Colonization of the Moon

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1986 artist concept of a lunar colony

Colonization of the Moon is the proposed establishment of a permanent human community or robotic industries on the Moon.

Discovery of lunar water at the lunar poles by Chandrayaan-1 has renewed interest in the Moon. Locating such a colony at one of the lunar poles would also avoid the problem of long lunar nights – about 354 hours long, a little more than two weeks – and allow the colony to take advantage of the continuous sunlight there for generating solar power.

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 vision of space colonization as an achievable and worthwhile goal has gained momentum. Because of its proximity to Earth, the Moon is seen as the best and most obvious location for the first permanent off-planet colony. Currently, the main problem hindering the development of such a colony is the high cost of spaceflight.

There are also several projects that have been proposed for the near future by space tourism startup companies for tourism on the Moon.

Proposals

Concept art from NASA showing astronauts entering a lunar outpost

The notion of a lunar colony 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 writer 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.^[11] The proposed design consisted of a half-cylinder with half-domes at both ends, with a micrometeoroid shield placed above the base.

Project Horizon

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. It was proposed that the first landing would be carried out by two "soldier-astronauts" in 1965 and that more construction workers would soon follow. It was posited that through numerous launches (61 Saturn Is and 88 Saturn C-2s), 245 tons of cargo could be transported to the outpost by 1966.

Lunex Project

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.

Sub-surface 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 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.

Recent proposals

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 2016, Johann-Dietrich Wörner, the Chief of ESA, proposed the International Moon Village as a non-governmental organization (NGO), and in November 2017, the Moon Village Association was created. This organization aims to promote international discussions to foster the implementation of a permanent human settlement near the lunar south pole.

Moon exploration

Exploration through 2017

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 robotic lunar explorations.

Responding to the Soviet program of space exploration, US President John F. Kennedy in 1961 told the US 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.

Crewed 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 Apollo 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 crewed missions in near Earth orbit.

In addition to its scientific returns, the Apollo program also provided valuable lessons about living and working in the lunar environment.

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 of India, carrying an ISRO instrument, discovered that the lunar soil contains 0.1% water by weight, overturning hypotheses that had stood for 40 years.

In 2004, US President George W. Bush called for a plan to return crewed missions to the Moon by 2020 (since cancelled – see Constellation program). On June 18, 2009, NASA's LCROSS/LRO mission to the moon was launched. The LCROSS mission was designed to acquire research information to assist with future lunar exploratory missions and was scheduled to conclude with a controlled collision of the craft on the lunar surface. LCROSS's mission concluded as scheduled with its controlled impact on October 9, 2009.

In 2010, due to reduced congressional NASA appropriations, President Barack Obama halted the Bush administration's earlier lunar exploration initiative and directed a generic focus on crewed missions to asteroids and Mars, as well as extending support for the International Space Station.

Planned crewed lunar missions 2021 - 2036

As of 2016, Russia is planning to begin building a human colony on the moon by 2030. Initially, the Moon base would be crewed by no more than 4 people, with their number later rising to maximum of 12 people. Japan also has plans to land a man on the moon by 2030, while the People's Republic of China is currently planning to land a human on the Moon by 2036 (see Chinese Lunar Exploration Program).

The United States currently (2017) has plans to send a crewed space mission to orbit (but not to land on) the Moon in 2021. While the US Trump administration has called for a return of crewed missions to the Moon, it has currently (2018) not authorized any funding for any such lunar missions in the next 20 years. The current administration has focused funding on Mars missions. What President Trump requests is the development of a lunar orbiting station called Lunar Orbital Platform-Gateway. A stated goal of aerospace company SpaceX is to enable the creation of a colony on the Moon using its upcoming BFR launch system. Billionaire Jeff Bezos has outlined his plans for a lunar base in the next decade.

Lunar water ice

 
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Beginning with a full-frame Moon in this video, the camera flies to the lunar south pole and shows areas of permanent shadow. Realistic shadows evolve through several months.

On 24 September 2009, Science magazine reported that the Moon Mineralogy Mapper (M³) on the Indian Space Research Organization's (ISRO) Chandrayaan-1 had detected water on the Moon. M³ detected absorption features near 2.8–3.0 μm (0.00011–0.00012 in) on the surface of the Moon. For silicate bodies, such features are typically attributed to hydroxyl- and/or water-bearing materials. On the Moon, the feature is seen as a widely distributed absorption that appears strongest at cooler high latitudes and at several fresh feldspathic craters. The general lack of correlation of this feature in sunlit M³ data with neutron spectrometer H abundance data suggests that the formation and retention of OH and H₂O is an ongoing surficial process. OH/H₂O production processes may feed polar cold traps and make the lunar regolith a candidate source of volatiles for human exploration.

The Moon Mineralogy Mapper (M³), an imaging spectrometer, was one of the 11 instruments on board Chandrayaan-1, whose mission came to a premature end on 29 August 2009. M³ was aimed at providing the first mineral map of the entire lunar surface.

Lunar scientists had discussed the possibility of water repositories for decades. They are now increasingly "confident that the decades-long debate is over" a report says. "The Moon, in fact, has water in all sorts of places; not just locked up in minerals, but scattered throughout the broken-up surface, and, potentially, in blocks or sheets of ice at depth." The results from the Chandrayaan mission are also "offering a wide array of watery signals."

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 impactor 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.

In 2018, it was announced that M³ infrared data from Chandrayaan-1 had been re-analyzed to confirm the existence of water across wide expanses of the Moon's polar regions.

