Lunar Habitats

Initial lunar base (inflatable spherical habitat) for up to twelve people, NASA

The initial lunar outpost would need to be self-sufficient. It would need to be designed for protection against extreme temperatures and high radiation levels on the Moon. The base life support system would be entirely closed. All waste gases, liquids, and solids are recycled, and there is a food production facility including a greenhouse.

NASA developed an updated concept of an initial lunar outpost in early 2009. The first structures are currently expected to be inflatable.

With certain sites near polar heights receiving sunlight more than 80% of the time, general power requirements could be satisfied by solar energy. At a more equatorial location, for the night period of 14.75 days, when the solar power would not function and for lunar manufacturing, fuel cells or the installation of a nuclear power generating station would probably be required.

First Moon Habitat

 

Conception of a lunar colony, NASA 1986

In recent years, the idea of lunar colonization gained some momentum in the form of national and international interest and actual moon missions. 

The initial moonbase habitation could be constructed by linking together metal habitat canisters and inflatable structures. For radiation protection, these units would need to be buried under a layer of lunar regolith (soil) using excavation equipment.  The basic infrastructure will include life-support structures and climate control system, a solar power plant, an oxygen generator plant, a lunar ice processing plant, a greenhouse to produce food, plus the excavation of a Lunar spaceport, and the preparation of a supply processing and storage area.

ESA Moon Village with regolith covered habitat as viewed from
the moon surface and structure cut-away below, Courtesy ESA

A major problem with inflatable structures is that in the moon's vacuum, there is little protection from micrometeorites. Catastrophic depressurization could occur if a high velocity projectile penetrates the membrane. A solution, as with radiation, would require covering the inflatable habitats with a layer of protective regolith dug out of the moon's surface.

The European Space Agency (ESA) has proposed a Moon Village approach along these lines. A cylinder with an inflatible structure would be brought to the moon and inflated. The inflated structure and cylinder would then be covered with lunar regolith. The resulting habitat interior would be protected from radiation and micrometeorites. The regolith would insulate the habitat interior from the surface environment, with its extreme day/night range in surface temperature.

A key habitation requirement will be water. The most likely source of water will be ice on the dark bottoms of craters near the poles. Shipping water to the Moon for use by humans would be extremely expensive ($2,000 to $20,000 per kg).

In addition, extensive exploration of the lunar environment will be made for nitrogen and carbon sources necessary for food production. Although present in lunar soil, the amounts are too small to be easily extracted. Carbon could be found in meteorites and nitrogen may be found locked in gas pockets under the surface. These elements must be found if the Lunar Station is to become truly self-sufficient.

Underground Approach

Illustration of a lunar robot surveying the entrance to underground lava tubes. 
(Image credit: William Whittaker, Carnegie Mellon University)

Building the lunar colony underground would give protection from radiation, extreme surface temperature changes and micrometeoroids. The area around prospective sites would first be explored for the presence of lunar lava tubes (drained conduits of underground lava rivers) for future underground habitation. Lava tubes could provide an environment naturally sheltered from radiation, thermal extremes, and micrometeoroid impact. Even more important at the early stage of human lunar settlement, there use would be less expensive than excavation of manmade tunnels.

Lava tube entrance on earth

In support of this approach, scientists at the Indian Space Research Organization in 2011 discovered a giant underground chamber on the moon, which they feel could be used as a base by astronauts on future manned missions to moon. An analysis by an instrument on Chandrayaan-1 revealed a 1.7-km long and 120-metre wide cave near the moon's equator. It is located in the Oceanus Procellarum area of the moon and has been proposed as a suitable 'base station' for future human missions. Scientists of the Space Applications Centre in Ahmedabad said that the cave provides "a safe environment from hazardous radiations, micro-meteoritic impacts, extreme temperatures and dust storms." Another possible skylight entrance to an underground cave has been discovered in the Marius Hills.

