May 25: "I believe that this nation should commit itself to achieving the goal before this decade is out of landing a man on the moon and returning him safely to the Earth." The same year the Soviet leadership made some of its first public pronouncements about landing a man on the Moon and establishing a lunar base.

In 1962, John DeNike and Stanley Zahn published their idea of a sub-surface base located at the Sea of Tranquility. This base would house a crew of 21, in modules placed 4 meters below the surface, which was believed to provide radiation shielding as well as the Earth's atmosphere does. They favored nuclear reactors for energy production, because they are more efficient than solar panels, and would also overcome the problems with the long lunar nights. For life support system, an algae-based gas exchanger was proposed.

Manned exploration of the lunar surface began in 1968 when the Apollo 8 spacecraft orbited the Moon with three astronauts on board. This was mankind's first direct view of the far side. The following year, the Apollo 11 lunar module landed two astronauts on the Moon, proving the ability of humans to travel to the Moon, perform scientific research work and bring back sample materials.

Additional missions to the Moon continued this exploration phase. In 1969 the Apollo 12 mission landed next to the Surveyor 3 spacecraft, demonstrating precision landing capability. Following the near-disaster of Apollo 13, Apollo 14 was the last mission on which astronauts were quarantined on their return from the Moon. The use of a manned vehicle was demonstrated in 1971 with the Lunar Rover during Apollo 15. Apollo 16 made the first landing within the rugged lunar highlands. However, interest in further exploration of the Moon was beginning to wane among the American public. In 1972 Apollo 17 was the final Apollo lunar mission, and further planned missions were scrapped at the directive of President Nixon. Instead, focus was turned to the Space Shuttle and manned missions in near Earth orbit.

The Soviet Luna program failed to send a manned mission to the Moon. However, in 1966 Luna 9 was the first probe to achieve a soft landing and return close-up shots of the lunar surface. Luna 16 in 1970 returned the first Soviet lunar soil samples, while in 1970 and 1973 during the Lunokhod program two robotic rovers landed on the Moon. Lunokhod 1 explored the lunar surface for 322 days, but the contact with Lunokhod 2 was lost after about 4 months of its operation. 1974 saw the end of the Soviet Moonshot, two years after the last American manned landing.

In the decades following, interest in exploring the Moon faded considerably, and only a few dedicated enthusiasts supported a return. However, evidence of lunar ice at the poles gathered by NASA's Clementine and Lunar Prospector missions rekindled some discussion, as did the potential growth of a Chinese space program that contemplated its own mission to the Moon. Subsequent research suggested that there was far less ice present (if any) than had originally been thought, but that there may still be some usable deposits of hydrogen in other forms.

In 2004, U.S. President George W. Bush called for a plan to return manned missions to the Moon by 2020. Propelled by this new initiative, NASA issued a new long-range plan that includes building a base on the Moon as a staging point to Mars. This plan envisions a Lunar outpost at one of the moon's poles by 2024 which, if well-sited, might be able to continually harness solar power; at the poles, temperature changes over the course of a lunar day are also less extreme, and reserves of water and useful minerals may be found nearby. The European Space Agency, also, has a plan for a permanently manned lunar base by 2025. Russia has also announced similar plans to send a man to the moon by 2025 and establish a permanent base there several years later.

A Chinese space scientist has said that the People's Republic of China could be capable of landing a human on the moon by 2022 (see Chinese Lunar Exploration Program), and Japan and India also have plans for a lunar base by 2030. Neither of these plans involves permanent residents on the Moon. Instead they call for sortie missions, in some cases followed by extended expeditions to the lunar base using rotating crew members, as is currently done for the International Space Station.

Putting aside the general questions of whether a human colony beyond the Earth is feasible or scientifically desirable in light of cost-efficiency, proponents of space colonization point out that the Moon offers both advantages and disadvantages as a site for such a colony.

Placing a colony on a natural body would provide an ample source of material for construction and other uses, including shielding from radiation. The energy required to send objects from the Moon to space is much less than from Earth to space. This could allow the Moon to serve as a construction site or fueling station for spacecraft. Some proposals include using electric acceleration devices (mass drivers) to propel objects off the Moon without building rockets. Others have proposed momentum exchange tethers (see below). Furthermore, the Moon does have some gravity, which, experience to date indicates, may be vital for fetal development and long-term human health. Whether the Moon's gravity (roughly one sixth of Earth's) is adequate for this purpose, however, is uncertain.

While a colony might be located anywhere, potential locations for a lunar colony fall into three broad categories.

NASA chose to use a south-polar site for the lunar outpost reference design in the Exploration Systems Architecture Study chapter on Lunar Architecture.

At the north pole, the rim of Peary crater has been proposed as a favorable location for a base. Examination of images from the Clementine mission appear to show that parts of the crater rim are permanently illuminated by sunlight (except during lunar eclipses). As a result, the temperature conditions are expected to remain very stable at this location, averaging −50 °C (−58 °F). This is comparable to winter conditions in Earth's the Poles of Cold in Siberia and Antarctica. The Peary crater interior may also harbor hydrogen deposits.

Although hydrogen appears to be concentrated at the poles, the presence of lunar ice has not yet been confirmed. A bistatic radar experiment performed during the Clementine mission suggested the presence of water ice around the south pole. The Lunar Prospector spacecraft reported enhanced hydrogen abundances not only at the south pole, but also at the north pole — actually more so. On the other hand, results reported using the Arecibo radio telescope have been interpreted by some to indicate that the anomalous Clementine radar signatures are not indicative of ice, but surface roughness. This interpretation, however, is not universally agreed upon.

The lunar equatorial regions are likely to have higher concentrations of helium-3 (rare on Earth but much sought after for use in nuclear fusion research) because the solar wind has a higher angle of incidence. They also enjoy an advantage in launching material from the Moon, but the advantage is slight due to the Moon's slow rotation.

Several probes have landed in the Oceanus Procellarum area. There are many areas and features that could be subject to long-term study, such as the Reiner Gamma anomaly and the dark-floored Grimaldi crater.

The lunar far side lacks direct communication with Earth, though a communication satellite at the L2 Lagrangian point, or a network of orbiting satellites, could enable communication between the far side of the Moon and Earth. The far side is also a good location for a large radio telescope because it is well shielded from the Earth. Due to the lack of atmosphere, the location is also suitable for an array of optical telescopes, similar to the Very Large Telescope in Chile. To date, there has been no ground exploration of the far side.

Scientists have estimated that the highest concentrations of helium-3 will be found in the maria on the far side, as well as near side areas containing concentrations of the titanium-based mineral ilmenite. On the near side the Earth and its magnetic field partially shields the surface from the solar wind during each orbit. But the far side is fully exposed, and thus should receive a somewhat greater proportion of the ion stream.

