Difference between revisions of "Space habitats"

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Stanford torus

space habitat

Inside a large landscaped cylindrical habitat (credit: Eric Bruneton)

Space is a very harsh environment and it may be hard to imagine wanting to live in such a seemingly bleak place, but taming it is only a matter of some (admittedly fairly serious) engineering and is entirely within our current technical capability.

There are many challenges to enable building these towns and cities off-Earth. Two prominent ones are raw materials and gravity. A colony of any size will have to be self-sufficient in materials as it will not be practical to ship them up from the surface of the Earth due to the enormous energy required to climb against its pull. Escaping this pull long-term also causes major problems for the human body. Muscles get very weak, including the heart, and bones de-mineralise. The only real solution is to generate artificial gravity by rotation – even for Moon-bases if people are going to stay for any length of time.

To balance these difficulties, living in space brings many advantages. One is reliable, abundant solar energy, not blocked by the atmosphere or subject to weather or time of day. Another is control of gravity; a rotating space colony would have normal Earthlike gravity at the periphery and zero gravity at the center. Large-scale construction and other industrial activities could be done at the center, reducing the energy they need to almost zero. When we build space habitats, we have the opportunity to tailor-make ecosystems that are optimized for human habitation. A space colony could be kept at any desired temperature, have any desired weather conditions and any desired seasonal and diurnal cycles. Careful controls could keep infectious diseases and agricultural pests from being introduced to the ecosystem.

A passion for space colonisation and a faith in its promise was evoked in thousands of people by an essay called Space Colonies: The High Frontier, written by Gerard O'Neill in 1976. The essay outlines a fairly modest proposal, using only technology then existing, to build a series of space stations orbiting the Sun and Earth. Materials would be mined from the moon and the asteroid belt, launched into space and assembled into space colonies. Launching material from the moon is an easy matter, as there is far less gravity and no air resistance. The first colony would require about as much building material as the world's biggest skyscrapers, and would become self-replicating, as each colony could be used as a factory to assemble the next. The network of space habitats could then expand to form a Dyson swarm (see below) which could house tens of trillions of people to an extremely high standard of living.

O'Neill sets out to challenge a fundamental assumption underlying all our thinking about mankind's resources: the assumption that "The material and energy resources of the human race are just those of our planet." His calculations show that by building such colonies with materials from the asteroid belt and the moon, we could create "a land area many thousands of times that of the entire earth", with ideal climate, temperature and conditions. This would certainly impact just about every pressing social problem we now face on Earth.

Places

'Islands' in space

A pair of O'Neill cylinder habitats

Some of the most feasible strategies for building space colonies involve building giant self-contained structures in the vacuum of space itself, rather than on some pre-existing heavenly body. Bernal spheres , Stanford torus and O'Neill cylinders described below fall into this category.

The first step to building such habitats would be to set up automated facilities on the moon to mine titanium and iron and launch it into space. Water could be mined from icy asteroids and perhaps from the moon also; as well as providing the water needs for colonists, it could be split to make breathable oxygen for the atmosphere of the colony. The habitat could then be assembled in space, and used to manufacture another habitat, making them self-replicating.

Using materials from the moon and asteroid belt alone, we could create artificial habitats with thousands of times the land area of Earth, able to comfortably accommodate tens of trillions of people.

Lagrangian points and Solar orbit

A Dyson swarm refers to a series of habitats build in space around the Sun. Lagrangian points are points in the Sun's orbit where a colony could orbit stably, keeping a constant comfortable distance from the Sun and the Earth.

Low Earth orbit

The easiest place to get to from Earth is low earth orbit, although due to the Earth's strong gravitational pull it requires enormous amounts of energy to bring material up from the surface and one could not be classed as self-sufficient here, however the views are pretty good. If we develop more efficient means of access to space than we currently have, low Earth orbit habitats become more feasible.

Geosynchronous orbit

In a geosynchronous orbit the habitat would hover the same spot on Earth which would mean that one would have the same day / night cycle as on the surface which is an important consideration with human physiology.

Lunar orbit

In lunar orbit the moon is within easy reach but the habitat will be subject to the Moon's two-week long day / night cycles which might not be to everyone's taste, although of course the normal light rhythms can be replicated internally with lighting.