Lava tube skylight candidates in Philolaus Crater.
(NASA/Lunar Reconnaissance Orbiter/SETI Institute/
Mars Institute/Pascal Lee)

Using an orbiting radar sounder system that can examine underground structures, scientists at the Japan Aerospace Exploration Agency (Jaxa) confirmed the presence of a cave under an opening 50 meters wide and 50 meters deep. Gravity data showed that the cavern is part of a larger chain that extends for 60 kilometers (37 miles). It is at least 1 kilometer (0.6 miles) high and wide, beginning a few tens to hundreds of meters below the surface. It appears to be structurally sound and its rocks may contain ice or water deposits that could be turned into fuel, according to data sent back by the orbiter, nicknamed Kaguya.

The problem with lunar caves far from the poles is that it is not where we want it.  Human presence on the Moon requires material and energy resources sufficient to support human life and operations around the Moon.  After years of study and exploration, we now know that these locations are near the poles of the Moon.  Both poles are in the highlands and finding a lava tube in such non-volcanic terrain has been thought highly unlikely. For this reason, a discovery announced in 2018 was like music to the ears of lunar explorers.

In 2018, the SETI Institute and the Mars Institute announced the discovery of small pits on the northeastern floor of Philolaus Crater, a large, 43 mile (70 km)-diameter impact crater located about 340 miles (550 km) from the North Pole of the Moon, on the lunar near side. These pits may be entrances to an underground network of lava tubes. The pits were identified through analysis of imaging data from NASA’s Lunar Reconnaissance Orbiter (LRO).

Potential lunar tubes and rills in the Oceanus Procellarum
that could contain future habitats

If water ice is present, these potential lava tube entrances or skylights might allow future explorers easier access to subsurface ice, and therefore water, than if they had to excavate the gritty ice-rich “regolith” (surface rubble) at the actual lunar poles. The candidate pits inside Philolaus are located at such a high latitude that sunlight would never enter the underlying caves. These would be so cold that ice could be cold-trapped in them, much like it is in the permanently shadowed regions at the actual poles of the moon.

The pits appear as small rimless depressions, typically 50 to 100 feet across (15 to 30 meters), with completely shadowed interiors. The pits are located along sections of winding channels, known on the Moon as “sinuous rilles,” that crisscross the floor of Philolaus Crater. Lunar sinuous rilles are generally thought to be collapsed, or partially collapsed, lava tubes, underground tunnels that were once streams of flowing lava.

Cities

Once lunar settlement has passed beyond the initial colony stage to permanent cities, construction of labyrinthine underground corridors and greater voids in the rocky crust becomes possible. Advances in technology would need to produce fast tunnel cutters that melt through the lunar rock and form a structurally sound finished surface.

The rock overhead shields the inhabitants from radiation, changes in temperature during the day/night cycle and virtually all meteors. As an additional safety measure, airlocks could separate neighborhoods. Parks and gardens could be created in excavated voids using artificial light or beneath transparent domes roofing craters. Agriculture produce would be grown hydroponically during the lunar day in excavated channels covered with a transparent skylight that could be covered with an insulated moveable roof during the lunar night.

Von Braun Moon City

The crater walls would be excellent places to locate windows, allowing the inhabitants to look out on the actual lunar surface. The actual windows would be set back within the wall to eliminate penetration by micrometeors. The rock overhang could provide radiation and meteor protection.

Adjacent domed craters could be parks containing trees growing to great size and height in the weak gravity. With the moon's light gravity, it would be feasible for the domes to be constructed of material able to shield against small meteors and radiation.

Within most of the city, outside views would be video perspectives of lunar or earthly scenes displayed on walls. Power would most likely be generated by fission or fusion energy or the sun. The latter source would come from solar cells located on heights at the lunar poles able to catch the maximum period of daylight. Power lines laid in trenches cut in the lunar surface would connect the solar power sources to the lunar cities in non-polar regions. Two hundred years in the future, the lunar cities could be connected by underground tubes through which trains would travel.

Given the relatively light gravity compared to earth, exercise for the able bodied would be mandatory for physical health. Should long-term presence in such a light gravity be ill advised (persons would weigh less than one fifth of their earth weight), periodic stays on earth may be required.