Advantages and disadvantages

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 advantages:

• A lunar base could be a site for launching rockets with locally manufactured fuel to distant planets such as Mars. Launching rockets from the Moon would be easier than from Earth because the Moon's gravity is lower, requiring a lower escape velocity. A lower escape velocity would require less propellant, but there is no guarantee that less propellant would cost less money than that required to launch from Earth. Asteroid mining, however, may prove useful in lowering various costs accrued during the construction and management of a lunar base and its activities.
• The energy required to send objects from Earth to the Moon is lower than for most other bodies.
• Transit time is short. The Apollo astronauts made the trip in three days and future technologies could improve on this time.
• The short transit time would also allow emergency supplies to quickly reach a Moon colony from Earth, or allow a human crew to evacuate relatively quickly from the Moon to Earth in case of emergency. This could be an important consideration when establishing the first human colony.
• If a long-term base were to be built on the Moon, the exposure would show the effects of low gravity on humans over an extended period of time. Those results could likely inform the viability of attempting a long-term base or a Mars colony.
• The round trip communication delay to Earth is less than three seconds, allowing near-normal voice and video conversation, and allowing some kinds of remote control of machines from Earth that are not possible for any other celestial body. The delay for other Solar System bodies is minutes or hours; for example, round trip communication time between Earth and Mars ranges from about eight to forty minutes. This, again, could be particularly valuable in an early colony, where life-threatening problems requiring Earth's assistance could occur.
• On the lunar near side, the Earth appears large and is always visible as an object 60 times brighter than the Moon appears from Earth, unlike more distant locations where the Earth would be seen merely as a star-like object, much as the planets appear from Earth. As a result, a lunar colony might feel less remote to humans living there.
• Building observatory facilities on the Moon from lunar materials allows many of the benefits of space based facilities without the need to launch these into space. The lunar soil, although it poses a problem for any moving parts of telescopes, can be mixed with carbon nanotubes and epoxies in the construction of mirrors up to 50 meters in diameter. It is relatively nearby; astronomical seeing is not a concern; certain craters near the poles are permanently dark and cold, and thus especially useful for infrared telescopes; and radio telescopes on the far side would be shielded from the radio chatter of Earth. A lunar zenith telescope can be made cheaply with ionic liquid.
• A farm at the lunar north pole could provide eight hours of sunlight per day during the local summer by rotating crops in and out of the sunlight which is continuous for the entire summer. A beneficial temperature, radiation protection, insects for pollination, and all other plant needs could be artificially provided during the local summer for a cost. One estimate suggested a 0.5 hectare space farm could feed 100 people.

There are several disadvantages to the Moon as a colony site:

• The long lunar night would impede reliance on solar power and require that a colony exposed to the sunlit equatorial surface be designed to withstand large temperature extremes (about 95 K (−178.2 °C) to about 400 K (127 °C)). An exception to this restriction are the so-called "peaks of eternal light" located at the lunar north pole that are constantly bathed in sunlight. The rim of Shackleton Crater, towards the lunar south pole, also has a near-constant solar illumination. Other areas near the poles that get light most of the time could be linked in a power grid. The temperature 1 meter below the surface of the Moon is estimated to be near constant over the period of a month varying with latitude from near 220 K (−53 °C) at the equator to near 150 K (−123 °C) at the poles.
• The Moon is highly depleted in volatile elements, such as nitrogen and hydrogen. Carbon, which forms volatile oxides, is also depleted. A number of robot probes including Lunar Prospector gathered evidence of hydrogen generally in the Moon's crust consistent with what would be expected from solar wind, and higher concentrations near the poles. There had been some disagreement whether the hydrogen must necessarily be in the form of water. The 2009 mission of the Lunar Crater Observation and Sensing Satellite (LCROSS) proved that there is water on the Moon. This water exists in ice form perhaps mixed in small crystals in the regolith in a colder landscape than people have ever mined. Other volatiles containing carbon and nitrogen were found in the same cold trap as ice. If no sufficient means is found for recovering these volatiles on the Moon, they would need to be imported from some other source to support life and industrial processes. Volatiles would need to be stringently recycled. This would limit the colony's rate of growth and keep it dependent on imports. The transportation cost of importing volatiles from Earth could be reduced by constructing the upper stage of supply ships using materials high in volatiles, such as carbon fiber and plastics. The 2006 announcement by the Keck Observatory that the binary Trojan asteroid 617 Patroclus, and possibly large numbers of other Trojan objects in Jupiter's orbit, are likely composed of water ice, with a layer of dust, and the hypothesized large amounts of water ice on the closer, main-belt asteroid 1 Ceres, suggest that importing volatiles from this region via the Interplanetary Transport Network may be practical in the not-so-distant future. However, these possibilities are dependent on complicated and expensive resource utilization from the mid to outer Solar System, which is not likely to become available to a Moon colony for a significant period of time.
• It is uncertain whether the low (~ one-sixth g) gravity on the Moon is strong enough to prevent detrimental effects to human health in the long term. Exposure to weightlessness over month-long periods has been demonstrated to cause deterioration of physiological systems, such as loss of bone and muscle mass and a depressed immune system. Similar effects could occur in a low-gravity environment, although virtually all research into the health effects of low gravity has been limited to micro gravity.
• The lack of a substantial atmosphere for insulation results in temperature extremes and makes the Moon's surface conditions somewhat like a deep space vacuum. It also leaves the lunar surface exposed to half as much radiation as in interplanetary space (with the other half blocked by the Moon itself underneath the colony), raising the issues of the health threat from cosmic rays and the risk of proton exposure from the solar wind. Lunar rubble can protect living quarters from cosmic rays. Shielding against solar flares during expeditions outside is more problematic.
• When the Moon passes through the magnetotail of the Earth, the plasma sheet whips across its surface. Electrons crash into the Moon and are released again by UV photons on the day side but build up voltages on the dark side. This causes a negative charge build up from −200 V to −1000 V. See Magnetic field of the Moon.
• The lack of an atmosphere increases the chances of the colony's being hit by meteors. Even small pebbles and dust (micrometeoroids) have the potential to damage or destroy insufficiently protected structures.
• Moon dust is an extremely abrasive glassy substance formed by micrometeorites and unrounded due to the lack of weathering. It sticks to everything, can damage equipment, and it may be toxic. Since it is bombarded by charged particles in the solar wind, it is highly ionized, and is extremely harmful when breathed in. During the 1960s and 70s Apollo missions, astronauts were subject to respiratory problems on return flights from the Moon, for this reason.
• Growing crops on the Moon faces many difficult challenges due to the long lunar night (354 hours), extreme variation in surface temperature, exposure to solar flares, nitrogen-poor soil, and lack of insects for pollination. Due to the lack of any atmosphere on the Moon, plants would need to be grown in sealed chambers, though experiments have shown that plants can thrive at pressures much lower than those on Earth. The use of electric lighting to compensate for the 354-hour night might be difficult: a single acre of plants on Earth enjoys a peak 4 megawatts of sunlight power at noon. Experiments conducted by the Soviet space program in the 1970s suggest it is possible to grow conventional crops with the 354-hour light, 354-hour dark cycle. A variety of concepts for lunar agriculture have been proposed, including the use of minimal artificial light to maintain plants during the night and the use of fast-growing crops that might be started as seedlings with artificial light and be harvestable at the end of one lunar day.
• One of the less obvious difficulties lies not with the Moon itself but rather with the political and national interests of the nations engaged in colonization. Assuming that colonization efforts were able to overcome the difficulties outlined above – there would likely be issues regarding the rights of nations and their colonies to exploit resources on the lunar surface, to stake territorial claims and other issues of sovereignty which would have to be agreed upon before one or more nations established a permanent presence on the Moon. The ongoing negotiations and debate regarding the Antarctic is a good case study for prospective lunar colonization efforts in that it highlights the numerous pitfalls of developing/inhabiting a location that is subject to the claims of multiple sovereign nations.