In other solar systems

Habitable planets may turn out to be rare in other solar systems. If this proves to be the case, it need not stop us from eventually settling other solar systems. The same designs of O'Neill cylinders etc. would work equally well around other stars as around our Sun. This is a much more long-term goal.

Moons and planets

Base on Mars

Before we can think about terraforming Mars, we need to land some people on it and have them live there for an extended stay. The 'Mars Direct ' plan has rigorously demonstrated that a manned mission to Mars has been possible since the 1990s for $20 billion (in other words, about 0.4% of the US military budget, over five years). With open collaborative design, it could be done at a fraction of this price.

A team of astronauts on the surface of Mars for years would be able to fill in the blanks in our knowledge of the Red Planet. The plan to terraform Mars is based on the assumption that there is no native life there. Before terraforming begins, we would need to confirm this assumption, as it is not ethically acceptable to deliberately destroy an entire planet's biosphere. More precise measurements of the chemical composition of Mars would allow us to perfect our terraforming plan.

Terraforming Mars

Terraforming refers to re-engineering the environment of a planet to make it more like Earth, that is, more friendly to life. We have some enormous advantages on our side in terraforming Mars: firstly, Mars has plenty of water, carbon, oxygen (though this is bound up as carbon dioxide), secondly, as we shall see, Mars will almost terraform itself once we give it a nudge in the right direction.

To terraform Mars we need add four things: heat, oxygen, water and atmospheric pressure. Happily, these are all connected; if we add heat, the others fall into place.

Step one: greenhousing Mars

Mars has a lot of frozen carbon dioxide, especially at its poles. Robert Zubrin has calculated^[1] that if we heat the planet just 4°C, some of this will sublimate into the atmosphere. This carbon dioxide is a greenhouse gas, which will lead to more warming, making the frozen carbon dioxide that remains at the colder parts sublimate, leading to more global warming. This will cause the frozen water below the surface to melt and some of it will vaporise. This water vapour is another greenhouse gas, leading to even more global warming. When the planet reaches higher temperatures, more carbon dioxide, trapped in minerals, will be released into the atmosphere. So an initial nudge of 4°C can set in motion a chain of events that raise the temperature of the planet 55°C and build enough atmosphere to raise the pressure from 0.006 to 0.296 atmospheres (similar to pressure at twice the altitude of Everest).

This whole process may take about 25 years. By the end of it, the average temperature of the planet is 0°C (though we can locate our colonies at places much warmer than average), the pressure allows people to walk around without a spacesuit and oceans cover large parts of the planet. The atmosphere is still nearly all carbon dioxide, so breathing apparatus will be required when colonists venture beyond sealed habitats. The planet is not fully terraformed, but we are off to a very good start and all it took was heating the planet 4°.

How do we create this 4°C increase? Ironically, the very scientific knowledge gained by studying the destruction of the Earth's biosphere can be applied to engineering a new biosphere on Mars; releasing enough CFCs or similar greenhouse gases into the atmosphere should do the trick. The building blocks of CFCs are available on the surface of Mars, and we could engineer some automated method of synthesizing them in-situ and releasing them into the atmosphere. About 39 million tons would be required, roughly three times what was produced on Earth during the 20 years CFCs were used in industry. Two other methods of warming the planet have been proposed: sending ammonia- or methane-rich asteroids to collide with it (these are greenhouse gases) or building gigantic mirrors in orbit to reflect more sunlight onto those frozen carbon dioxide deposits. Of the three methods, CFCs do seem the most feasible, but the other two may provide a helping hand. If a lunar mining colony is in place, launching a thin aluminium mirror of sufficient size would not be unfeasible.

Step two: life off-Earth

Once this process is complete, we must release the oxygen from carbon dioxide. The best way to do this would be with some automated technology that can perform the chemical processes necessary to split carbon dioxide molecules apart. Because it would take too much energy to build enough of these to transform an entire planet, the technology would have to be self-replicating, capable of colonizing the planet autonomously as it worked. Luckily, such a technology is already in existence: Life.

