Mars habitat

From Wikipedia, the free encyclopedia

Langley's Mars Ice Dome design from 2016 for a Mars base

 

Various components of the Mars Outpost proposal. (M. Dowman, 1989)

 

1990s era NASA design featuring 'spam can' type habitat landers. The downside may be minimal shielding for crew, and two ideas are to use Mars materials, such as ice to increase shielding, and another is move underground, perhaps caves

A Mars habitat is a place that humans can live in on Mars. Mars habitats must contend with surface conditions that include almost no oxygen in the air, extreme cold, low pressure, and high radiation. Alternatively, the habitat may be placed underground which helps solve some problems but creates new difficulties.

One aspect is the extreme cost of building materials for Mars, which by the 2010s was estimated be about 2 million USD per brick to the surface of Mars. While the gravity on Mars is lower compared to earth, there is increased solar radiation, temperature cycles, and for pressurized habitats high internal forces to contain the air.

To contend with these constraints, architects have worked to understand the right balance between in-situ materials and construction, and ex-situ to Mars. For example, one idea is to use the locally available regolith to shield against radiation exposure, and another idea is to use transparent ice to allow non-harmful light to enter the habitat. Mars habitat design can also involve the study of local conditions, including pressures, temperatures, and local materials especially water.

Overview

Significant challenges for Mars habitats are maintaining an artificial environment and shielding from intense solar radiation. Humans require a pressurized environment at all times and protection from the toxic Martian atmosphere. Connecting habitats is useful, as moving between separate structures requires a pressure suit or perhaps a mars rover. One of the largest issues lies in simply getting to Mars, which means escaping Earth's atmosphere, sustaining the journey to Mars, and finally landing on the surface of Mars. One helpful aspect is the Mars atmosphere which allows for aerobraking meaning less need for using propellant to slow a craft for safe landing. However, the amount of energy required to transfer material to the surface of Mars is an additional task beyond simply getting into orbit. During the late 1960s, the United States produced the Saturn V rocket which was capable of launching enough mass into orbit required for a single-launch trip holding a crew of three to the surface of the Moon and back again. This feat required a number of specially designed pieces of hardware and the development of a technique known as the Lunar Orbit Rendezvous. The Lunar Orbit Rendezvous was a plan to coordinate the descent and ascent vehicles for a rendezvous in Lunar orbit. Referring to Mars, a similar technique would require a Mars Excursion Module, which combines a manned Descent-Ascent vehicle and short stay surface habitat. Later plans have separated the descent-ascent vehicle and surface habitat, which further developed into separate descent, surface stay, and ascent vehicles using a new design architecture. In 2010 the Space Launch System, or growth variants therefore, is envisioned as having the payload capacity and qualities needed for human Mars missions, utilizing the Orion (spacecraft) capsule. 

On the surface of Mars some human needs:

• Air
• Food
• Water
• Shelter
• Toilet
• Sleep
• Temperature
• Bathing

One of the challenges for Mars habitats is maintaining the climate, especially the right temperature in the right places. Things like electronics and lights generate heat that rises in the air, even as there are extreme temperature fluctuation outside.

One idea for a Mars habitat, is to use a Martian cave or lava tube, and an inflatable air-lock was proposed by Caves of Mars project for making use of such a structure. The idea of living in lava tubes has been suggest later, noted for its potential to provide increased protection from radiation, temperature fluctuation, Martian sunlight, etc. An advantage of living underground is that it avoids the need to create a radiation shield above ground. Another idea is to use robots to construct the base in advance of human's arrival.

A mobile habitat on the move, such as for a circumnavigation of the planet

The use of living plants or other living biologicals to aid in the air and food supply if desired can have major impact on the design. An example of how engineering demands and operational goals can interact, is a reduced-pressure green house area. This would reduce the structural demands of maintain air pressure, but require the relevant plants to survive at that lower pressure.

Taken to an extreme, the question remains just how a low a pressure could a plant survive in and still be useful.

A Mars habitat may need to focus on keeping a certain type of plant alive for example, as part of supporting its inhabitants.

NASA Caves of Mars study suggested the following aspects to a bio support for inhabitant food in their example:

• Rapid growth
• survive in low light
• wide pH range
• high nutrition
• minimal waste

The study noted two plants, duckweed (Lemna minor) and water fern (Azolla filiculoides), as particularly suitable, and they grow on the surface of water. The Mars habitat would have support the conditions of this food source, possible incorporating elements from greenhouse design or farming.

Historically, space missions tend to have a non-growing food supply eating out of set amount of rations like Skylab, replenished with resupply from Earth. Using plants to effect the atmosphere and even enhance food supply was experimented with the 2010s aboard the International Space Station.

Another issue is waste management. On Skylab all waste was put in a big tank; on Apollo and the Space Shuttle urine could be vented out into space or pushed away in bags to re-enter Earth's atmosphere. 

Considerations for maintain the environment in a closed system included, removal of carbon dioxide, maintaining air pressure, supply of oxygen, temperature and humidity, and stopping fires. Another issue with closed system is keeping it free from contaminations from emissions from different materials, dust, or smoke. One concern on Mars is the effect of the fine Martian dust working its way into the living quarters or devices. The dust is very fine that collects on for example solar panels, as fine as tobacco smoke.

Relevant technologies

Structure of Orion spacecraft under construction

Some possible areas of needed technology or expertise:

• 3D Printing
• Mars atmospheric entry
• Caves of Mars Project
• Mars Excursion Module
• Aerospace engineering
• Space capsule
• Plants in space
• Etc.

Context

A Mars habitat is often conceived as part of an ensemble of Mars base and infrastructure technologies. Some examples include Mars EVA suits, Mars rover, aircraft, landers, storage tanks, communication structures, mining, and Mars-movers (e.g. Earth-moving equipment).

A Mars habitat might exist in the context of a human expedition, outpost, or colony on Mars.

Air

Bubbles of gas in a soft drink (soda pop)

 

People inside a clear diving bell on Earth

In creating a habitat for people, some considerations are maintaining the right air temperature, the right air pressure, and the composition of that atmosphere.