There have been numerous proposals regarding habitat modules. The designs have evolved throughout the years as mankind's knowledge about the Moon has grown, and as the technological possibilities have changed. The proposed habitats range from the actual spacecraft landers or their used fuel tanks, to inflatable modules of various shapes. Early on, some hazards of the lunar environment such as sharp temperature shifts, lack of atmosphere or magnetic field (which means higher levels of radiation and micrometeoroids) and long nights, were recognized and taken into consideration.

Some suggest building the lunar colony underground, which would give protection from radiation and micrometeoroids. This is not the only advantage to this option. The average temperature on the moon is about −5 °C. The day period (two weeks) has an average temperature of about 107 °C (225 F), although it can rise as high as 123 °C (253 F). The night period (also two weeks) has an average temperature of about −153 °C (−243 F). Underground, both periods would be around 24 °C (75 F), and humans could install ordinary air conditioners. The construction of such a base would probably be more complex; one of the first machines from Earth might be a remote controlled boring machine to excavate living quarters. Once created, some sort of hardening would be necessary to avoid collapse, possibly a spray-on concrete-like substance made from available materials. A more porous insulating material also made in situ could then be applied. Inflatable self-sealing fabric habitats might then be put in place to retain air. As an alternative to excavating, it is possible that large underground extinct lava tubes might exist on the Moon.

A possibly easier solution would be to build the lunar base on the surface, and cover the modules with lunar soil. The lunar regolith is composed of a unique blend of silica and iron-containing compounds that may be fused into a glass-like solid using microwave energy. This may allow for the use of "lunar bricks" in structural designs, or the "glassing" of loose dirt to form a hard, ceramic crust. Others have put forward the idea that the lunar base could be built on the surface and protected by other means, such as improved radiation and micrometeoroid shielding. Artificial magnetic fields have been proposed as a means to provide radiation shielding for long range deep space manned missions, and it might be possible to use similar technology on a lunar colony. Some regions on the Moon possess strong local magnetic fields that might partially mitigate against exposure to charged solar and galactic particles.

A lunar base would need power for its operations — from fuel production and communications to life support systems and scientific research.

A nuclear fission reactor might fulfill most of the base's power requirements. The advantage of a fission reactor over a fusion reactor is that the technology already exists. A fusion reactor has the advantage that helium-3, which is required for a certain type of fusion reaction, is abundant on the Moon. However, fusion reactors are far from being a practical power source at present and may not be available at the time of lunar colonization. Radioisotope thermoelectric generators could be used as backup and emergency power sources for solar powered colonies.

Solar energy is a strong candidate. It could prove to be a relatively cheap source of power for a lunar base, especially since many of the raw materials needed for solar panel production can be extracted on site. However, the long lunar night (14 Earth days) is a drawback for solar power on the Moon's surface. This might be solved by building several power plants, so that at least one of them is always in daylight. Another possibility would be to build such a power plant where there is constant or near-constant sunlight, such as at the Malapert mountain near the lunar south pole, or on the rim of Peary crater near the north pole. A third possibility would be to leave the panels in orbit, and beam the power down as microwaves.

The solar energy converters need not be silicon solar panels. It may be more advantageous to use the larger temperature difference between sun and shade to run heat engine generators. Concentrated sunlight could also be relayed via mirrors and used in Stirling engines or solar trough generators, or it could be used directly for lighting, agriculture and process heat. The focused heat might also be employed in materials processing to extract various elements from lunar surface materials.

In the early days, a combination of solar panels for 'day-time' operation and fuel cells for 'night-time' operation could be used.

Fuel cells on the Space Shuttle have operated reliably for up to 17 days at a time. On the Moon, they would only be needed for 13.7 days — the length of the lunar night. Fuel cells produce water directly as a waste product. Current fuel cell technology is more advanced than the Shuttle's cells — PEM (Proton Exchange Membrane) cells produce considerably less heat (though their waste heat would likely be useful during the lunar night) and are physically lighter, not to mention the reduced mass of the smaller heat-disapating radiators. This makes PEMs more economical to launch from Earth than the shuttle's cells, but PEMs have not yet been proven in space.

Combining fuel cells with electrolysis would provide a 'perpetual' source of electricity - solar energy could be used to provide power during the Lunar 'day', and fuel cells at night. During the Lunar 'day', solar energy would also be used to electrolise the water created in the fuel cells - although there would be small losses of gases that would have to be replaced.

Conventional rockets have been used for most lunar exploration to date. The ESA's SMART-1 mission from 2003 to 2006 used Hall effect thrusters. NASA will use chemical rockets on its Ares V booster and Lunar Surface Access Module, being developed for a planned return to the Moon around 2019. The construction workers, location finders, and other astronauts vital to building, will be taken in NASA's Orion spacecraft.

Lunar colonists will want the ability to move over long distances, to transport cargo and people to and from modules and spacecraft, and to carry out scientific study of a larger area of the lunar surface for long periods of time. Proposed concepts include a variety of vehicle designs, from small open rovers to large pressurised modules with lab equipment, and also a few flying or hopping vehicles.

Rovers could be useful if the terrain is not too steep or hilly. The only rovers to have operated on the surface of the Moon (as of 2008) are the Apollo Lunar Roving Vehicle (LRV), developed by Boeing, and the robotic Soviet Lunokhod. The LRV was an open rover for a crew of two, and a range of 92 km during one lunar day. One NASA study resulted in the Mobile Lunar Laboratory concept, a manned pressurised rover for a crew of two, with a range of 396 km. The Soviet Union developed different rover concepts in the Lunokhod series and the L5 for possible use on future manned missions to the Moon or Mars. These rover designs were all pressurised for longer sorties.

If multiple bases were established on the lunar surface, they could be linked together by permanent railway systems. Both conventional and magnetic levitation (Mag-Lev) systems have been proposed for the transport lines. Mag-Lev systems are particularly attractive as there is no atmosphere on the surface to slow down the train, so the vehicles could achieve velocities comparable to aircraft on the Earth. One significant difference with lunar trains, however, is that the cars would need to be individually sealed and possess their own life support systems. The trains would also need to be highly resistant to derailment, as a punctured car could lead to rapid loss of life.

For difficult areas, a flying vehicle may be more suitable. Bell Aerosystems proposed their design for the Lunar Flying Vehicle as part of a study for NASA. Bell also developed the Manned Flying System, a similar concept.

One way to get materials and products from the Moon to an interplanetary waystation might be with a mass driver, a magnetically accelerated projectile launcher. Cargo would be picked up from orbit or an Earth-Moon Lagrangian point by a shuttle craft using ion propulsion, solar sails or other means and delivered to Earth orbit or other destinations such as near-Earth asteroids, Mars or other planets, perhaps using the Interplanetary Transport Network. If a lunar space elevator is ever built, it could transport people, raw materials and products to and from an orbital station at Lagrangian points L1 or L2.