Locations

Russian astronomer Vladislav V. Shevchenko proposed in 1988 the following three criteria that a lunar outpost should meet:

• good conditions for transport operations;
• a great number of different types of natural objects and features on the Moon of scientific interest; and
• natural resources, such as oxygen. The abundance of certain minerals, such as iron oxide, varies dramatically over the lunar surface.

While a colony might be located anywhere, potential locations for a lunar colony 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 colony. 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 colonies 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 colony would therefore require a reserve energy supply that could temporarily sustain a colony 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:

• It is exposed to the Sun most of the time; two closely spaced arrays of solar panels would receive nearly continuous power.
• Its proximity to Shackleton Crater (116 km, or 69.8 mi) means that it could provide power and communications to the crater. This crater is potentially valuable for astronomical observation. An infrared instrument would benefit from the very low temperatures. A radio telescope would benefit from being shielded from Earth's broad spectrum radio interference.
• The nearby Shoemaker and other craters are in constant deep shadow, and might contain valuable concentrations of hydrogen and other volatiles.
• At around 5,000 meters (16,000 feet) elevation, it offers line of sight communications over a large area of the Moon, as well as to Earth.
• The South Pole-Aitken basin is located at the lunar south pole. This is the second largest known impact basin in the Solar System, as well as the oldest and biggest impact feature on the Moon, and should provide geologists access to deeper layers of the Moon's crust.

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 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, 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.

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 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 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 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

High Sun view of a 100 meter deep lunar pit crater that may provide access to a lava tube

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 heaters.

One such lava tube was discovered in early 2009.

Structure

Habitat

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. 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 colonies

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. An alternative solution is studied in Europe by students to excavate a habitat in the ice-filled craters of the moon.

Surface colonies

Variant for habitat creation on the surface or over lava tube

 

A NASA model of a proposed inflatable module

 

A possibly easier solution would be to build the lunar base on the surface, and cover the 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 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 crewed 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 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 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.

Moon Capital

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.

3D printed structures

On January 31, 2013, the ESA working with an independent architectural firm, tested a 3D-printed structure that could be constructed of lunar regolith for use as a Moon base.

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 colonies.

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." As of 2010, significant component hardware testing had been successfully completed, and a non-nuclear system demonstration test was being fabricated.

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. However, 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 ​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 lunar colonies 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.

Transport

Earth to 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 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.

Proposed concepts of Earth-Moon transportation are Space elevators.

On the surface

A lunar rover being unloaded from a cargo spacecraft. Conceptual drawing

Lunar colonists 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, 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 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 92 km 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 396 km. 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 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, while Bell proposes the Manned Flying System, a similar concept.

Surface to space

Launch technology

A lunar base with a mass driver (the long structure that goes toward the horizon). NASA conceptual illustration

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.

Launch costs

• Estimates of the cost per unit mass of launching cargo or people from the Moon vary and the cost impacts of future technological improvements are difficult to predict. An upper bound on the cost of launching material from the Moon might be about $40,000,000 per kilogram, based on dividing the Apollo program costs by the amount of material returned. At the other extreme, the incremental cost of launching material from the Moon using an electromagnetic accelerator could be quite low. The efficiency of launching material from the Moon with a proposed electric accelerator is suggested to be about 50%. If the carriage of a mass driver weighs the same as the cargo, two kilograms must be accelerated to orbital velocity for each kilogram put into orbit. The overall system efficiency would then drop to 25%. So 1.4 kilowatt-hours would be needed to launch an incremental kilogram of cargo to low orbit from the Moon. At $0.1/kilowatt-hour, a typical cost for electrical power on Earth, that amounts to $0.16 for the energy to launch a kilogram of cargo into orbit. For the actual cost of an operating system, energy loss for power conditioning, the cost of radiating waste heat, the cost of maintaining all systems, and the interest cost of the capital investment are considerations.
• Passengers cannot be divided into the parcel size suggested for the cargo of a mass driver, nor subjected to hundreds of gravities acceleration. However, technical developments could also affect the cost of launching passengers to orbit from the Moon. Instead of bringing all fuel and oxidizer from Earth, liquid oxygen could be produced from lunar materials and hydrogen should be available from the lunar poles. The cost of producing these on the Moon is yet unknown, but they would be more expensive than production costs on Earth. The situation of the local hydrogen is most open to speculation. As a rocket fuel, hydrogen could be extended by combining it chemically with silicon to form silane, which has yet to be demonstrated in an actual rocket engine. In the absence of more technical developments, the cost of transporting people from the Moon would be an impediment to colonization.

Surface to and from cis-lunar space

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.

Economic development

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 cargo would 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.

Space-based materials processing

In the long term, the Moon will likely play an important role in supplying space-based construction facilities with raw materials. Zero gravity in space 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. The annealing process 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.)