Some extremophiles have survived well in conditions that simulate current conditions on Mars^[2]. But by the time we set about bio-terraforming the Red Planet it would have much more comfortable conditions: liquid oceans, an average temperature of 0°, and pressure about a fifth of Earth's. Many anaerobic bacteria, algae, fungi and lichens could survive there. Genetic engineering could be used, but would probably not prove necessary. Genetic engineering may create organisms that can release another reservoir of oxygen: that which is trapped in metallic oxides on the planet's surface.

This plan does not address one major difference between Earth and Mars: gravity. Gravity on Mars is only 38% of Earth's. This may be an obstacle to making Mars really Earthlike. We know that after enough time in zero-gravity environments, astronauts suffer degeneration of the muscular and skeletal systems, but we do not know what would happen in a reduced-gravity environment like Mars. All of Man's experience so far has either been at zero-gravity or Earth's gravity. If this turns out to be an insurmountable issue then homes and other living spaces could be built within very large diameter rotating structures on the surface to simulate 'Earth gravity'.

Further info on terraforming by Martyn Fogg (an appropriate name for an expert on Martian gases...)

Surface of the Moon

The moon would be considerably harder to colonise than Mars, as there is no atmosphere and essential elements are lacking. On the other hand, there is an abundance of metals suitable for construction and, at the poles, there is ice that could provide water and oxygen.

Such a colony would have to be sealed in to create an atmosphere. A dome could be built over an ice-rich crater, or the colony could be built underground.

Venus atmosphere

There are formidable obstacles to establishing human life on the surface of Venus: the surface temperature is around 450°C and the pressure is around 90 atmospheres – equivalent to the pressure at around one kilometre under Earth's oceans. However, 50km above the surface of the planet we find Earthlike temperatures and pressures. It has therefore been proposed that the easiest way to establish a colony on Venus would be to fill a gigantic balloon with breathable air. Such a structure would float above the denser air of Venus in the same way that a helium balloon floats above Earth.

The gravity on Venus is 91% of that on Earth, so it is unlikely any special measures would need to be taken to create artificial gravity.

These floating habitats could also be equipped with devices to alter the atmosphere with the aim of eventually terraforming Venus.

Terraforming Venus

As noted above, the surface of Venus is a hostile place. However it has one big advantage: Earthlike gravity. If the heat, pressure and gaseous composition of the atmosphere can be changed through a terraforming project more radical than that on Mars, Venus could one day be settled.

http://www.terraformers.org.au/start.html

Other moons

Many to choose from.

Enceladus around Saturn has H₂O.

Europa around Jupiter has lots of H₂O and O₂. Underneath the ice are liquid oceans which may harbour life.

Titan around Saturn - Pressure of 1.45 atmospheres. Lots of atmospheric nitrogen, carbon (as methane) and H₂O.

Asteroid Belt

Ceres

(Water ice)

Habitat design

There have been many studies over the past thirty years to understand what kind of habitats could be built and what size constraints there are based on current engineering knowledge. Surprisingly in the 1970s the answer was that using bridge and ship-building techniques it would possible to build cylindrical mega-structures up to 30 kilometres long and 6 kilometres in diameter, with a single one able to comfortably house several million people. These habitats would be assembled in zero gravity, which would greatly reduce the energy costs for such large-scale engineering. Materials like carbon fiber have since allowed us to imagine safer, stronger, more efficient habitats. A more recent design for a space habitat is described here.

Being mega-scale engineering projects, it is not hard to see that similar techniques used in the habitat's construction could be used to make the interiors of these habitats be like beautiful places on Earth, such as rolling green English countryside. Some designs even have enough atmosphere inside them to make the sky appear blue.

Stanford torus
O'Neill cylinder
Hollowed out asteroid
Rotating dumbell
Crater bubble
Rotating moonbase for 1g
Zero-G station
Underground habitations on the Moon or Mars (perhaps in lava tubes). These would offer protection from radiation.

Life support

To survive in space, a person needs a minimum of 1kg of oxygen, 0.5kg of dehydrated food and 3.5kg of water per day ^[3]. Shuttle flights simply bring all the food they need, and bring their waste back with them, but for permanent space habitats, it is necessary to find a way to produce food in space.