While it is possible for humans to breathe pure oxygen, a pure oxygen atmosphere was implicated in the Apollo 1 fire. As such, Mars habitats may have a need for additional gases. One possibility is to take nitrogen and argon from the atmosphere of Mars; however, they are hard to separate from each other. As a result, a Mars habitat may use 40% argon, 40% nitrogen, and 20% oxygen.

A concept to scrub CO2 from the breathing air is to use re-usable amine bead carbon dioxide scrubbers. While one carbon dioxide scrubber filters the astronaut's air, the other can vent scrubbed CO2 to the Mars atmosphere, once that process is completed another one can be used, and the one that was used can take a break.

Mars habitats with astronauts

One unique structural force that Mars habitats must contend with if pressurized to Earth's atmosphere, is the force of air on the inside walls. This has been estimated at over 2000 pounds per square foot for a pressurized habitat on the surface of Mars, which is radically increased compared to Earth structures. A closer comparison can be made to manned high-altitude aircraft, which must withstand forces of 1100 to 1400 pounds per square foot when at altitude.

At about 150 thousand feet of altitude (28 miles (45 km)) on Earth, the atmospheric pressure starts to be equivalent to the surface of Mars.

Atmospheric pressure comparison

Location	Pressure
Olympus Mons summit	0.03 kPa (0.0044 psi)
Mars average	0.6 kPa (0.087 psi)
Hellas Planitia bottom	1.16 kPa (0.168 psi)
Armstrong limit	6.25 kPa (0.906 psi)
Mount Everest summit	33.7 kPa (4.89 psi)
Earth sea level	101.3 kPa (14.69 psi)
Surface of Venus	9,200 kPa (1,330 psi)

 

 

 

 

 

 

 

 

 

Temperature

One of the challenges for a Mars habitat is for it to maintain suitable temperatures in the right places in an habitat. Things like electronics and lights generate heat that rises in the air, even as there are extreme temperature fluctuation outside. There can be large temperature swings on Mars, for example at the equator it may reach 70 degrees F (20 degrees C) in the daytime but then go down to minus 100 degrees F (-73 C) at night.

Examples of Mars surface temperatures:

• Average -80 degrees Fahrenheit (-60 degrees Celsius).
• Polar locations in winter -195 degrees F (-125 degrees C).
• Equator in summer daytime High 70 degrees F (20 degrees C)

Temporary vs permanent habitation

Here is a vision for habitats published by NASA from CASE FOR MARS from the 1980s, featuring the re-use of landing vehicles, in-situ soil use for enhanced radiation shielding, and green houses. A bay for a mars rover is also visible.

 

A human landing on Mars would necessitate different levels of support for habitation

A short term stay on the surface of Mars does not require a habitat to have a large volume or complete shielding from radiation. The situation would be similar to the International Space Station, where individuals receive an unusually high amount of radiation for a short duration and then leave. A small and light habitat can be transported to Mars and used immediately.

Long term permanent habitats require much more volume (i.e.:greenhouse) and thick shielding to minimize the annual dose of radiation received. This type of habitat is too large and heavy to be sent to Mars, and must be constructed making use of some local resource. Possibilities include covering structures with ice or soil, excavating subterranean spaces or sealing the ends of an existing lava tube.

A larger settlement may be able to have a larger medical staff, increasing the ability to deal with health issues and emergencies. Whereas a small expedition of 4-6 may be able to have 1 medical doctor, an outpost of 20 might be able to have more than one and nurses, in addition to those with emergency or paramedic training. A full settlement may be able to achieve the same level of care as a contemporary Earth hospital.

Medical

One problem for medical care on Mars missions, is the difficulty in returning to Earth for advanced care, and providing adequate emergency care with a small crew size. A crew of six might have only trained at the level of Emergency medical and one physician, but for a mission that would last years. In addition, consultations with Earth would be hampered by a 7 to 40 minute time lag. Medical risks include exposure to radiation and reduced gravity, and one deadly risk is a Solar Particle Event that can generate a lethal dose over the course of several hours or days if the astronauts do not have enough shielding. The effect of radiation on stored pharmaceuticals and medical technology would have to be taken into account also.

One of the medical supplies that may be needed is intravenous fluid, which is mostly water but contains other things so it can be added directly to the human blood stream. If it can be created on the spot from existing water then it could spare the weight of hauling earth-produced units, whose weight is mostly water. A prototype for this capability was tested on the International Space Station in 2010.

On some of the first manned mission three types of pills that were taken into orbit were an anti-nausea medication, a pain-killer, and a stimulant. By the time of ISS, space crew-persons had almost 200 medications available, with separate pill cabinets for Russians and Americans. One of the many concerns for human Mars missions is what pills to bring and how the astronauts would respond to them in different conditions.

In 1999, NASA's Johnson Space Center published Medical Aspects of Exploration Missions as part of the Decadal Survey. On a small mission it might be possible to have one be a medical doctor and another be a paramedic, out of a crew of perhaps 4-6 people, however on a larger mission with 20 people there could also be a nurse and options like minor surgery might be possible. Two major categories for space would be emergency medical care and then more advanced care, dealing with a wide range concerns due to space-travel. For very small crews its difficult to treat a wide range issues with advanced care, whereas with a team with an overall size of 12-20 on Mars there could be multiple doctors and nurses, in addition to EMT-level certifications. While not at the level of a typical Earth hospital this would transition medical are beyond basic options typical of very small crew sizes (2-3) where the accepted risk is higher.

One idea to enhance under-qualified crew who might need to perform an advanced surgery, is to have a robotic surgery machine on Mars which would be operated by a crew-member with help via telecommunications from Earth. Two examples of medical-care situations that been mentioned in regard to people on Mars is how to deal with a broken leg and an appendicitis. One concern is to stop what would otherwise be a minor injury becoming life-threatening due to restrictions on the amount of medical equipment, training, and the time-delay in communication with Earth. The time delay for a one way message ranges from 4 to 24 minutes, depending. A response to a message takes that time, the delay processing the message and creating a reply, plus the time for that message to travel to Mars (another 4 to 24 minutes).