Exporting material to Earth

Exporting material to Earth in trade from the Moon is more problematic due to the cost of transportation, which would vary greatly if the Moon is industrially developed (see "Launch costs" above). One suggested trade commodity is helium-3 (³He) which is carried by the solar wind and accumulated on the Moon's surface over billions of years, but occurs only rarely on Earth. Helium-3 might be present in the lunar regolith in quantities of 0.01 ppm to 0.05 ppm (depending on soil). In 2006 it had a market price of about $1,500 per gram ($1.5M per kilogram), more than 120 times the value per unit weight of gold and over eight times the value of rhodium.

In the future ³He harvested from the Moon may have a role as a fuel in thermonuclear fusion reactors. It should require about 100 tonnes of helium-3 to produce the electricity that Earth uses in a year and there should be enough on the Moon to provide that much for 10,000 years.

Exporting propellant obtained from lunar water

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.

Solar power satellites

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 would vary greatly if the Moon is industrially developed (see "Launch costs" 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.

Superflare

From Wikipedia, the free encyclopedia

Superflares are very strong explosions observed on stars with energies up to ten thousand times that of typical solar flares. The stars in this class satisfy conditions which should make them solar analogues, and would be expected to be stable over very long time scales. The original nine candidates were detected by a variety of methods. No systematic study was possible until the launch of the Kepler satellite, which monitored a very large number of solar-type stars with very high accuracy for an extended period. This showed that a small proportion of stars had violent outbursts, up to 10,000 times as powerful as the strongest flares known on the Sun. In many cases there were multiple events on the same star. Younger stars were more likely to flare than old ones, but strong events were seen on stars as old as the Sun.

The flares were initially explained by postulating giant planets in very close orbits, such that the magnetic fields of the star and planet were linked. The orbit of the planet would warp the field lines until the instability released magnetic field energy as a flare. However, no such planet has showed up as a Kepler transit and this theory has been abandoned.

All superflare stars show quasi-periodic brightness variations interpreted as very large starspots carried round by rotation. Spectroscopic studies found spectral lines that were clear indicators of chromospheric activity associated with strong and extensive magnetic fields. This suggests that superflares only differ in scale from solar flares.

Attempts have been made to detect past solar superflares from nitrate concentrations in polar ice, from historical observations of auroras, and from those radioactive isotopes that can be produced by solar energetic particles. Although two promising events have been found in the carbon-14 records in tree rings, it is not possible to associate them definitely with a superflare event.

Solar superflares would have drastic effects, especially if they occurred as multiple events. Since they can occur on stars of the same age, mass and composition as the Sun this cannot be ruled out. However, solar-type superflare stars are very rare and are magnetically much more active than the Sun; if solar superflares do occur, it may be in well-defined episodes that occupy a small fraction of its time.

Superflare stars

A superflare star is not the same as a flare star, which usually refers to a very late spectral type red dwarf. The term is restricted to large transient events on stars that satisfy the following conditions:

• The star is in spectral class F8 to G8
• It is on or near the main sequence
• It is single or part of a very wide binary
• It is not a rapid rotator
• It is not exceedingly young

Essentially such stars may be regarded as solar analogues. Originally nine superflare stars were found, some of them similar to the Sun.

Original superflare candidates

The original paper  identified nine candidate objects from a literature search:

Star	Type	V (mag)	Detector	Flare Amplitude	Duration	Energy (erg)
Groombridge 1830	G8 V	6.45	Photography	ΔB = 0.62 mag	18 min	EB ~ 1035
Kappa1 Ceti	G5 V	4.83	Spectroscopy	EW(He) = 0.13Å	~ 40 min	E ~ 2 × 1034
MT Tauri	G5 V	16.8	Photography	ΔU = 0.7 mag	~ 10 min	EU ~ 1035
Pi1 Ursae Majoris	G1.5 Vb	5.64	X-ray	LX = 1029 erg/s	>~ 35 min	EX = 2 × 1033
S Fornacis	G1 V	8.64	Visual	ΔV ~ 3 mag	17 - 367 min	EV ~ 2 × 1038
BD +10°2783	G0 V	10.0	X-ray	LX = 2 × 1031 erg/s	~ 49 min	EX >> 3 × 1034
Omicron Aquilae	F8 V	5.11	Photometry	ΔV = 0.09 mag	~ 5 - 15 day	EBV ~ 9 × 1037
5 Serpentis	F8 IV-V	5.06	Photometry	ΔV = 0.09 mag	~ 3 - 25 day	EBV ~ 7 × 1037
UU Coronae Borealis	F8 V	8.86	Photometry	ΔI = 0.30 mag	>~ 57 min	Eopt ~ 7 × 1035

Type gives the spectral classification including spectral type and luminosity class.

V (mag) means the normal apparent visual magnitude of the star.

EW(He) is the equivalent width of the 5875.6Å He I D3 line seen in emission.

The observations vary for each object. Some are X-ray measurements, others are visual, photographic, spectroscopic or photometric. The energies for the events vary from 2 × 10³³ to 2 × 10³⁸ ergs.

Kepler discoveries

The Kepler spacecraft is a space observatory designed to find planets by the method of transits. A photometer continually monitors the brightness of 150,000 stars in a fixed area of the sky (in the constellations of Cygnus, Lyra and Draco) to detect changes in brightness caused by planets passing in front of the stellar disc. More than 90,000 are G-type stars (similar to the Sun) on or near the main sequence. The observed area corresponds to about 0.25% of the entire sky. The photometer is sensitive to wavelengths of 400–865 nm: the entire visible spectrum and part of the infrared. The photometric accuracy achieved by Kepler is typically 0.01% (0.1 mmag) for 30 minute integration times of 12th magnitude stars.