Biosphere 2

was an attempt to make a closed ecological system on Earth. Mastering this is an important step towards building permanent space habitats.

Modern space stations like the International Space Station use mechanical reactors to recycle the CO₂ and water that humans use. While this is quite an effective approach, it does generate waste that accumulates over time and requires shuttles to come every few months to remove waste and bring in new supplies (especially of hydrogen) which leaks out of the system over many iterations of the cycle. A major disadvantage of the systems is that they do not recycle solid waste like faeces. The ideal would be to create fully closed-loop systems for recycling water, air and waste. The only way to do this is to incorporate plants, to move from a mechanical life-support system to a partly biological one.

There have been several attempts on Earth to construct a closed ecological system and we have learned a lot from these. If we can build a fully closed ecological system on Earth, there is no reason why we cannot do so in space.

Space settlements will not have the easy access to the large store of resources we have on Earth. For this reason, they will have to be models of sustainable communities without pollution, waste or unsustainable harvesting of materials. The technologies of sustainable communities on Earth and sustainable communities in space must be considered together; they both require non-polluting industry, circular resource-chains and sophisticated recycling.

Areca palm, Snake plant and money plant are exceptional for their ability to create clean, fresh air for people ^[4]. A human can survive in a sealed environment with just four plants like these. The application of this to space habitats is obvious.

Aeroponics, as discussed here, can be used to grow plants in space. An aeroponic greenhouse could easily provide all the air and food space-dwelling humans need. Conservatively, this would require 20m² per person, perhaps as little as 5 m².

Biophilia is an important aspect of plant-based life-support systems. It is psychologically difficult to be surrounded by nothing but metal and machines for months or years on end, and psychological stability is a major issue on long-term space missions. Introducing a leafy, green, fresh stimulus to the space-dweller's sensorium is of immense benefit.

Material

See Resources in space. It would be far too inefficient to build these huge structures from material brought up from the surface of the Earth, so for orbiting colonies near Earth it has been proposed to use material mined from asteroids or the Moon.

Further information

Gerard O'Neill
O'Neill cylinder habitat
Resources in space
Space Habitats Gallery - Great art of Mars habitats, asteroid mining equipment, orbital habitats and more.
Marshall T. Savage's The Millenial Project: Colonizing the Galaxy in eight easy steps - employing known technology. (Forward by Arthur C. Clarke )