A cislunar transport system has been proposed using tethers to achieve momentum exchange. This system requires zero net energy input, and could not only retrieve payloads from the lunar surface and transport them to Earth, but could also soft land payloads on to the lunar surface.

For long term sustainability, a space colony should be close to self sufficient. On site mining and refining of the Moon's materials could provide an advantage over deliveries from Earth – for use both on the Moon and elsewhere in the solar system – as they can be launched into space at a much lower energy cost than from Earth. It is possible that vast sums of money will be spent in interplanetary exploration in the 21st century, and the cost of providing goods from the Moon might be attractive.

In the long term, the Moon is likely to be very important in supplying construction facilities with raw materials. Zero gravity allows materials to be processed in ways impossible or difficult on Earth, such as 'foaming' metals, where a gas is injected into a molten metal, and then the metal is annealed slowly. On Earth, the gas bubbles rise and burst, but in a zero gravity environment, that does not happen. Annealing is a process that requires large amounts of energy, as a material is kept very hot for an extended period of time. This allows the molecular structure to align in the strongest possible way. Materials which cannot be alloyed or mixed on Earth because of the gravity field effects on density differences could be combined in space, resulting in composites which could have exceptional qualities. No one knows, because no one has been able to experiment along these lines on any scale. However, it is possible that a material or process will be identified which will be highly valuable on Earth, but impossible to make here.

Exporting material to Earth in trade from the Moon is more problematic due to the high cost of transportation. One suggested candidate is Helium-3 from the solar wind, which has accumulated on the Moon's surface over billions of years, and which is rare on Earth. Helium is present in the lunar regolith in quantities of ten to a hundred (weight) parts per million, and 0.003 to 1 percent of this amount (depending on soil). 2006 market price for He-3 was about $46,500 per troy ounce ($1500/gram, $1.5M/kg), more than 120 times the value per unit weight of Gold and over eight times the value of Rhodium.

In the long term future He-3 may prove to be a desirable fuel in thermonuclear fusion reactors.

Gerald Kulcinski's group at the Fusion Technology Institute of the University of Wisconsin-Madison has operated an experimental He-3 fusion reactor for an extended period, on a non-governmental research budget, however the reactor has not achieved energy balance or breakeven.

Other economic possibilities include the tourism industry; manufacturing that requires a sterile, low-gravity environment in a vacuum; research and processing of potentially dangerous life forms or nanotechnology, and long-term storage of radioactive materials. The low gravity may find health uses such as allowing the physically disabled to continue to enjoy an active lifestyle. Large, pressurized domes or caverns would permit human-powered flight, which may result in new sports activities.

Technology developed for a Lunar colony would likely have application to other potential space venues, including near-Earth asteroids and Mercury, which has many similarities to the Moon.

Gerard O'Neill, noting the problem of high launch costs in the early 1970s, came up with the idea of building Solar Power Satellites in orbit with materials from the Moon. Launch costs from the Moon are about 100 times lower than from Earth, due to the lower gravity and lack of atmosphere. This 1970s proposal was predicated on the then advertised future launch costs of NASA's space shuttle.

On 30 April 1979 the Final Report "Lunar Resources Utilization for Space Construction" by General Dynamics Convair Division under NASA contract NAS9-15560 concluded that use of lunar resources would be cheaper than terrestrial materials for a system comprising as few as thirty Solar Power Satellites of 10 GW capacity each.

In 1980, when it became obvious NASA's launch cost estimates for the space shuttle were grossly optimistic, O'Neill et al published another route to manufacturing using lunar materials with much lower startup costs. This 1980s SPS concept relied less on human presence in space and more on partially self-replicating systems on the lunar surface under telepresence control of workers stationed on Earth.

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Full moon

The Moon (Latin: Luna) is Earth's only natural satellite and the fifth largest satellite in the Solar System.

The average centre-to-centre distance from the Earth to the Moon is 384,403 km, about thirty times the diameter of the Earth. The common centre of mass of the system (the barycentre) is located about 1,700 km—a quarter the Earth's radius—beneath the surface of the Earth. The Moon makes a complete orbit around the Earth every 27.3 days (the orbital period), and the periodic variations in the geometry of the Earth–Moon–Sun system are responsible for the lunar phases that repeat every 29.5 days (the synodic period).

The Moon's diameter is 3,474 km, a little more than a quarter of that of the Earth. Thus, the Moon's surface area is less than a tenth that of the Earth (about a quarter the Earth's land area, approximately as large as Russia, Canada, and the United States combined), and its volume is about 2 percent that of Earth. The pull of gravity at its surface is about 17 percent of that at the Earth's surface.

The Moon is the only celestial body to which humans have traveled and upon which humans have performed a manned landing. The first artificial object to pass near the Moon was the Soviet Union's Luna 1, the first artificial object to impact the lunar surface was Luna 2, and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959. The first spacecraft to perform a successful lunar soft landing was Luna 9, and the first unmanned vehicle to orbit the Moon was Luna 10, both in 1966. The United States (U.S.) Apollo program achieved the only manned missions to date, resulting in six landings between 1969 and 1972. Human exploration of the Moon ceased with the conclusion of the Apollo program, although a few robotic landers and orbiters have been sent to the Moon since that time. Several countries have announced plans to return humans to the surface of the Moon in the 2020s.

The proper English name for Earth's natural satellite is, simply, the Moon (capitalized). Moon is a Germanic word, related to the Latin mensis (month). It is ultimately a derivative of the Proto-Indo-European root me-, also represented in measure (time), with reminders of its importance in measuring time in words derived from it like Monday, month and menstrual. The related adjective is lunar, as well as an adjectival prefix seleno- and suffix -selene (from selēnē, the Ancient Greek word for the Moon). In English, the word moon exclusively meant "the Moon" until 1665, when it was extended to refer to the recently-discovered natural satellites of other planets. Subsequently, these objects were given distinct names in order to avoid confusion. The Moon is occasionally referred to by its Latin name Luna, primarily in science fiction.

The Moon is in synchronous rotation, which means that it rotates about its axis in the about the same time it takes to orbit the Earth. This results in it keeping nearly the same face turned towards the Earth at all times. The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and became locked in this orientation as a result of frictional effects associated with tidal deformations caused by the Earth.

Small variations (libration) in the angle from which the Moon is seen allow about 59% of its surface to be seen from the Earth (but only half at any instant).

The side of the Moon that faces Earth is called the near side, and the opposite side the far side. The far side is often inaccurately called the "dark side," but in fact, it is illuminated exactly as often as the near side: once per lunar day, during the new moon phase we observe on Earth when the near side is dark. The far side of the Moon was first photographed by the Soviet probe Luna 3 in 1959. One distinguishing feature of the far side is its almost complete lack of maria.