Examples of acute medical emergency possibilies for Mars missions:

• Wounds, lacerations, and burns
• Exposure to a toxin
• Acute allergic reactions (anaphylaxis)
• Acute radiation sickness
• Dental
• Eye (Ophthalmologic)
• Psychiatric

An example of spaceflight related health emergency was the inert gas asphyxiation with nitrogen gas aboard Space Shuttle Colombia in 1981, when it was undergoing preparations for its launch  In that case, a routine purge with nitrogen to decrease risk of fire lead to 5 medical emergencies and 2 deaths. Another infamous space related accident is the Apollo 1 incident, when a pure oxygen atmosphere ignited in the interior of space capsule during tests on the ground, three died. A 1997 study of about 280 space travelers between 1988 and 1995, found that only 3 did not have some sort of medical issue on their spaceflight. A medical risk for a Mars surface mission is how, after several months in zero gravity, they astronauts will handle operations on the surface. On Earth, astronauts must often be carted from the spacecraft and take a long time to recover.

Library

Library Tower of Biodome 2, an Earth analog space habitat tested in the 1990s

One idea for a Mars missions is a library sent to the surface of that planet. The Phoenix lander, which landed on the North polar surface of Mars in 2008, included a DVD library that was heralded as the first library on Mars. The Phoenix library DVD would be taken by future explorers who could access the content on the disk. The disc, both a memorial to the past and a message to the future, took 15 years to produce. The content on the disc includes Visions of Mars. One idea for exploration is knowledge arks for space, a sort of back-up of knowledge in case something happens to Earth.

The Biodome 2 spaceflight and closed-loop biosphere test included a library with the living quarters. The library was positioned at the top of a tower, and known as Library tower.

Meteor impacts

Fresh impact craters detected in the early 2000s by Mars satellites

Another consideration for Mars habitats, especially for long-term stay, is the need to potentially deal with a meteor impact. The atmosphere is thinner so more meteors make it to the surface, so one concern is if a meteor punctured the surface of the habitat causing a loss of pressure or damaging systems.

In the 2010s it was determined that something struck the surface of Mars, creating a spattering pattern of larger and smaller craters between 2008 and 2014. In this case the atmosphere only partially disintegrated the meteor before it struck the surface.

Radiation

Radiation exposure is a concern for astronauts even on the surface, as Mars lacks a strong magnetic field and atmosphere is thin to stop as much radiation as Earth. However, the planet does reduce the radiation significantly especially on the surface, and it is not detected to be radioactive itself.

It has been estimated that sixteen feet (5 meters) of Mars regolith stops the same amount of radiation as Earth's atmosphere.

Power

Space art illustrating a group approaching the Viking 2 lander probe, which were supported by RTG power

For a 500-day manned Mars mission NASA has studied using solar power and nuclear power for its base, as well as power storage systems (e.g. batteries). Some of the challenges for solar power include a reduction in solar intensity because Mars is farther from the sun, dust accumulation, and periodic dust storms, in addition to the usual challenges of solar power such as storing power for the night-time. One of the difficulties is enduring the global Mars dust storms, which cause lower temperatures and reduce sunlight reaching the surface. Two ideas for overcoming this are to use an additional array deployed during a dust storm and to use some nuclear power to provide a base-line power that is not effected by the storms. NASA has studied nuclear-power fission systems in the 2010s for Mars surface missions. One design was planned for an output of 40 kilowatts, and its more independent of the sunlight reaching the surface of Mars which can be effected by dust storms.

Another idea for power is to beam the power to the surface, a solar power satellite would send the power down to the surface to a rectifying antenna (aka rectenna) receiver. 245 GHz, laser, in-situ rectenna construction, and 5.8 GHz designs have been studied. One idea is combine this technology to with Solar Electric Propulsion to achieve a lower mass that the surface solar power. The big advantage is that the rectenna's should be immune to dust and weather changes, and with the right orbit, a solar power Mars satellite could beam power down continuously to the surface.

Technology to clean dust off the solar panels was considered for Mars Exploration Rover's development. In the 21st century there have been proposed ways of cleaning off solar panels on the surface Mars that are accumulating dust. The effects of Martian surface dust on solar cells was studied in the 1990s by the Materials Adherence Experiment on Mars Pathfinder.

Lander power (examples)

Name	Main Power
Viking 1 & 2	Nuclear - RTG
Mars Pathfinder	Solar panels
MER A & B	Solar panels
Phoenix	Solar panels
MSL	Nuclear - RTG

 

 

 

 

History

One early idea for a Mars habitat was to use put short stay accommodation in a Mars ascent-descent vehicle. This combination was called a Mars Excursion Module, and also typically featured other components such as basic rover and science equipment. Later missions tended to shift to a dedicated descent/ascent with a separate habitat.

In 2013 ZA architects proposed having digging robots build a Mars habitat underground. They chose an interior inspired by Fingal's Cave and noted the increased protection from high-energy radiation below ground. On the other hand, the issue of the difficulty of sending digging robots that must construct the habitat versus landing capsules on the surface was also noted. An alternative may be to build above ground, but use thick ice to shield from radiation but with advantage that it lets visible light in.

In 2015 the SHEE project noted the idea of autonomous construction and preparation for Mars habitat versus human construction.

NASA

NASA six-legged mobile habitat module (TRI-ATHLETE)

 

Habitat Demonstration Unit of the Desert Research and Technology Studies

In early 2015 NASA outlined a conceptual plan for three stage Mars habitat design and construction award program. The first stage is a design only, then in the next stage a construction technology based using discarded spacecraft components is conducted, and finally building an actual habitat for Mars using 3D printing technology.

In September 2015, NASA announced the winners of its 3-D Printed Habitat Challenge. The winning submission titled 'Mars Ice House' by Clouds Architecture Office / SEArch proposed a 3D-printed double ice shell surrounding a lander module core. Two European teams were awarded runner up prizes. The contenders explored many possibilities for materials, with one suggesting separately refining iron and silica from the Martian dust and using the iron to make a lattice-work filled in with silca panels. There were 30 finalists selected from an initial pool of 165 entries in the habitat challenge.