Detailed tour: previous page | next page

@@ Line 1: / Line 1: @@
 {{pagebg}}
-{{backto|Colonising Space}}
+{{early stage}}
-[[Image:Stanford torus.jpg|300px|right]]
+{{Colonising Space navbar}}
+[[Image:Stanford torus.jpg|250px|thumb|{{wp|Stanford_torus|Stanford torus}} space habitat]]
+[[Image:Rama-view1-n15.jpg|250px|thumb|Inside a large landscaped cylindrical habitat (credit: [http://ebruneton.free.fr/rama3/rama.html Eric Bruneton])]]
+Space is a very harsh environment and it may be hard to imagine wanting to live in such a seemingly bleak place, but taming it is only a matter of some (admittedly fairly serious) engineering and is entirely within our current technical capability.
+There are many challenges to enable building these towns and cities off-Earth. Two prominent ones are raw materials and gravity. A colony of any size will have to be self-sufficient in materials as it will not be practical to ship them up from the surface of the Earth due to the enormous energy required to climb against its pull. Escaping this pull long-term also causes major problems for the human body. Muscles get very weak, including the heart, and bones de-mineralise. The only real solution is to generate artificial gravity by rotation {{en}} even for Moon-bases if people are going to stay for any length of time.
+To balance these difficulties, living in space brings many advantages. One is reliable, abundant [[Energy|solar energy]], not blocked by the atmosphere or subject to weather or time of day. Another is control of gravity; a rotating space colony would have normal Earthlike gravity at the periphery and zero gravity at the center. Large-scale construction and other industrial activities could be done at the center, reducing the energy they need to almost zero. When we build space habitats, we have the opportunity to tailor-make ecosystems that are optimized for human habitation. A space colony could be kept at any desired temperature, have any desired weather conditions and any desired seasonal and diurnal cycles. Careful controls could keep infectious diseases and agricultural pests from being introduced to the ecosystem.
+A passion for space colonisation and a faith in its promise was evoked in thousands of people by an essay called [http://space.mike-combs.com/SCTHF.html ''Space Colonies: The High Frontier''], written by [http://ssi.org/the-life-of-gerard-k-oneill/ Gerard O'Neill] in 1976. The essay outlines a fairly modest proposal, using only technology then existing, to build a series of space stations orbiting the Sun and Earth. Materials would be [[Resources in space#Mining the moon|mined from the moon and the asteroid belt]], launched into space and assembled into space colonies. Launching material from the moon is an easy matter, as there is far less gravity and no air resistance. The first colony would require about as much building material as the world's biggest skyscrapers, and would become self-replicating, as each colony could be used as a factory to assemble the next. The network of space habitats could then expand to form a Dyson swarm ([[#Lagrangian points and Solar orbit|see below]]) which could house tens of trillions of people to an extremely high standard of living.
+O'Neill sets out to challenge a fundamental assumption underlying all our thinking about mankind's resources: the assumption that "The material and energy resources of the human race are just those of our planet." His calculations show that by building such colonies with [[Resources in space|materials]] from the asteroid belt and the moon, we could create "a land area many thousands of times that of the entire earth", with ideal climate, temperature and conditions. This would certainly impact just about every pressing social problem we now face on Earth.
 ==Places==
-There are many places to site a habitat in space, especially within the orbit of Jupiter.
+==='Islands' in space===
+[[Image:O'Neill cylinders.jpg|250px|thumb|A pair of O'Neill cylinder habitats]]
+Some of the most feasible strategies for building space colonies involve building giant self-contained structures in the vacuum of space itself, rather than on some pre-existing heavenly body. {{wp|Bernal_sphere|Bernal spheres}}, {{wp|Stanford_torus|Stanford torus}} and {{wp|O'Neill_cylinder|O'Neill cylinders}} described below fall into this category.
-The easiest to get to from Earth is '''low earth orbit''', although due to the Earth's strong gravitational pull it requires enormous amounts of energy to bring material up from the surface and one could not be classed as self-sufficient here, however the views are pretty good.
+The first step to building such habitats would be to set up automated [[Resources in space#Mining the moon|facilities on the moon to mine titanium and iron]] and launch it into space. Water could be mined from icy asteroids and perhaps from the moon also; as well as providing the [[Water|water needs]] for colonists, it could be split to make breathable oxygen for the atmosphere of the colony. The habitat could then be assembled in space, and used to manufacture another habitat, making them self-replicating.
-In a [http://en.wikipedia.org/wiki/Geosynchronous_orbit '''geosynchronous orbit'''] the habitat would hover the same spot on Earth which would mean that one would have the same day / night cycle as on the surface which is an important consideration with human physiology.