G-type stars

The high accuracy, the large number of stars observed and the long period of observation make Kepler ideal for detecting superflares. Studies published in 2012 and 2013 involved 83,000 stars over a period of 500 days (much of the data analysis was carried out with the help of five first-year undergraduates). The stars were selected from the Kepler Input Catalog to have T_eff, the effective temperature, between 5100 and 6000K (the solar value is 5750K) to find stars of similar spectral class to the Sun, and the surface gravity log g > 4.0 to eliminate sub-giants and giants. The spectral classes range from F8 to G8. The integration time was 30 min in the original study. 1547 superflares were found on 279 solar-type stars.The most intense events increased the brightness of the stars by 30% and had an energy of 10³⁶ ergs. White-light flares on the Sun change the brightness by about 0.01%, and the strongest flares have a visible-light energy of about 10³² ergs. (All energies quoted are in the optical bandpass and so are lower limits since some energy is emitted at other wavelengths.) Most events were much less energetic than this: flare amplitudes below 0.1% of the stellar value and energies of 2 × 10³³ ergs were detectable with the 30 minute integration. The flares had a rapid rise followed by an exponential decay on a time scale of 1–3 hours. The most powerful events corresponded to energies ten thousand greater than the largest flares observed on the Sun. Some stars flared very frequently: one star showed 57 events in 500 days, a rate of one every nine days. For the statistics of flares, the number of flares decreased with energy E roughly as E⁻², a similar behaviour to solar flares. The duration of the flare increased with its energy, again in accordance with the solar behaviour.

Some Kepler data is taken at one minute sampling, though inevitably with lower accuracy. Using this data, on a smaller sample of stars, reveals flares that are too brief for reliable detection with 30-min integrations, allowing detection of events as low as 10³² ergs, comparable with the brightest flares on the Sun. The occurrence frequency as a function of energy remains a power law E⁻ⁿ when extended to lower energies, with n around 1.5. At this time resolution some superflares show multiple peaks with separations of 100 to 1000 seconds, again comparable to the pulsations in solar flares. The star KIC 9655129 showed two periods, of 78 and 32 minutes, suggesting magnetohydrodynamic oscillations in the flaring region. These observations suggest that superflares are different only in scale and not in type to solar flares.

Superflare stars show a quasi-periodic brightness variation, which is interpreted as evidence of starspots carried around by solar rotation. This allows an estimate of the rotation period of the star; values range from less than one day up to tens of days (the value for the Sun is 25 days). On the Sun, radiometer monitoring from satellites shows that large sunspots can reduce the brightness by up to 0.2%. In superflare stars the most common brightness variations are 1-2%, though they can be as great as 7-8%, suggesting that the area of the starspots can be very much larger than anything found on the Sun. In some cases the brightness variations can be modelled by only one or two large starspots, though not all cases are so simple. The starspots could be groups of smaller spots or single giant spots.

Flares are more common in stars with short periods. However, the energy of the largest flares is not related to the period of rotation. Stars with larger variations also have much more frequent flares; there is as well a tendency for them to have more energetic flares. Large variations can be found on even the most slowly rotating stars: one star had a rotation period of 22.7 days and variations implying spot coverage of 2.5% of the surface, over ten times greater than the maximum solar value. By estimating the size of the starspots from the amplitude variation, and assuming solar values for the magnetic fields in the spots (1000 G), it is possible to estimate the energy available: in all cases there is enough energy in the field to power even the largest flares observed. This suggests that superflares and solar flares have essentially the same mechanism.

In order to determine whether superflares can occur on the Sun, it is important to narrow the definition of Sun-like stars. When the temperature range is divided into stars with T_eff above and below 5600K (early and late G-type stars), stars of lower temperature are about twice as likely to show superflare activity as those in the solar range and those that do so have more flares: the occurrence frequency of flares (number per star per year) is about five times as great in the late-type stars. It is well known that both the rotation rate and the magnetic activity of a star decrease with age in G-type stars. When flare stars are divided into fast and slow rotators, using the rotation period estimated from brightness variations, there is a general tendency for the fastest-rotating (and presumably youngest) stars to show a greater probability of activity: in particular, stars rotating in less than 10 days are 20-30 times more likely to have activity. Nevertheless, 44 superflares were found on 19 stars with similar temperatures to the Sun and periods greater than 10 days (out of 14000 such stars examined); four superflares with energies in the range 1-5 × 10³³ ergs were detected on stars rotating more slowly than the Sun (of about 5000 in the sample). The distribution of flares with energy has the same shape for all classes of star: although Sun-like stars are less likely to flare, they have the same proportion of very energetic flares as younger and cooler stars.

K and M type stars

Kepler data have also been used to search for flares on stars of later spectral types than G. A sample of 23,253 stars with effective temperature T_eff less than 5150K and surface gravity log g > 4.2, corresponding to main sequence stars later than K0V, was examined for flares over a time period of 33.5 days. 373 stars were identified as having obvious flares. Some stars had only one flare, while others showed as many as fifteen. The strongest events increased the brightness of the star by 7-8%. This is not radically different from the peak brightness of flares on G-type stars; however, since K and M stars are less luminous than type G, this suggests that flares on these stars are less energetic. Comparing the two classes of stars studied, it seems that M stars flare more frequently than K stars but the duration of each flare tends to be shorter. It is not possible to draw any conclusions about the relative proportion of G and K type stars showing superflares, or about the frequency of flares on those stars that do show such activity, since the flare detection algorithms and criteria in the two studies are quite different.

Most (though not all) of the K and M stars show the same quasi-periodic brightness variations as the G stars. There is a tendency for more energetic flares to occur on more variable stars; however flare frequency is only weakly related to variability.

Hot Jupiters as an explanation

When superflares were originally discovered on solar-type stars it was suggested that these eruptions may be produced by the interaction of the star's magnetic field with the magnetic field of a gas-giant planet orbiting so close to the primary that the magnetic fields were linked. Rotation or orbital motion would wind up the magnetic fields until a reconfiguration of the fields would cause an explosive release of energy. The RS Canum Venaticorum variables are close binaries, with orbital periods between 1 and 14 days, in which the primary is an F- or G-type main sequence star, and with strong chromospheric activity at all orbital phases. These systems have brightness variations attributed to large starspots on the primary; some show large flares thought to be caused by magnetic reconnection. The companion is close enough to spin up the star by tidal interactions.

A gas giant however would not be massive enough to do this, leaving the various measurable properties of the star (rotation speed, chromospheric activity) unchanged. If the giant and the primary were close enough for the magnetic fields to be linked, the orbit of the planet would wrap the field lines until the configuration became unstable followed by a violent release of energy in the form of a flare. Kepler discovered a number of closely orbiting gas giants, known as hot Jupiters; studies of two such systems showed periodic variations of the chromospheric activity of the primary synchronised to the period of the companion.