The second-place winner proposed the printing robots build a shield out of in-situ materials around inflatable modules. Another NASA project that has developed extraterrestrial surface habitats is the X-Hab challenge and the Habitation Systems Project.

The Sfero House by Fabulous also a contender in the 3D Mars Habitat program, featured levels above and below ground level. The proposed location was Gale crater (of Curiosity rover fame) with a focus on using both in-situ iron and water which would hopefully be available there. It has a double walled spherical design filled with water to both keep the higher-pressure of Mars habitat in, but help protect against radiation.

In 2016, NASA awarded the first prize of its In-Situ Materials Challenge to University of Southern California engineering professor Behrokh Khoshnevis "for Selective Separation Sintering -- a 3D-printing process that makes use of powder-like materials found on Mars." 

In 2016 NASA Langley showed the Mars Ice Dome, which used in-situ water to make an ice structure part of the design a Mars habitat.

In June 2018, NASA selected the top ten finalists of Phase 3: Level 1 in the 3D-Printed Habitat Challenge.

Phase 3: Level 1 Winners:

1. ALPHA Team - Marina Del Rey, California
2. Colorado School of Mines and ICON – Golden, Colorado
3. Hassell & EOC - San Francisco, California
4. Kahn-Yates - Jackson, Mississippi
5. Mars Incubator - New Haven, Connecticut
6. AI. SpaceFactory - New York, New York
7. Northwestern University - Evanston, Illinois
8. SEArch+/Apis Cor - New York, New York
9. Team Zopherus - Rogers, Arkansas
10. X-Arc - San Antonio, Texas

Mars analogs and analog habitat studies

Biosphere 2 tested a closed-loop greenhouse and accommodation in the early 1990s

Mock Mars missions or Mars analog missions typically construct terrestrial habitats on Earth and conduct mock missions, taking steps to solve some of the problems that could be faced for one on Mars. An example of this was the original mission of Biosphere 2, which was meant to test closed ecological systems to support and maintain human life in outer space. Biosphere 2 tested several people living in closed loop biological system, with several biological areas in support including rainforest, savannah, ocean, desert, marsh, agriculture, and in an area in support of a living space.

An example of Mars analog comparison mission is HI-SEAS of the 2010s. Other Mars analog studies include Mars Desert Research Station and Arctic Mars Analog Svalbard Expedition and:

• Australia Mars Analog Research Station
• Flashline Mars Arctic Research Station
• MARS-500
• Concordia Station

The ISS has also been described as a predecessor to Mars expedition, and in relation to a Mars habitat the study importance and nature of operation a closed system was noted.

At about 28 miles (45 km, 150 thousand feet ) Earth altitude the pressure starts to be equivalent to Mars surface pressure.

An example of regolith simulant is Martian regolith simulant.

Biodomes

Interior of the ESO Hotel which has been called a "boarding house on Mars", because the desert surroundings are Mars-like; it houses observatory staff at an observatory in the high Chilean desert.

 

Illustration of plants growing in a Mars base.

One example concept that is or is in support of habitat is a Mars biodome, a structure that could hold life generating needed oxygen and food for humans. An example of activity in support of this goals, was a program to develop bacteria that could convert the Martian regolith or ice into oxygen. Some issues with biodomes are the rate at which gas leaks out and the level of oxygen and other gases inside it.

One question for Biodomes is how low the pressure could be lowered to, and the plants still be useful. In one study where air pressure was lowered to 1/10 of Earth's air pressure at the surface, the plants had a higher rate of evaporation from its leaves. This triggered the plant to think there was drought, despite it having a steady supply of water. An example of a crop NASA tested growing at lower pressure is lettuce, and in another test green beans were grown at a standard air pressure, but in low Earth orbit inside the International Space Station.

The DLR found that some lichen and bacteria could survive in simulated Martian conditions, including air composition, pressure, and solar radiation spectrum. The Earth organisms survived for over 30 days under Mars conditions, and while it was not known if they would survive beyond this, it was noted they seemed to be performing photosynthesis under those conditions. For organisms like this the need would be to stop them spreading all over Mars 

To convert the entirety of Mars into a biodome directly, scientists have suggested the cyanobacteria Chroococcidiopsis. This would help convert the regolith into soil by creating an organic element. That bacteria is known to survive in extremely cold and dry conditions on Earth, so might provide a basis for bioengineering Mars into a more habitable place. As the bacteria reproduces the dead ones would create an organic layer in the regolith potentially paving the way for more advanced life.

A study published in 2016 showed that cryptoendolithic fungi survived for 18 months in simulated Mars conditions.

On Earth, plants that utilize the C4 photosynthesis reaction account for 3% of flowering plant species but 23% of carbon that is fixed, and includes species popular for human consumption including corn (aka maize) and sugar cane; certain types of plants may be more productive at producing food for a given amount of light. Plants noted for colonizing the barren landscape in the aftermath of the Mt Saint Helen's eruption included Asteraceae and Epilobium, and especially Lupinus lepidus for its (symbiotic) ability to fix its own nitrogen.

In-site resources

A concept for a combined surface habitat and ascent vehicle from the 1990s era Design Reference Mission 3.0-based mission, that integrated in-situ resources production in this case for propellant

 

Pine trees have been suggested, in combination with other techniques for creating more hospitable atmosphere on Mars.

 

In situ resource utilization involves using materials encountered on Mars to produce materials needed. One idea for supporting a Mars habitat is to extract subterranean water, which with sufficient power could then be split into hydrogen and oxygen, with the intention of mixing the oxygen with nitrogen and argon for breathable air. The hydrogen can be combined with carbon dioxide to make plastics or methane for rocket fuel. Iron has also been suggested as a building material for 3D printed Mars habitats.

In the 2010s the idea of using in-situ water to build an ice for protection from radiation and temperature, etc. appeared in designs.

A material processing plant would use Mars resources to reduce reliance on Earth provided material.