The dark and relatively featureless lunar plains which can clearly be seen with the naked eye are called maria (singular mare), Latin for seas, since they were believed by ancient astronomers to be filled with water. These are now known to be vast solidified pools of ancient basaltic lava. The majority of these lavas erupted or flowed into the depressions associated with impact basins that formed by the collisions of meteors and comets with the lunar surface. (Oceanus Procellarum is a major exception in that it does not correspond to a known impact basin). Maria are found almost exclusively on the near side of the Moon, with the far side having only a few scattered patches covering about 2% of its surface, compared with about 31% on the near side. The most likely 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. Several provinces containing shield volcanoes and volcanic domes are found within the near side maria.

The lighter-colored regions of the Moon are called terrae, or more commonly just highlands, since they are higher than most maria. Several prominent mountain ranges on the near side are found along the periphery of the giant impact basins, many of which have been filled by mare basalt. These are believed to be the surviving remnants of the impact basin's outer rims. In contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events.

From images taken by the Clementine mission in 1994, it appears that four mountainous regions on the rim of the 73 km-wide Peary crater at the Moon's north pole remain illuminated for the entire lunar day. These peaks of eternal light are possible because of the Moon's extremely small axial tilt to the ecliptic plane. No similar regions of eternal light were found at the south pole, although the rim of Shackleton crater is illuminated for about 80% of the lunar day. Another consequence of the Moon's small axial tilt is regions that remain in permanent shadow at the bottoms of many polar craters.

The Moon's surface is marked by impact craters which form when asteroids and comets collide with the lunar surface. There are about half a million craters with diameters greater than 1 km on the moon. Since impact craters accumulate at a nearly constant rate, the number of craters per unit area superposed on a geologic unit can be used to estimate the age of the surface (see crater counting). The lack of an atmosphere, weather and recent geological processes ensures that many of these craters have remained relatively well preserved in comparison to those on Earth.

The largest crater on the Moon, which also has the distinction of being one of the largest known craters in the Solar System, is the South Pole-Aitken basin. It is on the far side, between the South Pole and equator, and is some 2,240 km in diameter and 13 km in depth. Prominent impact basins on the near side include Imbrium, Serenitatis, Crisium, and Nectaris.

Blanketed atop the Moon's crust is a highly comminuted (broken into ever smaller particles) and "impact gardened" surface layer called regolith. Since the regolith forms by impact processes, the regolith of older surfaces is generally thicker than for younger surfaces. In particular, it has been estimated that the regolith varies in thickness from about 3–5 m in the maria, and by about 10–20 m in the highlands. Beneath the finely comminuted regolith layer is what is generally referred to as the megaregolith. This layer is much thicker (on the order of tens of kilometres) and comprises highly fractured bedrock.

The continuous bombardment of the Moon by comets and meteoroids has most likely added small amounts of water to the lunar surface. If so, sunlight would split much of this water into its constituent elements of hydrogen and oxygen, both of which would ordinarily escape into space over time, because of the Moon's weak gravity. However, because of the slightness of the axial tilt of the Moon's spin axis to the ecliptic plane—only 1.5°—some deep craters near the poles never receive direct light from the Sun and are thus in permanent shadow (see Shackleton crater). Water molecules that ended up in these craters could be stable for long periods of time.

Clementine has mapped craters at the lunar south pole that are shadowed in this way, and computer simulations suggest that up to 14 000 km² might be in permanent shadow. Results from the Clementine mission bistatic radar experiment are consistent with small, frozen pockets of water close to the surface, and data from the Lunar Prospector neutron spectrometer indicate that anomalously high concentrations of hydrogen are present in the upper metre of the regolith near the polar regions. Estimates for the total quantity of water ice are close to one cubic kilometre.

Water ice can be mined and then split into its constituent hydrogen and oxygen atoms by means of nuclear generators or electric power stations equipped with solar panels. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation cost-effective, since transporting water from Earth would be prohibitively expensive. However, recent observations made with the Arecibo planetary radar suggest that some of the near-polar Clementine radar data that were previously interpreted as being indicative of water ice might instead be a result of rocks ejected from young impact craters. The question of how much water there is on the Moon has not been resolved.

In July 2008, small amounts of water were found in the interior of volcanic pearls from the Moon (brought to Earth by Apollo 15).

The Moon is a differentiated body, being composed of a geochemically distinct crust, mantle, and core. This structure is believed to have resulted from the fractional crystallization of a magma ocean shortly after its formation, at about 4.4 billion years ago. The energy required to melt the outer portion of the Moon is commonly attributed to a giant impact event that is postulated to have formed the Earth-Moon system, and the subsequent reaccretion of material in Earth orbit. Crystallization of this magma ocean would have given rise to a mafic mantle and a plagioclase-rich crust (see Origin and geologic evolution below).

Geochemical mapping from orbit implies that the crust of the Moon is largely anorthositic in composition, consistent with the magma ocean hypothesis. In terms of elements, the crust is composed primarily of oxygen, silicon, magnesium, iron, calcium, and aluminium. Based on geophysical techniques, its thickness is estimated to be on average about 50 km.

Partial melting within the mantle of the Moon gave rise to the eruption of mare basalts on the lunar surface. Analyses of these basalts indicate that the mantle is composed predominantly of the minerals olivine, orthopyroxene and clinopyroxene, and that the lunar mantle is more iron rich than that of the Earth. Some lunar basalts contain high abundances of titanium (present in the mineral ilmenite), suggesting that the mantle is highly heterogeneous in composition. Moonquakes have been found to occur deep within the mantle of the Moon about a thousand kilometres below the surface. These occur with monthly periodicities and are related to tidal stresses caused by the eccentric orbit of the Moon about the Earth.

The Moon has a mean density of 3 346.4 kg/m³, making it the second densest moon in the Solar System after Io. Nevertheless, several lines of evidence imply that the core of the Moon is small, with a radius of about 350 km or less. This corresponds to only about 20% the size of the Moon, in contrast to about 50% as is the case for most other terrestrial bodies. The composition of the lunar core is not well constrained, but most believe that it is composed of metallic iron alloyed with a small amount of sulfur and nickel. Analyses of the Moon's time-variable rotation indicate that the core is at least partly molten.

The topography of the Moon has been measured by the methods of laser altimetry and stereo image analysis, most recently from data obtained during the Clementine mission. The most visible topographic feature is the giant far side South Pole-Aitken basin, which possesses the lowest elevations of the Moon. The highest elevations are found just to the north-east of this basin, and it has been suggested that this area might represent thick ejecta deposits that were emplaced during an oblique South Pole-Aitken basin impact event. Other large impact basins, such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and elevated rims. Another distinguishing feature of the Moon's shape is that the elevations are on average about 1.9 km higher on the far side than the near side.

