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Recent indication that water might be present in noteworthy quantities at the lunar poles has renewed interest in the Moon. Polar colonies could also avoid the problem of long lunar nights – about 354 hours, a little more than two weeks – and take advantage of the sun continuously, at least during the local summer (there is no data for the winter yet).
Permanent human habitation on a planetary body other than the Earth is one of science fiction's most prevalent 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 the most obvious natural expansion after Earth.
The notion of siting a colony on the Moon originated before the Space Age. In 1638 Bishop John Wilkins wrote A Discourse Concerning a New World and Another Planet, in which he predicted a human colony on the Moon. Konstantin Tsiolkovsky (1857–1935), among others, also suggested such a step. From the 1950s onwards, a number of concepts and designs have been suggested by scientists, engineers and others.
In 1954, science-fiction author Arthur C. Clarke proposed a lunar base of inflatable modules covered in lunar dust for insulation. A spaceship, assembled in low Earth orbit, would launch to 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 "[float] 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 proposed design consisted of a half-cylinder with half-domes at both ends, with a micrometeoroid shield placed above the base.
Project Horizon was a 1959 study regarding the United States Army's plan to establish a fort on the Moon by 1967. Heinz-Hermann Koelle, a German rocket engineer of the Army Ballistic Missile Agency (ABMA) led 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 II), 245 tons of cargo would be transported to the outpost by 1966.
Lunex Project was a US Air Force plan for a manned lunar landing prior to the Apollo Program in 1961. It envisaged a 21-airman underground Air Force base on the Moon by 1968 at a total cost of $7.5 billion.
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 four meters below the surface, which was believed to provide radiation shielding on par with Earth's atmosphere. DeNike and Zahn favored nuclear reactors for energy production, because they were more efficient than solar panels, and would also overcome the problems with the long Lunar nights. For the life support system, an algae-based gas exchanger was proposed.
As of 2006, Japan planned to have a Moon base in 2030.
As of 2007, Russia planned to have a Moon base in 2027–2032.
In 2007 Jim Burke of the International Space University in France said people should plan to preserve humanity's culture in the event of a civilization-stopping asteroid impact with Earth. A Lunar Noah's Ark was proposed. Subsequent planning may be taken up by the International Lunar Exploration Working Group (ILEWG).
In a January 2012 speech Newt Gingrich, Republican candidate for President, proposed a plan to build a U.S. moon colony by the year 2020. However, some consider this plan economically unfeasible and nationalistic, and an attempt to garner votes through public enthusiasm for space travel. 
Exploration of the Lunar surface by spacecraft began in 1959 with the Soviet Union's Luna program. Luna 1 missed the Moon, but Luna 2 made a hard landing (impact) into its surface, and became the first artificial object on an extraterrestrial body. 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.
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 there, 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. The use of a manned vehicle on the Moon's surface 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 manned lunar programs 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, and Lunokhod 2 operated on the Moon about four months only but covered a third more distance. 1974 saw the end of the Soviet Moonshot, two years after the last American manned landing. Besides the manned landings, an abandoned Soviet moon program included building the moonbase "Zvezda", which was the first detailed project with developed mockups of expedition vehicles and surface modules.
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 (1994) and Lunar Prospector (1998) 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. However, in September 2009, the Chandrayaan probe, carrying an ISRO instrument, discovered that the Lunar regolith contains 0.1% water by weight, overturning theories that had stood for 40 years.
In 2004, U.S. President George W. Bush called for a plan to return manned missions to the Moon by 2020 (since cancelled – see Constellation program). 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. In addition, the European Space Agency 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 by rotating crew members, as is currently done for the International Space Station.
NASA’s LCROSS/LRO mission had been scheduled to launch in October 2008. The launch was delayed until 18 June 2009, resulting in LCROSS's impact with the Moon at 11:30 UT on 9 October 2009. The purpose is preparing for future Lunar exploration.
On September 24, 2009 NASA announced the discovery of water on the Moon. The discovery was made by three instruments on board Chandrayaan-1. These were the ISRO's Moon Impact Probe (MIP), the Moon Mineralogy Mapper (M3) and Mini-Sar, belonging to NASA.
On November 13, 2009 NASA announced that the LCROSS mission had discovered large quantities of water ice on the Moon around the LCROSS impact site at Cabeus. Robert Zubrin, president of the Mars Society, relativized the term 'large': "The 30 m crater ejected by the probe contained 10 million kilograms of regolith. Within this ejecta, an estimated 100 kg of water was detected. That represents a proportion of ten parts per million, which is a lower water concentration than that found in the soil of the driest deserts of the Earth. In contrast, we have found continent sized regions on Mars, which are 600,000 parts per million, or 60% water by weight." Although the Moon is very dry on the whole, the spot where the LCROSS impacter hit was chosen for a high concentration of water ice. Dr. Zubrin's computations are not a sound basis for estimating the percentage of water in the regolith at that site. Researchers with expertise in that area estimated that the regolith at the impact site contained 5.6 ± 2.9% water ice, and also noted the presence of other volatile substances. Hydrocarbons, material containing sulfur, carbon dioxide, carbon monoxide, methane and ammonia were present.