Not all planetary transits can be detected by Kepler, since the planetary orbit may be out of the line of sight to Earth. However, the hot Jupiters orbit so close to the primary that the chance of a transit is about 10%. If superflares were caused by close planets the 279 flare stars discovered should have about 28 transiting companions; none of them actually showed evidence of transits, effectively excluding this explanation.

Spectroscopic observations of superflare stars

Spectroscopic studies of superflares allow their properties to be determined in more detail, in the hope of detecting the cause of the flares. The first studies were made using the high dispersion spectrograph on the Subaru telescope in Hawaii. Some 50 apparently solar-type stars, known from the Kepler observations to show superflare activity, have been examined in detail. Of these, only 16 showed evidence of being visual or spectroscopic binaries; these were excluded since close binaries are frequently active, while in the case of visual binaries there is the chance of activity taking place on the companion. Spectroscopy allows accurate determinations of the effective temperature, the surface gravity and the abundance of elements beyond helium ('metallicity'); most of the 34 single stars proved to be main sequence stars of spectral type G and similar composition to the Sun. Since properties such as temperature and surface gravity change over the lifetime of a star, stellar evolution theory allows an estimate of the age of a star: in most cases the age appeared to be above several hundred million years. This is important since very young stars are known to be much more active. Nine of the stars conformed to the narrower definition of solar-type given above, with temperatures greater than 5600K and rotation periods longer than 10 days; some had periods above 20 or even 30 days. Only five of the 34 could be described as fast rotators.

Observations from LAMOST have been used to measure chromospheric activity of 5,648 solar-like stars in the Kepler field, including 48 superflare stars. These observations show that superflare stars are generally characterized by larger chromospheric emissions than other stars, including the Sun. However, superflare stars with activity levels lower than, or comparable to, the Sun do exist, suggesting that solar flares and superflares most likely share the same origin. The very large ensemble of solar-like stars included in this study enables detailed and robust estimates of the relation between chromospheric activity and the occurrence of superflares.

All the stars showed the quasi-periodic brightness variations, ranging from 0.1% to nearly 10%, interpreted as the rotation of large starspots. When large spots exist on a star, the activity level of the chromosphere becomes high; in particular, large chromospheric plages form around sunspot groups. The intensities of certain solar and stellar lines generated in the chromosphere, particularly the lines of ionised calcium (Ca II) and the Hα line of hydrogen, are known to be indicators of magnetic activity. Observations of the Ca lines in stars of similar age to the Sun even show cyclic variations reminiscent of the 11 year solar cycle. By observing certain infrared lines of Ca II for the 34 superflare stars it was possible to estimate their chromospheric activity. Measurements of the same lines at points within an active region on the Sun, together with simultaneous measurements of the local magnetic field, show that there is a general relation between field and activity.

Although the stars show a clear correlation between rotational speed and activity, this does not exclude activity on slowly rotating stars: even stars as slow as the Sun can have high activity. All the superflare stars observed had more activity than the Sun, implying larger magnetic fields. There is also a correlation between the activity of a star and its brightness variations (and therefore the starspot coverage): all stars with large amplitude variations showed high activity.

Knowing the approximate area covered by starspots from the size of the variations, and the field strength estimated from the chromospheric activity, allows an estimate of the total energy stored in the magnetic field; in all cases there was enough energy stored in the field to account for even the largest superflares. Both the photometric and the spectroscopic observations are consistent with the theory that superflares are different only in scale from solar flares, and can be accounted for by the release of magnetic energy in active regions very much larger than those on the Sun. Nevertheless, these regions can appear on stars with masses, temperatures, compositions, rotation speeds and ages similar to the Sun.

Detecting past superflares on the Sun

Since stars apparently identical to the Sun can produce superflares it is natural to ask if the Sun itself can do so, and to try to find evidence that it has done in the past. Large flares are invariably accompanied by energetic particles, and these particles produce effects if they reach the earth. The Carrington event of 1859, the largest flare of which we have direct observation, produced global auroral displays extending close to the equator. Energetic particles can produce chemical changes in the atmosphere, which can be permanently recorded in the polar ice. Fast protons generate distinctive isotopes, particularly carbon-14, which can be taken up and preserved by living creatures.

Nitrate concentrations in polar ice

When solar energetic particles reach the Earth's atmosphere they cause ionisation that creates nitric oxide (NO) and other reactive nitrogen species, which then precipitate out in the form of nitrates. Since all energetic particles are deflected to a greater or lesser extent by the geomagnetic field, they enter preferentially at the polar latitudes; since high latitudes also contain permanent ice, it is natural to look for the nitrate signature of particle events in ice cores. A study of a Greenland ice core extending back to 1561 AD achieved resolutions of 10 or 20 samples a year, allowing in principle the detection of single events. Precise dates (within one or two years) can be achieved by counting annual layers in the cores, checked by identification of deposits associated with known volcanic eruptions. The core contained an annual variation of nitrate concentration, accompanied by a number of 'spikes' of different amplitudes. The strongest of these in the entire record was dated to within a few weeks of the Carrington event of 1859. However, other events can produce nitrate spikes, including biomass burning which also produces enhanced ammonium concentrations. An examination of fourteen ice cores from Antarctic and Arctic regions showed large nitrate spikes: however, none of them were dated to 1859 (the closest was 1863). All such spikes were associated with ammonium and other chemical indicators of combustion. There is no evidence that nitrate concentrations can be used as indicators of historic solar activity.

Single events from cosmogenic isotopes

When energetic protons enter the atmosphere they create isotopes by reactions with the major components; the most important of these is carbon-14 (¹⁴C), which is created when secondary neutrons react with nitrogen. ¹⁴C, which has a half-life of 5,730 years, reacts with oxygen to form carbon dioxide which is taken up by plants; dating wood by its ¹⁴C content is the basis of radiocarbon dating. If wood of known age is available the process can be reversed. Measuring the ¹⁴C content and using the half-life allows estimation of the content when the wood was formed. The growth rings of trees show patterns, caused by various environmental factors: dendrochronology uses these growth rings of trees, compared across overlapping sequences, to establish accurate dates. Applying this method shows that atmospheric ¹⁴C does indeed vary with time, due to solar activity. This is the basis of the carbon dating calibration curve. Clearly, it can also be used to detect any peaks in production caused by solar flares, if those flares create enough energetic particles to produce a measurable increase in ¹⁴C.