The gravitational field of the Moon has been determined through tracking of radio signals emitted by orbiting spacecraft. The principle used depends on the Doppler effect, whereby the spacecraft acceleration in the line-of-sight direction can be determined by means of small shifts in frequency of the radio signal, and the distance from the spacecraft to a station on Earth. However, because of the Moon's synchronous rotation it is not possible to track spacecraft much over the limbs of the Moon, and the farside gravity field is thus only poorly characterised.

The major characteristic of the Moon's gravitational field is the presence of mascons, which are large positive gravitational anomalies associated with some of the giant impact basins. These anomalies greatly influence the orbit of spacecraft about the Moon, and an accurate gravitational model is necessary in the planning of both manned and unmanned missions. The mascons are in part due to the presence of dense mare basaltic lava flows that fill some of the impact basins. However, lava flows by themselves can not explain the entirety of the gravitational signature, and uplift of the crust-mantle interface is required as well. Based on Lunar Prospector gravitational models, it has been suggested that some mascons exist that do not show evidence for mare basaltic volcanism. The huge expanse of mare basaltic volcanism associated with Oceanus Procellarum does not possess a positive gravitational anomaly.

The Moon has an external magnetic field of the order of one to a hundred nanotesla—less than one hundredth that of the Earth, which is 30–60 microtesla. Other major differences are that the Moon does not currently have a dipolar magnetic field (as would be generated by a geodynamo in its core), and the magnetizations that are present are almost entirely crustal in origin. One hypothesis holds that the crustal magnetizations were acquired early in lunar history when a geodynamo was still operating. The small size of the lunar core, however, is a potential obstacle to this theory. Alternatively, it is possible that on an airless body such as the Moon, transient magnetic fields could be generated during large impact events. In support of this, it has been noted that the largest crustal magnetizations appear to be located near the antipodes of the giant impact basins. It has been proposed that such a phenomenon could result from the free expansion of an impact generated plasma cloud around the Moon in the presence of an ambient magnetic field.

The Moon has an atmosphere so thin as to be almost negligible, with a total atmospheric mass of less than 104 kg. The effective surface pressure of this small mass is around 3  × 10-15 atm . This pressure varies, of course, with the diurnal moon cycle. One source of its atmosphere is outgassing—the release of gases such as radon that originate by radioactive decay processes within the crust and mantle. Another important source is generated through the process of sputtering, which involves the bombardment of micrometeorites, solar wind ions, electrons, and sunlight. Gases that are released by sputtering can either reimplant into the regolith as a result of the Moon's gravity, or can be lost to space either by solar radiation pressure or by being swept away by the solar wind magnetic field if they are ionised. The elements sodium (Na) and potassium (K) have been detected using earth-based spectroscopic methods, whereas the element radon–222 (222Rn) and polonium-210 (210Po) have been inferred from data obtained from the Lunar Prospector alpha particle spectrometer. Argon–40 (40Ar), helium-4 (4He), oxygen (O2) and/or methane (CH4), nitrogen (N2) and/or carbon monoxide (CO), and carbon dioxide (CO2) were detected by in-situ detectors placed by the Apollo astronauts.

During the lunar day, the surface temperature averages 107°C, and during the lunar night, it averages -153°C.

Several mechanisms have been suggested for the Moon's formation. The formation of the Moon is believed to have occurred 4.527 ± 0.010 billion years ago, about 30–50 million years after the origin of the Solar System.

As a result of the large amount of energy liberated during both the giant impact event and the subsequent reaccretion of material in Earth orbit, it is commonly believed that a large portion of the Moon was once initially molten. The molten outer portion of the Moon at this time is referred to as a magma ocean, and estimates for its depth range from about 500 km to the entire radius of the Moon.

As the magma ocean cooled, it fractionally crystallised and differentiated, giving rise to a geochemically distinct crust and mantle. The mantle is inferred to have formed largely by the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene. After about three-quarters of magma ocean crystallisation was complete, the mineral anorthite is inferred to have precipitated and floated to the surface because of its low density, forming the crust.

The final liquids to crystallise from the magma ocean would have been initially sandwiched between the crust and mantle, and would have contained a high abundance of incompatible and heat-producing elements. This geochemical component is referred to by the acronym KREEP, for potassium (K), rare earth elements (REE), and phosphorus (P), and appears to be concentrated within the Procellarum KREEP Terrane, which is a small geologic province that encompasses most of Oceanus Procellarum and Mare Imbrium on the near side of the Moon.

A large portion of the Moon's post–magma-ocean geologic evolution was dominated by impact cratering. The lunar geologic timescale is largely divided in time on the basis of prominent basin-forming impact events, such as Nectaris, Imbrium, and Orientale. These impact structures are characterised by multiple rings of uplifted material, and are typically hundreds to thousands of kilometres in diameter. Each multi-ring basin is associated with a broad apron of ejecta deposits that forms a regional stratigraphic horizon. While only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages on the basis of stratigraphic grounds. The continuous effects of impact cratering are responsible for forming the regolith.

The other major geologic process that affected the Moon's surface was mare volcanism. The enhancement of heat-producing elements within the Procellarum KREEP Terrane is thought to have caused the underlying mantle to heat up, and eventually, to partially melt. A portion of these magmas rose to the surface and erupted, accounting for the high concentration of mare basalts on the near side of the Moon. Most of the Moon's mare basalts erupted during the Imbrian period in this geologic province 3.0–3.5 billion years ago. Nevertheless, some dated samples are as old as 4.2 billion years, and the youngest eruptions, based on the method of crater counting, are believed to have occurred only 1.2 billion years ago.

There has been controversy over whether features on the Moon's surface undergo changes over time. Some observers have claimed that craters either appeared or disappeared, or that other forms of transient phenomena had occurred. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or the inadequacy of earlier drawings. Nevertheless, it is known that the phenomenon of outgassing does occasionally occur, and these events could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km diameter region of the lunar surface was modified by a gas release event about a million years ago.

Moon rocks fall into two main categories, based on whether they underlie the lunar highlands (terrae) or the maria. The lunar highlands rocks are composed of three suites: the ferroan anorthosite suite, the magnesian suite, and the alkali suite (some consider the alkali suite to be a subset of the mg-suite). The ferroan anorthosite suite rocks are composed almost exclusively of the mineral anorthite (a calic plagioclase feldspar), and are believed to represent plagioclase flotation cumulates of the lunar magma ocean. The ferroan anorthosites have been dated using radiometric methods to have formed about 4.4 billion years ago.