In March 2010, NASA reported that the findings of its mini-SAR radar aboard Chandrayaan-1 were consistent with ice deposits at the Moon's north pole. It is estimated there is at least 600 million tons of ice at the north pole in sheets of relatively pure ice at least a couple of meters thick.
In March 2014, researchers who had previously published reports on possible abundance of water on the Moon, reported new findings that refined their predictions substantially lower.
|This article contains a pro and con list, which is sometimes inappropriate. (November 2012)|
Placing a colony on a natural body would provide an ample source of material for construction and other uses in space, including shielding from cosmic 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 source of construction materials within cis-lunar space. Rockets launched from the Moon would require less locally produced propellant than rockets launched from Earth. 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.
In addition, the Moon is the closest large body in the Solar System to Earth. While some Earth-crosser asteroids occasionally pass closer, the Moon's distance is consistently within a small range close to 384,400 km. This proximity has several benefits:
There are several disadvantages to the Moon as a colony site:
Three criteria that a Lunar outpost should meet are:
While a colony might be located anywhere, potential locations for a Lunar colony fall into three broad categories.
There are two reasons why the north pole and south pole of the moon might be attractive locations for a human colony. First, there is evidence that water may be present in some continuously shaded areas near the poles. Second, the Moon's axis of rotation is sufficiently close to being perpendicular to the ecliptic plane that the radius of the Moon's polar circles is less than 50 km. Power collection stations could therefore be plausibly located so that at least one is exposed to sunlight at all times, thus making it possible to power polar colonies almost exclusively with solar energy. Moreover, due to the Moon's uneven surface some sites have nearly continuous sunlight. For example, Malapert mountain, located near the Shackleton crater at the Lunar south pole, offers several advantages as a site:
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 Poles of Cold in Siberia and Antarctica. The interior of Peary Crater may also harbor hydrogen deposits.
A 1994 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 at the south pole and even more at the north pole, in 2008. 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.
A potential limitation of the polar regions is that the inflow of solar wind can create an electrical charge on the leeward side of crater rims. The resulting voltage difference can affect electrical equipment, change surface chemistry, erode surfaces and levitate Lunar dust.
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 extra-Lunar traffic: The rotation advantage for launching material is slight due to the Moon's slow rotation, but the corresponding orbit coincides with the ecliptic, nearly coincides with the Lunar orbit around Earth, and nearly coincides with the equatorial plane of Earth.
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.
Lunar lava tubes are a potential location for constructing a Lunar base. Any intact lava tube on the moon could serve as a shelter from the severe environment of the Lunar surface, with its frequent meteorite impacts, high-energy ultra-violet radiation and energetic particles, and extreme diurnal temperature variations. Lava tubes provide ideal positions for shelter because of their access to nearby resources. They also have proven themselves as a reliable structure, having withstood the test of time for billions of years.
An underground colony would escape the extreme of temperature on the Moon's surface. The average temperature on the surface of the moon is about −5 °C. The day period (about 354 hours) 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 354 hours) has an average temperature of about −153 °C (−243 °F). Underground, both periods would be around −23 °C (−9 °F), and humans could install ordinary air conditioners.
One such lava tube was discovered in early 2009.
The central peaks of large lunar craters may contain material that rose from as far 19 kilometers beneath the surface when the peaks formed by rebound of the compressed rock under the crater. Material moved from the interior of craters is piled in their rims. These and other processes make possibly novel concentrations of minerals accessible to future prospectors from lunar colonies.
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 would also greatly reduce the risk of air leakage, as the colony would be fully sealed from the outside except for a few exits to the surface.
The construction of an underground base would probably be more complex; one of the first machines from Earth might be a remote-controlled excavating machine. 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. Rowley & Neudecker have suggested "melt-as-you-go" machines that would leave glassy internal surfaces. Mining methods such as the room and pillar might also be used. Inflatable self-sealing fabric habitats might then be put in place to retain air. Eventually an underground city can be constructed. Farms set up underground would need artificial sunlight. As an alternative to excavating, a lava tube could be covered and insulated, thus solving the problem of radiation exposure.
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. Blacic has studied the mechanical properties of lunar glass and has shown that it is a promising material for making rigid structures, if coated with metal to keep moisture out. This may allow for the use of "Lunar bricks" in structural designs, or the vitrification of loose dirt to form a hard, ceramic crust.
A Lunar base built on the surface would need to be protected by improved radiation and micrometeoroid shielding. Building the Lunar base inside a deep crater would provide at least partial shielding against radiation and micrometeoroids. 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 exposure to charged solar and galactic particles.