An examination of the calibration curve, which has a time resolution of five years, showed three intervals in the last 3,000 years in which ¹⁴C increased significantly. On the basis of this two Japanese cedar trees were examined with a resolution of a single year, and showed an increase of 1.2% in AD 774, some twenty times larger than anything expected from the normal solar variation. This peak steadily diminished over the next few years. The result was confirmed by studies of German oak, bristlecone pine from California, Siberian larch, and Kauri wood from New Zealand. All determinations agreed on both the time and amplitude of the effect. In addition, measurements of coral skeletons from the South China Sea showed substantial variations in ¹⁴C over a few months around the same time; however, the date could only be established to within a period of ±14 years around 783 AD.

Carbon-14 is not the only isotope that can be produced by energetic particles. Beryllium-10 (¹⁰Be) is also formed from nitrogen and oxygen, and deposited in polar ice. However, ¹⁰Be deposition can be strongly related to local weather and shows extreme geographic variability; it is also more difficult to assign dates. Nevertheless, a ¹⁰Be increase during the 770s was found in an ice core from the Antarctic, though the signal was less striking because of the lower time resolution (several years); another smaller increase was seen in Greenland. When data from two sites in North Greenland and one in the West Antarctic, all taken with a one-year resolution, were compared they all showed a strong signal: the time profile also matched well with the ¹⁴C results (within the uncertainty of dating for the ¹⁰Be data).^[20] Chlorine-36 (³⁶Cl) can be produced from argon and deposited in polar ice; because argon is a minor atmospheric constituent the abundance is low. The same ice cores which showed ¹⁰Be also provided increases of ³⁶Cl, though with a resolution of five years a detailed match was impossible.

A second event in AD 993/4 has also been found from ¹⁴C in tree rings, but at a lower intensity. This event also produced measurable increases in ¹⁰Be and ³⁶Cl in Greenland ice cores.

If these events are presumed to be produced by fast particles from large flares, it is not easy to estimate the particle energy in the flare or compare it with known events. The Carrington event does not appear in the ¹⁴C record, and neither did any other large particle event that has been directly observed. The flux of particles must be estimated by calculating production rates of radiocarbon, and then modelling the behaviour of the CO₂ once it has entered the carbon cycle; the fraction of the created radiocarbon taken up by trees depends to some extent on that cycle. As an extra complication, the cosmogenic isotopes are preferentially created by energetic protons (several hundred MeV). The energetic particle spectrum of a solar flare varies considerably between events; one with a 'hard' spectrum, with more high-energy protons, will be more efficient at producing a ¹⁴C increase. The most powerful flare which also had a hard spectrum that has been observed instrumentally took place in February 1956 (the beginning of nuclear testing obscures any possible effects in the ¹⁴C record); it has been estimated that if a single flare were responsible for the AD 774/5 event it would need to be 25-50 times more powerful than this. A sunspot group may produce several flares over its lifetime, and the effects of such a sequence would be aggregated over the one-year period covered by a single ¹⁴C measurement; however, the total effect would still be ten times greater than anything observed in a similar period in modern times.

Solar flares are not the only possibility for producing the cosmogenic isotopes. A long or short gamma-ray burst has been proposed as being consistent with all the details of the AD 774/5 event if it was sufficiently close. However, as known presently, this explanation is very unlikely.

Historical records

A number of attempts have been made to find additional evidence supporting the superflare interpretation of the isotope peak around AD 774/5 by studying historical records. The Carrington event produced auroral displays as far south as Caribbean and Hawaii, corresponding to geomagnetic latitude of about 22°; if the event of 774/5 corresponded to an even more energetic flare there should have been a global auroral event.

Usoskin et al. cited references to aurorae in Chinese chronicles for AD 770 (twice), 773 and 775. They also quote a “red cross” in the sky in AD 773/4 from the Anglo-Saxon Chronicle; “inflamed shields” or “shields burning with a red colour” seen in the sky over Germany in AD 776 recorded in the Royal Frankish Annals; “fire in heaven” seen in Ireland in AD 772; and an apparition in Germany in AD 773 interpreted as riders on white horses. Even if the dates do not precisely conform to the ¹⁴C increase this might suggest a period of high solar activity. Zhou et al.^[24] add further details from the Chinese chronicles. On a date which they give as 17 January AD 775, there were more than ten bands of white lights “like the spread silk” stretching across eight Chinese constellations; the display lasted for several hours. The observations, made during the Tang dynasty, were made from the capital Xian; although geomagnetic latitudes change over time, this would correspond to the lower twenties.

There are a number of difficulties involved when trying to link the ¹⁴C results to historical chronicles. Tree ring dates may be in error because there is no discernible ring for a year (unusually cold weather), or two rings (a second growth during a warm autumn). If the cold weather were global, following a large volcanic eruption, it is conceivable that the effects could also be global: the apparent ¹⁴C date may not always match the chronicles.

For the isotope peak in AD 993/994 studied by Hayakawa et al. surveyed contemporary historical documents show clustering auroral observations in late 992, while their relationship with the isotope peak is still under discussion.

General solar activity in the past

Superflares seem to be associated with a general high level of magnetic activity. As well as looking for individual events, it is possible to examine the isotope records to find the activity level in the past and identify periods when it may have been much higher than now. Lunar rocks provide a record unaffected by geomagnetic shielding and transport processes. Both cosmic rays and solar particle events can create isotopes in rocks, and both are affected by solar activity. The cosmic rays are much more energetic and penetrate more deeply, and can be distinguished from the solar particles which affect the outer layers. Several different radioisotopes can be produced with very different half-lives; the concentration of each may be regarded as representing an average of particle flux over its half-life. Since fluxes must be converted into isotope concentrations by simulations there is a certain model-dependence here. The data are consistent with the view that the flux of energetic solar particles with energies above a few tens of MeV has not changed over periods ranging from five thousand to five million years. Of course, a period of intense activity over a time scale short with respect to the half-life would not be detected.