The mg- and alkali-suite rocks are predominantly mafic plutonic rocks. Typical rocks are dunites, troctolites, gabbros, alkali anorthosites, and more rarely, granite. In contrast to the ferroan anorthosite suite, these rocks all have relatively high Mg/Fe ratios in their mafic minerals. In general, these rocks represent intrusions into the already-formed highlands crust (though a few rare samples appear to represent extrusive lavas), and they have been dated to have formed about 4.4–3.9 billion years ago. Many of these rocks have high abundances of, or are genetically related to, the geochemical component KREEP.

The lunar maria consist entirely of mare basalts. While similar to terrestrial basalts, they have much higher abundances of iron, are completely lacking in hydrous alteration products, and have a large range of titanium abundances.

Astronauts have reported that the dust from the surface felt like snow and smelled like spent gunpowder. The dust is mostly made of silicon dioxide glass (SiO2), most likely created from the meteors that have crashed into the Moon's surface. It also contains calcium and magnesium.

The Moon makes a complete orbit around the Earth with respect to the fixed stars (its sidereal period) about once every 27.3 days. However, since the Earth is moving in its orbit about the Sun at the same time, it takes slightly longer for the Moon to show its same phase to Earth, which is about 29.5 days (its synodic period). Unlike most satellites of other planets, the Moon orbits near the ecliptic and not the Earth's equatorial plane. It is the largest moon in the solar system relative to the size of its planet. (Charon is larger relative to the dwarf planet Pluto.) The natural satellites orbiting other planets are called "moons", after Earth's Moon.

Most of the tidal effects seen on the Earth are caused by the Moon's gravitational pull, with the Sun making only a small contribution. Tidal effects result in an increase of the mean Earth-Moon distance of about 3.8 m per century, or 3.8 cm per year. As a result of the conservation of angular momentum, the increasing semimajor axis of the Moon is accompanied by a gradual slowing of the Earth's rotation by about 0.002 seconds per day per century.

The Moon is exceptionally large relative to the Earth, being a quarter the diameter of the planet and 1/81 its mass. However, the Earth and Moon are still commonly considered a planet-satellite system, rather than a double-planet system, since the common centre of mass of the system (the barycentre) is located about 1 700 km beneath the surface of the Earth, or about a quarter of the Earth's radius. The surface of the Moon is less than one-tenth that of the Earth, and only about a quarter the size of the Earth's land area (or about as large as Russia, Canada, and the U.S. combined). Some people refer to the Earth–Moon system as a double planet system rather than a planet–moon system. Isaac Asimov proposed such a description, in part because the Sun's gravitational pull on the Moon is stronger than Earth's. Because of this, when the Moon is between the Sun and the Earth, it does not move away from the Sun and toward the Earth, as with most natural satellites. Instead, it keeps moving toward the Sun, but it slows down. That allows the Earth to pass it, which creates the appearance that the Moon is circling the Earth.

In 1997, the asteroid 3753 Cruithne was found to have an unusual Earth-associated horseshoe orbit. However, astronomers do not consider it to be a second moon of Earth, and its orbit is not stable in the long term. Three other near-Earth asteroids, (54509) 2000 PH5, (85770) 1998 UP1 and 2002 AA29, which exist in orbits similar to Cruithne's, have since been discovered.

Earth's ocean tides are initiated by the tidal force (a gradient in intensity) of Moon's gravity and are magnified by a host of effects in Earth's oceans. The gravitational tidal force arises because the side of Earth facing the Moon (nearest it) is attracted more strongly by the Moon's gravity than is the center of the Earth and—even less so—the Earth's far side. The gravitational tide stretches the Earth's oceans into an ellipse with the Earth in the center. The effect takes the form of two bulges—elevated sea level relative to the Earth; one nearest the Moon and one farthest from it. Since these two bulges rotate around the Earth once a day as it spins on its axis, ocean water is continuously rushing towards the ever-moving bulges. The effects of the two bulges and the massive ocean currents chasing them are magnified by an interplay of other effects; namely frictional coupling of water to Earth's rotation through the ocean floors, inertia of water's movement, ocean basins that get shallower near land, and oscillations between different ocean basins. The magnifying effect is a bit like water sloshing high up the sloped end of a bathtub after a relatively small disturbance of one's body in the deep part of the tub.

Gravitational coupling between the Moon and the ocean bulge nearest the Moon affects its orbit. The Earth rotates on its axis in the very same direction, and roughly 27 times faster, than the Moon orbits the Earth. Thus, frictional coupling between the sea floors and ocean waters, as well as water's inertia, drags the peak of the near-Moon tidal bulge slightly forward of the imaginary line connecting the centers of the Earth and Moon. From the Moon's perspective, the center of mass of the near-Moon tidal bulge is perpetually slightly ahead of the point about which it is orbiting. Precisely the opposite effect occurs with the bulge farthest from the Moon; it lags behind the imaginary line. However it is 12,756 km farther away and has slightly less gravitational coupling to the Moon. Consequently, the Moon is constantly being gravitationally attracted forward in its orbit about the Earth. This gravitational coupling drains kinetic energy and angular momentum from the Earth's rotation (see also, Day and Leap second). In turn, angular momentum is added to the Moon's orbit, which lifts the Moon into a higher orbit with a longer period. The effect on the Moon's orbital radius is a small one, just 0.10 ppb/year, but results in a measurable 3.82 cm annual increase in the Earth-Moon distance. Cumulatively, this effect becomes ever more significant over time; since astronauts first landed on the Moon approximately 40 years ago, it is 1.51 metres farther away.

Eclipses can occur only when the Sun, Earth, and Moon are all in a straight line. Solar eclipses occur near a new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur near a full moon, when the Earth is between the Sun and Moon.

Because the Moon's orbit around the Earth is inclined by about 5° with respect to the orbit of the Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes.

The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by the Earth, is described by the saros cycle, which has a period of approximately 6 585.3 days (18 years 11 days 8 hours).

The angular diameters of the Moon and the Sun as seen from Earth overlap in their variation, so that both total and annular solar eclipses are possible. In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Since the distance between the Moon and the Earth is very slightly increasing over time, the angular diameter of the Moon is decreasing. This means that hundreds of millions of years ago the Moon could always completely cover the Sun on solar eclipses so that no annular eclipses were possible. Likewise, about 600 million years from now (assuming that the angular diameter of the Sun will not change), the Moon will no longer cover the Sun completely and only annular eclipses will occur.

A phenomenon related to eclipse is occultation. The Moon is continuously blocking our view of the sky by a 1/2 degree-wide circular area. When a bright star or planet passes behind the Moon it is occulted or hidden from view. A solar eclipse is an occultation of the Sun. Because the Moon is close to Earth, occultations of individual stars are not visible everywhere, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted.

