Lunar habitation
Lunar habitation is any human social and material structure and activity on the Moon making human habitation on the Moon. Therefore it is more than the particular surface space habitats and more specific than the moonbases which can consist of any facility. History. Lunar habitats have been designed for many different proposed moonbases. The only lunar habitats that sofar have been erected were the temporary Apollo Lunar Modules, such as "Eagle" of Tranquility Base, the very first. Advantages, disadvantages, problems and potential solutions. Setting up structures on a natural body would provide ample sources of material for their  construction, particularly for purposes such as 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 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 advantages: There are several disadvantages and/or problems to the Moon as a site: Energy. Nuclear power. 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 facilities. The needed radioisotopes could either be imported from earth as they are relatively energy-dense, or produced "in situ" by neutron irradiation of suitable materials (e.g. - an unavoidably produced minor actinide in fission reactors operating with thermal neutrons - to produce the commonly used ) or separated chemically from the high level waste of the nuclear reactor (e.g. Americium-241). Such nuclear batteries have been used for decades in spacecraft by all major spacefaring nations and some have even been implanted in humans as power sources for medical devices  such as pacemakers, meaning their behavior and tradeoffs are well understood. The Japanese spacecraft Kaguya confirmed the existence of both Uranium and Thorium near the lunar surface, which might in the future allow for the local production of nuclear fission fuel from lunar resources. However, concentrations are relatively low and unless the thorium fuel cycle is used, uranium would likely have to be enriched to produce fuel usable in light water reactors. The isotopic composition of lunar uranium is not known, but there is little reason to assume it would differ much from that found on earth (99% , ~0.72% , 55  ppm ). Nuclear reprocessing "in situ" would reduce the need for enrichment or imported fuel from earth. Pyroprocessing, which has been demonstrated at the Integral Fast Reactor prototype operated by Argonne National Laboratory, could be used instead of the "de facto" standard PUREX which requires large amounts of organic solvents. One specific development program in the 2000s was the Fission Surface Power (FSP) project of NASA and DOE, a fission power system focused on "developing and demonstrating a nominal 40 kWe power system to support human exploration missions. The FSP system concept uses conventional low-temperature stainless steel, liquid metal-cooled  reactor technology coupled with Stirling power conversion." , significant component hardware testing had been successfully completed, and a non-nuclear system demonstration test was being fabricated.<ref name="nasa/doe2010"> </ref> In 2017 NASA started the Kilopower project that tested the KRUSTY reactor. Japan has the RAPID-L conceptual design. Helium-3 mining could be used to provide a substitute for tritium for potential production of fusion power in the future. Solar energy. 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. The  long lunar night (354 hours or 14.75 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. Since lunar regolith contains structural metals like iron and aluminum, solar panels could be mounted high up on locally-built  towers that might rotate to follow the Sun. 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. Energy storage. 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 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 lunar facilities could provide themselves access to a near-continuous source of solar energy, they  would still need to maintain fuel cells or an alternate energy storage system to sustain themselves during lunar eclipses and emergency situations. Locations. Soviet astronomer Vladislav V. Shevchenko proposed in 1988 the following three criteria that a lunar outpost should meet: While a habitat might be located anywhere, potential locations for a lunar habitat fall into three broad categories. Polar regions. There are two reasons why the north pole and south pole of the Moon might be attractive locations for a human facility. First, there is evidence for the presence of water 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 facilities almost exclusively with solar energy. Solar power would be unavailable only during a lunar eclipse, but these events are relatively brief and absolutely predictable. Any such habitat would therefore require a reserve energy supply that could temporarily sustain a habitat during  lunar eclipses or in the event of any incident or malfunction affecting solar power collection. Hydrogen fuel cells would be ideal for this purpose, since the hydrogen needed could be sourced locally using the Moon's polar water and surplus solar power. 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: 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 in 1994 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 . 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 in 2008 enhanced hydrogen abundances at the south pole and even more at the north pole. 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 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. Equatorial regions. 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. Far side. The lunar far side lacks direct communication with Earth, though a communication satellite at the 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. Scientists have estimated that the highest concentrations of helium-3 can 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 shield 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. 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 to be reliable structures, having withstood the test of time for billions of years. An underground habitat would escape the extreme temperatures on the Moon's surface. The day period (about 354 hours) has an average temperature of about , although  it can rise as high as . The night period (also 354 hours) has an average temperature of about . Underground, both day and night periods would be around , and humans could install ordinary heaters for warmth. One such lava tube was discovered in early 2009. Habitat construction. There have been numerous proposals regarding lunar habitats. The designs have evolved throughout the years as 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. 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 unknown early on. Proposals have shifted as these hazards were recognized and taken into consideration. Underground habitat. Some suggest building the lunar habitats underground, which would give protection from radiation and micrometeoroids. This would also greatly reduce the risk of air leakage, as the habitat would be fully sealed from the outside except for a few exits to the surface. These underground habitats would be akin to bunkers\fallout shelters The construction of an underground habitat 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. An alternative solution is studied  in Europe by students to excavate a habitat in the ice-filled craters of the Moon. Underground farming. 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. Surface habitats. A possibly easier solution would be to build the lunar habitat on the surface, and cover modules with lunar soil. The lunar soil 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 habitat 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 crewed missions, and it might be possible to use similar technology on a lunar habitat. 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 would provide both "radiation and temperature insulation. Inside, a lightweight pressurized inflatable with the same dome shape would be the living environment for the first human Moon settlers." The building technology would include mixing lunar material with magnesium oxide, which would 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 of building material per hour with the next-generation printers capable of per hour, sufficient to complete a building in a week. 3D-printed structures. On January 31, 2013, the ESA working with Foster + Partners, tested a 3D-printed structure that could be constructed of lunar regolith for use as a Moon base. Transportation. Earth to the Moon. Conventional rockets have been used for most lunar explorations 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 Altair lander, 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. Proposed concepts of Earth-Moon transportation are Space elevators. On the surface. Lunar habitation would need 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, such as the Toyota rover concept. 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 () are the three Apollo Lunar Roving Vehicles (LRV), developed by Boeing, the two robotic Soviet Lunokhods and the Chinese Yutu rover in 2013. The LRV was an open rover for a crew of two, and a range of during one  lunar day. One NASA study resulted in the Mobile Lunar Laboratory concept, a crewed pressurized rover for a crew of two, with a range of . The Soviet Union developed different rover concepts in the Lunokhod series and the L5 for possible use on future crewed 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 (Maglev) 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 - or even higher than - aircraft on Earth. In essence any maglev on the moon would behave similar to a vactrain without the need to provide an artificial vacuum. One significant difference with lunar trains 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, while Bell proposed  the Manned Flying System, a similar concept. Surface to space. Launch technology. 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 or . Chemical rockets would take a payload from Earth to the L 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. Surface to and from cis-lunar space. 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.