The planned Mars 2020 mission includes Mars Oxygen ISRU Experiment (MOXIE) which would convert Mars carbon dioxide into oxygen. 

To convert the whole of Mars into a habitat, increased would air could come from vaporizing materials in the planet. In time lichen and moss might be established, and then eventually pine trees.

There is an theory to make rocket fuel on Mars, by the Sabatier process. In this process hydrogen and carbon dioxide are used to make methane and water. In the next step the water is split into hydrogen and oxygen, with the oxygen and methane being used for a Methane-Oxygen rocket engine, and the hydrogen could be re-used. This process requires a large input of energy, so an appropriate power source would be needed in addition to the reactants.

Terraforming of Venus

From Wikipedia, the free encyclopedia

Artist's conception of a terraformed Venus. The cloud formations are depicted assuming the planet's rotation has not been accelerated.

The terraforming of Venus is the hypothetical process of engineering the global environment of the planet Venus in such a way as to make it suitable for human habitation. Terraforming Venus was first scholarly proposed by the astronomer Carl Sagan in 1961, although fictional treatments, such as The Big Rain of The Psychotechnic League by novelist Poul Anderson, preceded it. Adjustments to the existing environment of Venus to support human life would require at least three major changes to the planet's atmosphere: Reducing Venus' surface temperature of 462 °C (735 K; 864 °F), eliminating most of the planet's dense 9.2 MPa (91 atm) carbon dioxide and sulfur dioxide atmosphere via removal or conversion to some other form, and the addition of breathable oxygen to the atmosphere. These three changes are closely interrelated, because Venus' extreme temperature is due to the high pressure of its dense atmosphere, and the greenhouse effect.

History

Prior to the early 1960s, the atmosphere of Venus was believed by astronomers to have an Earth-like temperature. When Venus was understood to have a thick carbon dioxide atmosphere with a consequence of a very large greenhouse effect, some scientists began to contemplate the idea of altering the atmosphere to make the surface more Earth-like. This hypothetical prospect, known as terraforming, was first proposed by Carl Sagan in 1961, as a final section of his classic article in the journal Science discussing the atmosphere and greenhouse effect of Venus. Sagan proposed injecting photosynthetic bacteria into the Venus atmosphere, which would convert the carbon dioxide into reduced carbon in organic form, thus reducing the carbon dioxide from the atmosphere.

Unfortunately, the knowledge of Venus' atmosphere was still inexact in 1961, when Sagan made his original proposal for terraforming. Thirty-three years after his original proposal, in his 1994 book Pale Blue Dot, Sagan conceded his original proposal for terraforming would not work because the atmosphere of Venus is far denser than was known in 1961:

Here's the fatal flaw: In 1961, I thought the atmospheric pressure at the surface of Venus was a few bars ... We now know it to be 90 bars, so if the scheme worked, the result would be a surface buried in hundreds of meters of fine graphite, and an atmosphere made of 65 bars of almost pure molecular oxygen. Whether we would first implode under the atmospheric pressure or spontaneously burst into flames in all that oxygen is open to question. However, long before so much oxygen could build up, the graphite would spontaneously burn back into CO₂, short-circuiting the process.

Following Sagan's paper, there was little scientific discussion of the concept until a resurgence of interest in the 1980s.

Proposed approaches to terraforming

A number of approaches to terraforming are reviewed by Martyn J. Fogg (1995) and by Geoffrey A. Landis (2011).

Eliminating the dense carbon dioxide atmosphere

The main problem with Venus today, from a terraformation standpoint, is the very thick carbon dioxide atmosphere. The ground level pressure of Venus is 9.2 MPa (1,330 psi). This also, through the greenhouse effect, causes the temperature on the surface to be several hundred degrees too warm for any known lifeforms. Basically, all approaches to the terraforming of Venus include somehow getting rid of practically all the carbon dioxide in the atmosphere.

Biological approaches

The method proposed in 1961 by Carl Sagan involves the use of genetically engineered bacteria to fix carbon into organic compounds. Although this method is still proposed in discussions of Venus terraforming, later discoveries showed that biological means alone would not be successful.

Difficulties include the fact that the production of organic molecules from carbon dioxide requires hydrogen, which is very rare on Venus. Because Venus lacks a protective magnetosphere, the upper atmosphere is exposed to direct erosion by the solar wind and has lost most of its original hydrogen to space. And, as Sagan noted, any carbon that was bound up in organic molecules would quickly be converted to carbon dioxide again by the hot surface environment. Venus would not begin to cool down until after most of the carbon dioxide has already been removed.

Although it is generally conceded that Venus could not be terraformed by introduction of photosynthetic biota alone, use of photosynthetic organisms to produce oxygen in the atmosphere continues to be a component of other proposed methods of terraforming.

Capture in carbonates

On Earth nearly all carbon is sequestered in the form of carbonate minerals or in different stages of the carbon cycle, while very little is present in the atmosphere in the form of carbon dioxide. On Venus, the situation is the opposite. Practically all of the carbon is present in the atmosphere, while very little is sequestered in the lithosphere. Many approaches to terraforming therefore focus on getting rid of carbon dioxide by chemical reactions trapping and stabilising it in the form of carbonate minerals. 

Modelling by astrobiologists Mark Bullock and David Grinspoon of Venus' atmospheric evolution suggests that the equilibrium between the current 92 bar atmosphere and existing surface minerals, particularly calcium and magnesium oxides is quite unstable, and that the latter could serve as a sink of carbon dioxide and sulfur dioxide through conversion to carbonates. If these surface minerals were fully converted and saturated, then the atmospheric pressure would decline and the planet would cool somewhat. One of the possible end states modelled by Bullock and Grinspoon was a 43 bars (620 psi) atmosphere and 400 K (127 °C) surface temperature. To convert the rest of the carbon dioxide in the atmosphere, a larger portion of the crust would have to be artificially exposed to the atmosphere to allow more extensive carbonate conversion. In 1989, Alexander G. Smith proposed that Venus could be terraformed by lithosphere overturn, allowing crust to be converted into carbonates. Landis 2011 calculated that it would require the involvement of the entire surface crust down to a depth of over 1 km to produce enough rock surface area to convert enough of the atmosphere.