The most recent lunar eclipse was on February 20, 2008. It was a total eclipse. The entire event was visible from South America and most of North America (on Feb. 20), as well as Western Europe, Africa, and western Asia (on Feb. 21). The most recent solar eclipse took place on September 11, 2007, visible from southern South America and parts of Antarctica. The last total solar eclipse, on August 1, 2008, had a path of totality beginning in northern Canada and passed through Russia and China.

During its brightest phase, at "full moon", the Moon has an apparent magnitude of about −12.6. By comparison, the Sun has an apparent magnitude of −26.8. When the Moon is in a quarter phase, its brightness is not half of a full moon, but only about a tenth. This is because the lunar surface is not a perfect Lambertian reflector. When the Moon is full the opposition effect makes it appear brighter, but away from full there are shadows projected onto the surface which diminish the amount of reflected light.

The Moon appears larger when close to the horizon. This is a purely psychological effect (see Moon illusion). It is actually about 1.5% smaller when the Moon is near the horizon than when it is high in the sky (because it is farther away by up to one Earth radius).

The moon appears as a relatively bright object in the sky, in spite of its low albedo. The Moon is about the poorest reflector in the solar system and reflects only about 7% of the light incident upon it (about the same proportion as is reflected by a lump of coal). Color constancy in the visual system recalibrates the relations between the colours of an object and its surroundings, and since the surrounding sky is comparatively dark the sunlit Moon is perceived as a bright object.

The highest altitude of the Moon on a day varies and has nearly the same limits as the Sun. It also depends on the Earth season and lunar phase, with the full moon being highest in winter. Moreover, the 18.6 year nodes cycle also has an influence, as when the ascending node of the lunar orbit is in the vernal equinox, the lunar declination can go as far as 28° each month (which happened most recently in 2006). This results that the Moon can go overhead on latitudes up to 28 degrees from the equator (e.g. Florida, Canary Islands or in the southern hemisphere Brisbane). Slightly more than 9 years later (next time in 2015) the declination reaches only 18° N or S each month. The orientation of the Moon's crescent also depends on the latitude of the observation site. Close to the equator, an observer can see a boat Moon.

Like the Sun, the Moon can give rise to atmospheric effects, including a 22° halo ring, and the smaller coronal rings seen more often through thin clouds. For more information on how the Moon appears in Earth's sky, see lunar phase.

The first leap in lunar observation was prompted by the invention of the telescope. Galileo Galilei made good use of this new instrument and observed mountains and craters on the Moon's surface.

The Cold War-inspired space race between the Soviet Union and the U.S. led to an acceleration of interest in the Moon. Unmanned probes, both flyby and impact/lander missions, were sent almost as soon as launcher capabilities would allow. The Soviet Union's Luna program was the first to reach the Moon with unmanned spacecraft. The first man-made object to escape Earth's gravity and pass near the Moon was Luna 1, the first man-made object to impact the lunar surface was Luna 2, and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959. The first spacecraft to perform a successful lunar soft landing was Luna 9 and the first unmanned vehicle to orbit the Moon was Luna 10, both in 1966. Moon samples have been brought back to Earth by three Luna missions (Luna 16, 20, and 24) and the Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing).

The landing of the first humans on the Moon in 1969 is seen by many as the culmination of the space race. Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on July 21, 1969. The American Moon landing and return was enabled by considerable technological advances, in domains such as ablation chemistry and atmospheric re-entry technology, in the early 1960s.

Scientific instrument packages were installed on the lunar surface during all of the Apollo missions. Long-lived ALSEP stations (Apollo lunar surface experiment package) were installed at the Apollo 12, 14, 15, 16, and 17 landing sites, whereas a temporary station referred to as EASEP (Early Apollo Scientific Experiments Package) was installed during the Apollo 11 mission. The ALSEP stations contained, among others, heat flow probes, seismometers, magnetometers, and corner-cube retroreflectors. Transmission of data to Earth was terminated on September 30, 1977 because of budgetary considerations. Since the lunar laser ranging (LLR) corner-cube arrays are passive instruments, they are still being used. Ranging to the LLR stations is routinely performed from earth-based stations with an accuracy of a few centimetres, and data from this experiment are being used to place constraints on the size of the lunar core.

From the mid-1960s to the mid-1970s, there were 65 instances of artificial objects reaching the Moon (both manned and robotic, with ten in 1971 alone), with the last being Luna 24 in 1976. Only 18 of these were controlled moon landings, with nine completing a round trip from Earth and returning samples of moon rocks. The Soviet Union then turned its primary attention to Venus and space stations, and the U.S. to Mars and beyond. In 1990, Japan orbited the Moon with the Hiten spacecraft, becoming the third country to place a spacecraft into lunar orbit. The spacecraft released a smaller probe, Hagormo, in lunar orbit, but the transmitter failed, thereby preventing further scientific use of the mission.

In 1994, the U.S. finally returned to the Moon, robotically at least, sending the Joint Defense Department/NASA spacecraft Clementine. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface. This was followed by the Lunar Prospector mission in 1998. The neutron spectrometer on Lunar Prospector indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few metres of the regolith within permanently shadowed craters. The European spacecraft Smart 1 was launched September 27, 2003 and was in lunar orbit from November 15, 2004 to September 3, 2006.

On January 14, 2004, U.S. President George W. Bush called for a plan to return manned missions to the Moon by 2020 (see Vision for Space Exploration). NASA is now planning for the construction of a permanent outpost at one of the lunar poles. The People's Republic of China has expressed ambitious plans for exploring the Moon and has started the Chang'e program for lunar exploration, successfully launching its first spacecraft, Chang'e-1, on October 24, 2007. Like NASA, China hopes to land people on the Moon by 2020. The U.S. will launch the Lunar Reconnaissance Orbiter and the Lunar Crater Observation and Sensing Satellite in late April 2009 (the two missions are co-manifested). Russia also announced to resume its previously frozen project Luna-Glob, consisting of an unmanned lander and orbiter, which is slated to land in 2012.

The Google Lunar X Prize, announced September 13, 2007, hopes to boost and encourage privately funded lunar exploration. The X Prize Foundation is offering anyone US$20 million who can land a robotic rover on the Moon and meet other specified criteria.

On September 14, 2007 the Japan Aerospace Exploration Agency launched SELENE, also known as Kaguya, a lunar orbiter which is fitted with a high-definition camera and two small satellites. The mission is expected to last one year.

On October 22, 2008 India successfully launched the Chandrayaan I (a Sanskrit word literally meaning the 'Moon-craft') unmanned mission to the Moon and intends to launch several further unmanned missions. The country plans to launch Chandrayaan II in 2010 or 2011, which is slated to include a robotic lunar rover. India also has expressed its hope for a manned mission to the Moon by 2020.

The Moon has been the subject of many works of art and literature and the inspiration for countless others. It is a motif in the visual arts, the performing arts, poetry, prose and music. A 5000-year-old rock carving at Knowth, Ireland may represent the Moon, which would be the earliest depiction discovered. In many prehistoric and ancient cultures, the Moon was thought to be a deity or other supernatural phenomenon, and astrological views of the Moon continue to be propagated today.