+Using materials from the moon and asteroid belt alone, we could create artificial habitats with thousands of times the land area of Earth, able to comfortably accommodate tens of trillions of people.
-In '''lunar orbit''' the moon is within easy reach but the habitat will be subject to the moons two-week long day / night cycles which might not be to everyone's taste, although of course the normal light rythms can be replicated internally with lighting.
+====Lagrangian points and Solar orbit====
+A Dyson swarm refers to a series of habitats build in space around the Sun. {{wp|Lagrangian_point|Lagrangian points}} are points in the Sun's orbit where a colony could orbit stably, keeping a constant comfortable distance from the Sun and the Earth.
-The '''surface of the moon''' provides a continent-sized area to inhabit and somewhere to 'get outside'. However it is not clear that the one-sixth of Earth's gravity here is enough to stop degredation of the human over extended periods, but this could be overcome by having large circular habitats that rotate simulating a 1G environment.
+====Low Earth orbit====
+The easiest place to get to from Earth is '''low earth orbit''', although due to the Earth's strong gravitational pull it requires enormous amounts of energy to bring material up from the surface and one could not be classed as self-sufficient here, however the views are pretty good. If we develop more efficient means of [[Colonising Space/Access to space|access to space]] than we currently have, low Earth orbit habitats become more feasible.
-*Low Earth orbit
+====Geosynchronous orbit====
-*Higher earth orbits
+In a {{wp|Geosynchronous_orbit|geosynchronous orbit}} the habitat would hover the same spot on Earth which would mean that one would have the same day / night cycle as on the surface which is an important consideration with human physiology.
-*Lunar orbit(?)
-*The surface of the Moon
+====Lunar orbit====
-*Lagrangian points
+In '''lunar orbit''' the moon is within easy reach but the habitat will be subject to the Moon's two-week long day / night cycles which might not be to everyone's taste, although of course the normal light rhythms can be replicated internally with lighting.
-*Near Earth asteroids
-*Solar orbit
+====In other solar systems====
-*Mars moons
+Habitable planets may turn out to be rare in other solar systems. If this proves to be the case, it need not stop us from eventually settling other solar systems. The same designs of O'Neill cylinders etc. would work equally well around other stars as around our Sun. This is a much more long-term goal.
-*Mars orbit
-*The surface of Mars
+===Moons and planets===
-*The asteroid belt
-**Ceres - lots of water ice
+====Base on Mars====
-*Other moons (and there are x number of them)
+Before we can think about terraforming Mars, we need to land some people on it and have them live there for an extended stay. The '{{wp|Mars_Direct|Mars Direct}}' plan has rigorously demonstrated that a manned mission to Mars has been possible since the 1990s for $20 billion (in other words, about 0.4% of the US military budget, over five years).  With [[Open collaborative design|open collaborative design]], it could be done at a fraction of this price.
+A team of astronauts on the surface of Mars for years would be able to fill in the blanks in our knowledge of the Red Planet. The plan to terraform Mars is based on the assumption that there is no native life there. Before terraforming begins, we would need to confirm this assumption, as it is not ethically acceptable to deliberately destroy an entire planet's biosphere. More precise measurements of the chemical composition of Mars would allow us to perfect our terraforming plan.
+====Terraforming Mars====
+[[Image:152px-MarsTransitionV.jpg|right]]Terraforming refers to re-engineering the environment of a planet to make it more like Earth, that is, more friendly to life. We have some enormous advantages on our side in terraforming Mars: firstly, Mars has plenty of water, carbon, oxygen (though this is bound up as carbon dioxide), secondly, as we shall see, Mars will almost terraform itself once we give it a nudge in the right direction.
+To terraform Mars we need add four things: heat, oxygen, water and atmospheric pressure. Happily, these are all connected; if we add heat, the others fall into place.
+=====Step one: greenhousing Mars=====
+Mars has a lot of frozen carbon dioxide, especially at its poles. Robert Zubrin has calculated<sup>[http://www.users.globalnet.co.uk/~mfogg/zubrin.htm]</sup> that if we heat the planet just 4°C, some of this will sublimate into the atmosphere. This carbon dioxide is a greenhouse gas, which will lead to more warming, making the frozen carbon dioxide that remains at the colder parts sublimate, leading to more global warming. This will cause the frozen water below the surface to melt and some of it will vaporise. This water vapour is another greenhouse gas, leading to even more global warming. When the planet reaches higher temperatures, more carbon dioxide, trapped in minerals, will be released into the atmosphere. So an initial nudge of 4°C can set in motion a chain of events that raise the temperature of the planet 55°C and build enough atmosphere to raise the pressure from 0.006 to 0.296 atmospheres (similar to pressure at twice the altitude of Everest).