Natural formation of carbonate rock from minerals and carbon dioxide is a very slow process. Recent research into sequestering carbon dioxide into carbonate minerals in the context of mitigating global warming on Earth however points out that this process can be considerably accelerated (from hundreds or thousands of years to just 75 days) through the use of catalysts such as polystyrene microspheres. It could therefore be theorised that similar technologies might also be used in the context of terraformation on Venus. It can also be noted that the chemical reaction that converts minerals and carbon dioxide into carbonates is exothermic, in essence producing more energy than is consumed by the reaction. This opens up the possibility of creating self-reinforcing conversion processes with potential for exponential growth of the conversion rate until most of the atmospheric carbon dioxide can be converted. 

Bombardment of Venus with refined magnesium and calcium from off-world could also sequester carbon dioxide in the form of calcium and magnesium carbonates. About 8×10²⁰ kg of calcium or 5×10²⁰ kg of magnesium would be required to convert all the carbon dioxide in the atmosphere, which would entail a great deal of mining and mineral refining (perhaps on Mercury which is notably mineral rich). 8×10²⁰ kg is a few times the mass of the asteroid 4 Vesta (more than 500 kilometres (310 mi) in diameter).

Injection into volcanic basalt rock

Research projects in Iceland and Washington (state) have recently shown that potentially large amounts of carbon dioxide could be removed from the atmosphere by high-pressure injection into subsurface porous basalt formations, where carbon dioxide is rapidly transformed into solid inert minerals. Other recent studies predict that one cubic meter of porous basalt has the potential to sequester 47 kilograms of injected carbon dioxide. According to these estimates a volume of about 9.86 × 10⁹ km³ of basalt rock would be needed to sequester all the carbon dioxide in the Venusian atmosphere. This is equal to the entire crust of Venus down to a depth of about 21.4 kilometers. Another study concluded that under optimal conditions, on average, 1 cubic meter of basalt rock can sequester 260 kg of carbon dioxide. Venus's crust appears to be 70 kilometres (43 mi) thick and the planet is dominated by volcanic features. The surface is about 90% basalt, and about 65% consists of a mosaic of volcanic lava plains. There should therefore be ample volumes of basalt rock strata on the planet with very promising potential for carbon dioxide sequestration.

Recent research has also demonstrated that under the high temperature and high pressure conditions in the mantle, silicon dioxide, the most abundant mineral in the mantle (on Earth and probably also on Venus) can form carbonates that are stable under these conditions. This opens up the possibility of carbon dioxide sequestration in the mantle.

Introduction of hydrogen

According to Birch, bombarding Venus with hydrogen and reacting it with carbon dioxide could produce elemental carbon (graphite) and water by the Bosch reaction. It would take about 4 × 10¹⁹ kg of hydrogen to convert the whole Venusian atmosphere, and such a large amount of hydrogen could be obtained from the gas giants or their moons' ice. Another possible source of hydrogen could be somehow extracting it from possible reservoirs in the interior of the planet itself. According to some researchers, the Earth's mantle and/or core might hold large quantities of hydrogen left there since the original formation of Earth from the nebular cloud. Since the original formation and inner structure of Earth and Venus are generally believed to be somewhat similar, the same might be true for Venus. 

Iron aerosol in the atmosphere will also be required for the reaction to work, and iron can come from Mercury, asteroids, or the Moon. (Loss of hydrogen due to the solar wind is unlikely to be significant on the timescale of terraforming.) Due to the planet's relatively flat surface, this water would cover about 80% of the surface, compared to 70% for Earth, even though it would amount to only roughly 10% of the water found on Earth.

The remaining atmosphere, at around 3 bars (about three times that of Earth), would mainly be composed of nitrogen, some of which will dissolve into the new oceans of water, reducing atmospheric pressure further, in accordance with Henry's law. To bring down the pressure even more, hydrogen could also be fixated into nitrates.

Direct removal of atmosphere

The thinning of the Venerian atmosphere could be attempted by a variety of methods, possibly in combination. Directly lifting atmospheric gas from Venus into space would probably prove difficult. Venus has sufficiently high escape velocity to make blasting it away with asteroid impacts impractical. Pollack and Sagan calculated in 1994 that an impactor of 700 km diameter striking Venus at greater than 20 km/s, would eject all the atmosphere above the horizon as seen from the point of impact, but because this is less than a thousandth of the total atmosphere and there would be diminishing returns as the atmosphere's density decreases, a very great number of such giant impactors would be required. Landis calculated that to lower the pressure from 92 bar to 1 bar would require a minimum of 2,000 impacts, even if the efficiency of atmosphere removal was perfect. Smaller objects would not work, either, because more would be required. The violence of the bombardment could well result in significant outgassing that would replace removed atmosphere. Most of the ejected atmosphere would go into solar orbit near Venus, and, without further intervention, could be captured by the Venerian gravitational field and become part of the atmosphere once again. 

Removal of atmospheric gas in a more controlled manner could also prove difficult. Venus' extremely slow rotation means that space elevators would be very difficult to construct because the planet's geostationary orbit lies an impractical distance above the surface, and the very thick atmosphere to be removed makes mass drivers useless for removing payloads from the planet's surface. Possible workarounds include placing mass drivers on high-altitude balloons or balloon-supported towers extending above the bulk of the atmosphere, using space fountains, or rotovators. 

In addition, if the density of the atmosphere (and corresponding greenhouse effect) were dramatically reduced, the surface temperature (now effectively constant) would probably vary widely between day side and night side. Another side effect to atmospheric-density reduction could be the creation of zones of dramatic weather activity or storms at the terminator because large volumes of atmosphere would undergo rapid heating or cooling.

Cooling planet by solar shades

Venus receives about twice the sunlight that Earth does, which is thought to have contributed to its runaway greenhouse effect. One means of terraforming Venus could involve reducing the insolation at Venus' surface to prevent the planet from heating up again.