Among the first in the Western world to offer a scientific explanation for the Moon was the Greek philosopher Anaxagoras (d. 428 BC), who reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. His atheistic view of the heavens was one cause for his imprisonment and eventual exile.

In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether. This separation was held to be part of physics for many centuries after.

During the Warring States of China, astronomer Shi Shen (fl. 4th century BC) gave instructions for predicting solar eclipse and lunar eclipse based on the relative positions of the moon and sun. Although the Chinese of the Han Dynasty (202 BC–202 AD) believed the moon to be energy equated to qi, their 'radiating influence' theory recognized that the light of the moon was merely a reflection of the sun (mentioned by Anaxagoras above). This was supported by mainstream thinkers such as Jing Fang (78–37 BC) and Zhang Heng (78–139 AD), but it was also opposed by the influential philosopher Wang Chong (27–97 AD). Jing Fang noted the sphericity of the moon, while Zhang Heng accurately described lunar eclipse and solar eclipse. These assertions were supported by Shen Kuo (1031–1095) of the Song Dynasty (960–1279) who created an allegory equating the waxing and waning of the moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent. He also noted that the reason for the sun and moon not eclipsing every time their paths met was because of a small obliquity in their orbital paths.

By the Middle Ages, before the invention of the telescope, more and more people began to recognise the Moon as a sphere, though they believed that it was "perfectly smooth". In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius and noted that it was not smooth but had mountains and craters. Later in the 17th century, Giovanni Battista Riccioli and Francesco Maria Grimaldi drew a map of the Moon and gave many craters the names they still have today.

On maps, the dark parts of the Moon's surface were called maria (singular mare) or seas, and the light parts were called terrae or continents. The possibility that the Moon contains vegetation and is inhabited by selenites was seriously considered by major astronomers even into the first decades of the 19th century. The contrast between the brighter highlands and darker maria create the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, among others.

In 1835, the Great Moon Hoax fooled some people into thinking that there were exotic animals living on the Moon. Almost at the same time however (during 1834–1836), Wilhelm Beer and Johann Heinrich Mädler were publishing their four-volume Mappa Selenographica and the book Der Mond in 1837, which firmly established the conclusion that the Moon has no bodies of water nor any appreciable atmosphere.

The far side of the Moon remained completely unknown until the Luna 3 probe was launched in 1959, and was extensively mapped by the Lunar Orbiter program in the 1960s.

Although several pennants of the Soviet Union were scattered by Luna 2 in 1959 and by later landing missions, and U.S. flags have been symbolically planted on the Moon, no nation currently claims ownership of any part of the Moon's surface. Russia and the U.S. are party to the Outer Space Treaty, which places the Moon under the same jurisdiction as international waters (res communis). This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction (including nuclear weapons).

A second treaty, the Moon Treaty, was proposed to restrict the exploitation of the Moon's resources by any single nation, but it has not been signed by any of the space-faring nations. Several individuals have made claims to the Moon in whole or in part, although none of these are generally considered credible.

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Asteroid moon

243 Ida and its moon Dactyl.

An asteroid moon is an asteroid that orbits another asteroid as its natural satellite. It is thought that many asteroids may possess moons, in some cases quite substantial in size. Discoveries of asteroid moons (and binary objects, in general) are important because the determination of their orbits provides estimates on the mass and density of the primary, allowing insights of their physical properties that is generally not otherwise possible.

The moons of Trans-Neptunian objects will also be covered in this article.

In addition to the terms satellite and moon, the term binary is sometimes used for asteroids with moons (or triple for asteroids with two moons). If one object is much bigger it is usually referred to as the primary and its companion as secondary. The term double asteroid is sometimes used for systems in which the asteroid and its moon are roughly the same size, while binary tends to be used independently from the relative sizes of the components.

As early as 1978, following a stellar occultation, 532 Herculina had been suggested to have a moon and there were reports of other asteroids having companions (usually referred to as satellites) in the following years. A letter in Sky & Telescope magazine at this time pointed to pairs of large craters (e.g. the Clearwater Lakes in Quebec) also suggesting asteroids having companions. However, it was not until 1993 that the first asteroid moon was confirmed when the Galileo probe discovered Dactyl orbiting 243 Ida. The second was discovered around 45 Eugenia in 1998. The first Trans-Neptunian binary, 1998 WW31 was optically resolved in 2002.

As of September 2008, 104 asteroid moons had been discovered, 60 in the main belt, 2 orbiting Trojan asteroids, 42 near-Earth objects and Mars-crossers. There were at that time also 58 Trans-Neptunian moons. In 2005, the asteroid 87 Sylvia was discovered to have two moons, making it the first known triple asteroid. This was followed by the discovery of a second moon orbiting 45 Eugenia. Also in 2005, the KBO Haumea was discovered to have two moons, making it the second KBO after Pluto known to have more than one moon.

An example of a double asteroid is 90 Antiope, where two roughly equal-sized components orbit the common centre of gravity. 617 Patroclus and its same-sized companion Menoetius is the only known binary system in the Trojan population.

The data about the populations of binary objects are still patchy. In addition to the inevitable observational bias (dependence on the distance from Earth, size, albedo and separation of the components) the frequency appears to be different among different categories of objects. Among asteroids, an estimated 2% would have satellites. Among trans-Neptunian objects (TNO), an estimated 11% are believed to be binary or multiple objects, but three of the four known large TNO (75%) have at least one satellite.

More than 20 binaries are known in each of the main groupings: Near Earth asteroids, Main belt asteroids, and Trans-Neptunians, not including numerous claims based solely on the light curve variation. No binaries have been found so far among Centaurs, presumably due to the much smaller number and relative faintness of these objects.

The origin of asteroid moons is not currently known with certainty, and a variety of theories exist. A widely accepted theory is that asteroid moons are formed from debris knocked off of the primary asteroid by an impact. Other pairings may be formed when a small object is captured by the gravity of a larger one.

Formation by collision is constrained by the angular momentum of components i.e. by the masses and their separation. Close binaries fit this model (e.g. Pluto/Charon). Distant binaries however, with components of comparable size, are unlikely to have followed this scenario, unless considerable mass has been lost in the event.

The distances of the components for the known binaries vary from a few hundreds of kilometres (243 Ida, 3749 Balam) to more than 3000 km (379 Huenna) for the asteroids. Among TNOs, the known separations vary from 3,000 to 50,000 km.

What is "typical" for a binary asteroid system tends to depend on its location in the Solar System (presumably because of different modes of origin and lifetimes of such systems in different populations of minor planets).

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Source : Wikipedia