¹⁴C measurements, even with low time resolution, can indicate the state of solar activity over the last 11,000 years until about 1900. Although radiocarbon dating has been applied as far back as 50,000 years, during the deglaciations at the start of the Holocene the biosphere and its carbon uptake changed dramatically making estimation before this impractical; after about 1900 the Suess effect makes interpretation difficult. ¹⁰Be concentrations in stratified polar ice cores provide an independent measure of activity. Both measures agree reasonably with each other and with the Zurich sunspot number of the last two centuries. As an additional check, it is possible to recover the isotope Titanium-44 (⁴⁴Ti) from meteorites; this provides a measurement of activity that is not affected by changes in transport process or the geomagnetic field. Although it is limited to about the last two centuries, it is consistent with all but one of the ¹⁴C and ¹⁰Be reconstructions and confirms their validity. The energetic flare events discussed above are rare; on long time scales (significantly more than a year), the radiogenic particle flux is dominated by cosmic rays. The inner solar system is shielded by the general magnetic field of the sun, which is strongly dependent on the time within a cycle and the strength of the cycle. The result is that times of powerful activity show up as decreases in the concentrations of all these isotopes. Because cosmic rays are also influenced by the geomagnetic field, difficulties in reconstructing this field set a limit to the accuracy of the reconstructions.

The ¹⁴C reconstruction of activity over the last 11,000 years shows no period significantly higher than the present; in fact, the general level of activity in the second half of the 20th century was the highest since 9000 BC. In particular, the activity in the period around the AD 774 ¹⁴C event (averaged over decades) was somewhat lower than the long-term average, while the AD 993 event coincided with a small minimum. A more detailed scrutiny of the period AD 731 to 825, combining several ¹⁴C datasets of one- and two-year resolution with auroral and sunspot accounts does show a general increase in solar activity (from a low level) after about AD 733, reaching its highest level after 757 and remaining high in the 760s and 770s; there were several aurorae around this time, and even a low-latitude aurora in China.

Effects of a hypothetical solar superflare

The effect of the sort of superflare apparently found on the original nine candidate stars would be catastrophic for the Earth and would leave traces on the Solar System; the event on S Fornacis for example involved an increase in the stars' luminosity by a factor of about twenty. Thomas Gold suggested that the glaze on the top surface of certain lunar rocks might be caused by a solar outburst involving a luminosity increase of over a hundred times for 10 to 100 seconds at some time in the last 30,000 years. Apart from the terrestrial effects, this would cause local ice melting followed by refreezing as far out as the moons of Jupiter. There is no evidence of superflares on this scale having occurred in the Solar System.
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Even for much smaller superflares, at the lower end of the Kepler range, the effects would be serious. In 1859 the Carrington event caused failures in the telegraph system in Europe and North America. Possible consequences today would include:

• Damage to or loss of all artificial satellites
• Airline passengers on trans-polar flights would receive high radiation doses from the energetic particles (as would any astronauts or the crew of the International Space Station)
• Significant depletion of the ozone layer with increased risk of cataracts, sunburn and skin cancer, as well as damage to growing plants. The recovery time would be of the order of months to years. In the strongest cases there would be severe damage to the biosphere, especially to primary photosynthesis in the oceans
• Failure of the electricity distribution system (as in the March 1989 geomagnetic storm), possibly with damage to transformers and switching equipment
• Loss of power to the cooling systems of spent fuel rods stored at nuclear power stations
• Loss of most radio communication because of increased ionisation in the atmosphere

It is evident that superflares often repeat rather than occurring as isolated events. The NO and other odd nitrogens created by flare particles catalyse the destruction of ozone without being consumed themselves, and have a long lifetime in the stratosphere. Flares at a frequency of one a year or even less would have a cumulative effect; the destruction of the ozone layer could be permanent and lead to at least a low-level extinction event.

Superflares have also been suggested as a solution to the Faint young Sun paradox.

Can superflares occur on the Sun?

Since superflares can occur on stars apparently equivalent in every way to the Sun, it is natural to ask if they can occur on the Sun itself. An estimate based on the original Kepler photometric studies suggested a frequency on solar-type stars (early G-type and rotation period more than 10 days) of once every 800 years for an energy of 10³⁴ erg and every 5000 years at 10³⁵ erg. One-minute sampling provided statistics for less energetic flares and gave a frequency of one flare of energy 10³³ erg every 5–600 years for a star rotating as slowly as the Sun; this would be rated as X100 on the solar flare scale. This is based on a straightforward comparison of the number of stars studied with the number of flares observed. An extrapolation of the empirical statistics for solar flares to an energy of 10³⁵ erg suggests a frequency of one in 10,000 years.

However, this does not match the known properties of superflare stars. Such stars are extremely rare in the Kepler data; one study showed only 279 such stars in 31,457 studied, a proportion below 1%; for older stars this fell to 0.25%. Also, about half of the stars which were active showed repeating flares: one had as many as 57 events in 500 days. Concentrating on solar-type stars, the most active averaged one flare every 100 days; the frequency of superflare occurrence in the most active Sun-like stars is 1000 times larger than that of the general average for such stars. This suggests that such behaviour is not present throughout a star's lifetime, but is confined to episodes of extraordinary activity. This is also suggested by the clear relation between the magnetic activity of a star and its superflare activity; in particular, superflare stars are much more active (based on starspot area) than the Sun.

There is no evidence for any flare greater than the Carrington event (about 10³² erg, or 1/10,000 of the largest superflares) in the last 200 years. Although larger events from the ¹⁴C record ca. 775 AD is unambiguously identified as a solar event, its association to the flare energy is unclear, and it is unlikely to exceed 10³² erg.

The more energetic superflares seem to be ruled out by energetic considerations for our sun, which suggest it is not capable of a flare of more than 10³⁴ ergs. A calculation of the free energy in magnetic fields in active regions that could be released as flares gives a lower upper bound of around 3×10³² erg suggesting the most energetic a super flare can be is three times that of the Carrington event.

Some stars have a magnetic field 5 times that of Earth and rotate much faster and these could theoretically have a flare of up to 10³⁴ ergs. This could explain some superflares at the lower end of the range. To go higher than this may require an anti-solar rotation curve - one in which the polar regions rotate faster than the equatorial regions.