Space-based

Solar shades could be used to reduce the total insolation received by Venus, cooling the planet somewhat. A shade placed in the Sun–Venus L₁ Lagrangian point also would serve to block the solar wind, removing the radiation exposure problem on Venus. 

A suitably large solar shade would be four times the diameter of Venus itself if at the L₁ point. This would necessitate construction in space. There would also be the difficulty of balancing a thin-film shade perpendicular to the Sun's rays at the Sun–Venus Lagrangian point with the incoming radiation pressure, which would tend to turn the shade into a huge solar sail. If the shade were simply left at the L₁ point, the pressure would add force to the sunward side and the shade would accelerate and drift out of orbit. The shade could instead be positioned nearer to the sun, using the solar pressure to balance the gravitational forces, in practice becoming a statite. 

Other modifications to the L₁ solar shade design have also been suggested to solve the solar-sail problem. One suggested method is to use polar-orbiting, solar-synchronous mirrors that reflect light toward the back of the sunshade, from the non-sunward side of Venus. Photon pressure would push the support mirrors to an angle of 30 degrees away from the sunward side.

Paul Birch proposed a slatted system of mirrors near the L₁ point between Venus and the Sun. The shade's panels would not be perpendicular to the Sun's rays, but instead at an angle of 30 degrees, such that the reflected light would strike the next panel, negating the photon pressure. Each successive row of panels would be +/- 1 degree off the 30-degree deflection angle, causing the reflected light to be skewed 4 degrees from striking Venus.

Solar shades could also serve as solar power generators. Space-based solar shade techniques, and thin-film solar sails in general, are only in an early stage of development. The vast sizes require a quantity of material that is many orders of magnitude greater than any human-made object that has ever been brought into space or constructed in space.

Atmospheric or surface-based

Venus could also be cooled by placing reflectors in the atmosphere. Reflective balloons floating in the upper atmosphere could create shade. The number and/or size of the balloons would necessarily be great. Geoffrey A. Landis has suggested that if enough floating cities were built, they could form a solar shield around the planet, and could simultaneously be used to process the atmosphere into a more desirable form, thus combining the solar shield theory and the atmospheric processing theory with a scalable technology that would immediately provide living space in the Venusian atmosphere. If made from carbon nanotubes or graphene (a sheet-like carbon allotrope), then the major structural materials can be produced using carbon dioxide gathered in situ from the atmosphere. The recently synthesised amorphous carbonia might prove a useful structural material if it can be quenched to Standard Temperature and Pressure (STP) conditions, perhaps in a mixture with regular silica glass. According to Birch's analysis, such colonies and materials would provide an immediate economic return from colonizing Venus, funding further terraforming efforts.

 

Increasing the planet's albedo by deploying light-colored or reflective material on the surface (or at any level below the cloud tops) would not be useful, because the Venerian surface is already completely enshrouded by clouds, and almost no sunlight reaches the surface. Thus, it would be unlikely to be able to reflect more light than Venus' already-reflective clouds, with Bond albedo of 0.77.

Combination of solar shades and atmospheric condensation

Birch proposed that solar shades could be used to not merely cool the planet but that this could be used to reduce atmospheric pressure as well, by the process of freezing of the carbon dioxide. This requires Venus's temperature to be reduced, first to the liquefaction point, requiring a temperature less than 304 K (31 °C; 88 °F) and partial pressures of CO₂ to bring the atmospheric pressure down to 73.8 bar (carbon dioxide's critical point); and from there reducing the temperature below 217 K (−56 °C; −69 °F) (carbon dioxide's triple point). Below that temperature, freezing of atmospheric carbon dioxide into dry ice will cause it to deposit onto the surface. He then proposed that the frozen CO₂ could be buried and maintained in that condition by pressure, or even shipped off-world (perhaps to provide greenhouse gas needed for terraforming of Mars or the moons of Jupiter). After this process was complete, the shades could be removed or solettas added, allowing the planet to partially warm again to temperatures comfortable for Earth life. A source of hydrogen or water would still be needed, and some of the remaining 3.5 bar of atmospheric nitrogen would need to be fixed into the soil. Birch suggests disrupting an icy moon of Saturn, for example Hyperion (moon) and bombarding Venus with its fragments.

Cooling planet by heat pipes, atmospheric vortex engines or radiative cooling

Paul Birch suggests that, in addition to cooling the planet with a sunshade in L1, "heat pipes" could be built on the planet to accelerate the cooling. The proposed mechanism would transport heat from the surface to colder regions higher up in the atmosphere, similar to a solar updraft tower, thereby facilitating radiation of excess heat out into space. A newly proposed variation of this technology is the atmospheric vortex engine, where in stead of physical chimney pipes, the atmospheric updraft is achieved through the creation of a vortex, similar to a stationary tornado. In addition to this method being less material intensive and potentially more cost effective, this process also produces a net surplus of energy, which could be utilised to power venusian colonies or other aspects of the terraforming effort, while simultaneously contributing to speeding up the cooling of the planet. Another method to cool down the planet could be with the use of radiative cooling. This technology could utilise the fact that in certain wavelengths, thermal radiation from the lower atmosphere of Venus can "escape" to space through partially transparent atmospheric “windows” – spectral gaps between strong CO2 and H2O absorption bands in the near infrared range (0.8–2.4 μm). The outgoing thermal radiation is wavelength dependent and varies from the very surface at 1 μm to ~35 km at 2.3 μm. Nanophotonics and construction of metamaterials opens up new possibilities to tailor the emittance spectrum of a surface via properly designing periodic nano/micro-structures. Recently there has been proposals of a device named a "emissive energy harvester" that can transfer heat to space through radiative cooling and convert part of the heat flow into surplus energy, opening up possibilities of a self replicating system that could exponentially cool the planet.