In a turn from the usual engineer-designed lunar habitats, London-based Foster + Partners architectural firm proposed a building construction 3D-printer technology in January 2013 that would use Lunar regolith raw materials to produce Lunar building structures while using enclosed inflatable habitats for housing the human occupants inside the hard-shell Lunar structures. Overall, these habitats would require only ten percent of the structure mass to be transported from Earth, while using local Lunar materials for the other 90 percent of the structure mass. "Printed" Lunar soil will provide both "radiation and temperature insulation. Inside, a lightweight pressurized inflatable with the same dome shape will be the living environment for the first human Moon settlers." The building technology will include mixing Lunar material with magnesium oxide, which will turn the "moonstuff into a pulp that can be sprayed to form the block" when a binding salt is applied that "converts [this] material into a stone-like solid." Terrestrial versions of this 3D-printing building technology are already printing 2 metres (6 ft 7 in) of building material per hour with the next-generation printers capable of 3.5 metres (11 ft) per hour, sufficient to complete a building in a week.
In 2010, The Moon Capital Competition offered a prize for a design of a Lunar habitat intended to be an underground international commercial center capable of supporting a residential staff of 60 people and their families. The Moon Capital is intended to be self-sufficient with respect to food and other material required for life support. Prize money was provided primarily by the Boston Society of Architects, Google Lunar X Prize and The New England Council of the American Institute of Aeronautics and Astronautics.
A nuclear fission reactor might fulfill most of a Moon base's power requirements. With the help of fission reactors, one could overcome the difficulty of the 354 hour Lunar night. According to NASA, a nuclear fission power station could generate a steady 40 kilowatts, equivalent to the demand of about eight houses on Earth. An artist’s concept of such a station published by NASA envisages the reactor being buried below the Moon's surface to shield it from its surroundings; out from a tower-like generator part reaching above the surface over the reactor, radiators would extend into space to send away any heat energy that may be left over.
Radioisotope thermoelectric generators could be used as backup and emergency power sources for solar powered colonies.
Solar energy is a possible source of power for a Lunar base. Many of the raw materials needed for solar panel production can be extracted on site. However, the long Lunar night (354 hours) 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 Earth days at a time. On the Moon, they would only be needed for 354 hours (14 3⁄4 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 lighter, not to mention the reduced mass of the smaller heat-dissipating radiators. This makes PEMs more economical to launch from Earth than the shuttle's cells. 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 electrolyze the water created in the fuel cells – although there would be small losses of gases that would have to be replaced.
Even if (and when) lunar colonies had provided themselves access to a near-continuous source of solar energy, they would still need to maintain fuel cells (or some other energy storage system) to sustain themselves during lunar eclipses and for emergencies.
Conventional rockets have been used for most Lunar exploration to date. The ESA's SMART-1 mission from 2003 to 2006 used conventional chemical rockets to reach orbit and Hall effect thrusters to arrive at the Moon in 13 months. NASA would have used chemical rockets on its Ares V booster and Lunar Surface Access Module, that were being developed for a planned return to the Moon around 2019, but this was cancelled. The construction workers, location finders, and other astronauts vital to building, would have been taken four at a time in NASA's Orion spacecraft.
Lunar colonists will want the ability 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 pressurized 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[update]) are the three Apollo Lunar Roving Vehicles (LRV), developed by Boeing, and the two robotic Soviet Lunokhods. 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 pressurized 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 pressurized 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.
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.
Experience so far indicates that launching human beings into space is much more expensive than launching cargo.
One way to get materials and products from the Moon to an interplanetary way station 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.
A Lunar space elevator could transport people, raw materials and products to and from an orbital station at Lagrangian points L1 or L2. Chemical rockets would take a payload from Earth to the L1 Lunar Lagrange location. From there a tether would slowly lower the payload to a soft landing on the lunar surface.
Other possibilities include a momentum exchange tether system.
A cis-Lunar 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. Mining and refining the Moon's materials on-site – for use both on the Moon and elsewhere in the Solar System – could provide an advantage over deliveries from Earth, as they can be launched into space at a much lower energy cost than from Earth. It is possible that large amounts of matter will need to be launched into space for interplanetary exploration in the 21st century, and the lower cost of providing goods from the Moon might be attractive.
In the long term, the Moon will likely play an important role in supplying space-based construction facilities with raw materials. Zero gravity allows for the processing of materials 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 realign.) Materials which cannot be alloyed or mixed on Earth because of gravity-field effects on density differences could be combined in space, resulting in composites which could have exceptional qualities. (This is the foundation of the free MoonBaseOne game that teaches children about space.)
Exporting material to Earth in trade from the Moon is more problematic due to the cost of transportation, which will vary greatly if the Moon is industrially developed (see above). One suggested trade commodity, Helium-3 (3He) from the solar wind, is thought to have accumulated on the Moon's surface over billions of years, but occurs only rarely on Earth. Helium might be present in the Lunar regolith in quantities of 0.01 ppm to 0.05 ppm (depending on soil). In 2006 3He had a market price of 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.
To reduce the cost of transport, the Moon could store propellants produced from lunar water at one or several depots between the Earth and the Moon, to resupply rockets or satellites in earth orbit. The Shackleton Energy Company estimate investment in this infrastructure could cost around $25 billion.
Gerard K. 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 will vary greatly if the Moon is industrially developed (see above). This proposal was based on the contemporary estimates of future launch costs of the 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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