Artificial mountains

As an alternative to changing the atmosphere of Venus, it has been proposed that a large artificial mountain, dubbed the "Venusian Tower of Babel", could be built on the surface of Venus that would reach up to 50 kilometres (31 mi) into the atmosphere where the temperature and pressure conditions are similar to Earth and where a colony could be built on the peak of this artificial mountain. Such a structure could be built using autonomous robotic bulldozers and excavators that have been hardened against the extreme temperature and pressure of the Venus atmosphere. Such robotic machines would be covered in a layer of heat and pressure shielding ceramics, with internal helium-based heat pumps inside of the machines to cool both an internal nuclear power plant and to keep the internal electronics and motor actuators of the machine cooled to with in operating temperature. Such a machine could be designed to operate for years without external intervention for the purpose of building colossal mountains on Venus to serve as islands of colonization in the skies of Venus.

Introduction of water

Since Venus only has a fraction of the water on earth (less than half the earth's water content in the atmosphere, and none on the surface), water would have to be introduced either by the aforementioned method of introduction of hydrogen, or from some other extraplanetary source.

Capture of ice moon

Paul Birch suggests the possibility of colliding Venus with one of the ice moons from the outer solar systems, thereby bringing in all the water needed for terraformation in one go. This could be achieved through gravity assisted capture of for example Saturn's moons Enceladus (moon) and Hyperion (moon) or Uranus' moon Miranda (moon). Simply changing the velocity enough of these moons to move them from their current orbit and enable gravity assisted transport to Venus would require large amounts of energy. However, through complex gravity assisted chain reactions the propulsion requirements could be reduced by several orders of magnitude. As Birch puts it "Theoretically one could flick a pebble in to the asteroid belt and end up dumping Mars into the Sun".

Altering day–night cycle

Venus rotates once every 243 Earth days—by far the slowest rotation period of any known object in the Solar System. A Venusian sidereal day thus lasts more than a Venusian year (243 versus 224.7 Earth days). However, the length of a solar day on Venus is significantly shorter than the sidereal day; to an observer on the surface of Venus, the time from one sunrise to the next would be 116.75 days. Therefore, the slow Venerian rotation rate would result in extremely long days and nights, similar to the day-night cycles in the polar regions of earth, only shorter. The slow rotation might also account for the lack of a significant magnetic field.

Arguments for keeping the current day-night cycle unchanged

It has until recently been assumed that the rotation rate or day-night cycle of Venus would have to be increased for successful terraformation to be achieved. More recent research has, however, shown that the current slow rotation rate of Venus is not at all detrimental to the planet's capability to support an Earth-like climate. Rather, the slow rotation rate would, given an Earth-like atmosphere, enable the formation of thick cloud layers on the side of the planet facing the sun. This in turn would raise planetary albedo and act to cool the global temperature to Earth-like levels, despite the greater proximity to the Sun. According to calculations, maximum temperatures would be just around 35 °C, given an Earth-like atmosphere. Speeding up the rotation rate would therefore be both impractical and detrimental to the terraforming effort. A terraformed Venus with the current slow rotation would result in a global climate with "day" and "night" periods each about 58 days long, resembling the seasons at higher latitudes on Earth. The "day" would resemble a short summer with a warm, humid climate, a heavy overcast sky and ample rainfall. The "night" would resemble a short, very dark winter with quite cold temperature and snowfall. There would be periods with more temperate climate and clear weather at sunrise and sunset resembling a "spring" and "autumn".

Space mirrors

The problem of very dark conditions during the 58 earth days long "night" period could be solved through the use of a space mirror in a 24-hour orbit (the same distance as a geostationary orbit on earth) similar to the Znamya (satellite) project experiments. Extrapolating the numbers from those experiments and applying them to Venerian conditions would mean that a space mirror just under 1700 meters in diameter could illuminate the entire nightside of the planet with the luminosity of 10-20 full moons and create a artficial 24-hour light cycle. An even bigger mirror could potentially create even stronger illumination conditions. Further extrapolation suggests that to achieve illumination levels of about 400 lux (similar to normal office lighting or a sunrise on a clear day on earth) a circular mirror about 55 kilometers across would be needed. Paul Birchs suggested keeping the entire planet protected from sunlight by a permanent system of slated shades in L1, and the surface illuminated by a rotating soletta mirror in a polar orbit, which would produce a 24-hour light cycle.

Changing rotation speed

If increasing the rotation speed of the planet would be desired (despite the above-mentioned potentially positive climatic effects of the current rotational speed), it would require energy of a magnitude many orders greater than the construction of orbiting solar mirrors, or even than the removal of the Venerian atmosphere. Birch calculates that increasing the rotation of Venus to an Earth-like solar cycle would require about 1.6 × 10²⁹ Joules (50 billion petawatt-hours). 

Scientific research suggests that close flybys of asteroids or cometary bodies larger than 100 kilometres (60 mi) across could be used to move a planet in its orbit, or increase the speed of rotation. The energy required to do this is large. In his book on terraforming, one of the concepts Fogg discusses is to increase the spin of Venus using three quadrillion objects circulating between Venus and the Sun every 2 hours, each traveling at 10% of the speed of light.

G. David Nordley has suggested, in fiction, that Venus might be spun up to a day length of 30 Earth days by exporting the atmosphere of Venus into space via mass drivers. A proposal by Birch involves the use of dynamic compression members to transfer energy and momentum via high-velocity mass streams to a band around the equator of Venus. He calculated that a sufficiently high-velocity mass stream, at about 10% of the speed of light, could give Venus a day of 24 hours in 30 years.

Creating an artificial magnetosphere

One key aspect of terraforming Venus is to protect the new atmosphere from the solar wind, so as to avoid the loss of hydrogen which likely created the current runaway greenhouse effect. Since Venus (like Mars, but unlike Earth) lacks a magnetic field some scientists hypothesize (in the context of terraforming on Mars) that creating a planet-wide artificial magnetosphere would be helpful in resolving this issue. According to two NIFS Japanese scientists, it is feasible to do that with current technology by building a system of refrigerated latitudinal superconducting rings, each carrying a sufficient amount of direct current. In the same report, it is claimed that the economic impact of the system can be minimized by using it also as a planetary energy transfer and storage system (SMES). Another study proposes the possibility of deployment of a magnetic dipole shield at the L1 Lagrange point, thereby creating an artificial magnetosphere that would protect the whole planet from solar wind and radiation.