+This whole process may take about 25 years. By the end of it, the average temperature of the planet is 0°C (though we can locate our colonies at places much warmer than average), the pressure allows people to walk around without a spacesuit and oceans cover large parts of the planet. The atmosphere is still nearly all carbon dioxide, so breathing apparatus will be required when colonists venture beyond sealed habitats. The planet is not fully terraformed, but we are off to a very good start and all it took was heating the planet 4°.
+How do we create this 4°C increase? Ironically, the very scientific knowledge gained by studying the destruction of the Earth's biosphere can be applied to engineering a new biosphere on Mars; releasing enough CFCs or similar greenhouse gases into the atmosphere should do the trick. The building blocks of CFCs are available on the surface of Mars, and we could engineer some automated method of synthesizing them in-situ and releasing them into the atmosphere. About 39 million tons would be required, roughly three times what was produced on Earth during the 20 years CFCs were used in industry. Two other methods of warming the planet have been proposed: sending ammonia- or methane-rich asteroids to collide with it (these are greenhouse gases) or building gigantic mirrors in orbit to reflect more sunlight onto those frozen carbon dioxide deposits. Of the three methods, CFCs do seem the most feasible, but the other two may provide a helping hand. If a [[Resources in space#Mining the moon|lunar mining colony]] is in place, launching a thin aluminium mirror of sufficient size would not be unfeasible.
+=====Step two: life off-Earth=====
+Once this process is complete, we must release the oxygen from carbon dioxide. The best way to do this would be with some automated technology that can perform the chemical processes necessary to split carbon dioxide molecules apart. Because it would take too much energy to build enough of these to transform an entire planet, the technology would have to be self-replicating, capable of colonizing the planet autonomously as it worked. Luckily, such a technology is already in existence: Life.
+Some extremophiles have survived well in conditions that simulate current conditions on Mars<sup>[http://www.skymania.com/wp/2012/04/lichen-survives-harsh-martian-setting.html/5806/]</sup>. But by the time we set about bio-terraforming the Red Planet it would have much more comfortable conditions: liquid oceans, an average temperature of 0°, and pressure about a fifth of Earth's. Many anaerobic bacteria, algae, fungi and lichens could survive there. Genetic engineering could be used, but would probably not prove necessary. Genetic engineering may create organisms that can release another reservoir of oxygen: that which is trapped in metallic oxides on the planet's surface.
+This plan does not address one major difference between Earth and Mars: gravity. Gravity on Mars is only 38% of Earth's. This may be an obstacle to making Mars really Earthlike. We know that after enough time in zero-gravity environments, astronauts suffer degeneration of the muscular and skeletal systems, but we do not know what would happen in a reduced-gravity environment like Mars. All of Man's experience so far has either been at zero-gravity or Earth's gravity. If this turns out to be an insurmountable issue then homes and other living spaces could be built within very large diameter rotating structures on the surface to simulate 'Earth gravity'.
+* [http://www.users.globalnet.co.uk/~mfogg/paper1.htm Further info on terraforming by Martyn Fogg] (an appropriate name for an expert on Martian gases...)
+====Surface of the Moon====
+The moon would be considerably harder to colonise than Mars, as there is no atmosphere and essential elements are lacking. On the other hand, there is an abundance of [[Resources in space#Mining the moon|metals suitable for construction]] and, at the poles, there is [http://www.nasa.gov/mission_pages/Mini-RF/multimedia/feature_ice_like_deposits.html ice] that could provide water and oxygen.
+Such a colony would have to be sealed in to create an atmosphere. A dome could be built over an ice-rich crater, or the colony could be built underground.
+====Venus atmosphere====
+There are formidable obstacles to establishing human life on the surface of Venus: the surface temperature is around 450°C and the pressure is around 90 atmospheres {{en}} equivalent to the pressure at around one kilometre under Earth's oceans. However, 50km above the surface of the planet we find Earthlike temperatures and pressures. It has therefore been proposed that the easiest way to establish a colony on Venus would be to fill a gigantic balloon with breathable air. Such a structure would float above the denser air of Venus in the same way that a helium balloon floats above Earth.
+The gravity on Venus is 91% of that on Earth, so it is unlikely any special measures would need to be taken to create artificial gravity.
+These floating habitats could also be equipped with devices to alter the atmosphere with the aim of eventually terraforming Venus.
+*[http://spacemonitor.blogspot.com/2007/05/floating-city-on-venus.html A floating city on Venus]
+*[http://spacemonitor.blogspot.com/2007/05/copy-of-pdf-file.html Geoffrey A. Landis' original proposal for such a habitat]
+====Terraforming Venus====
+As noted above, the surface of Venus is a hostile place. However it has one big advantage: Earthlike gravity. If the heat, pressure and gaseous composition of the atmosphere can be changed through a terraforming project more radical than that on Mars, Venus could one day be settled.
+*http://www.terraformers.org.au/start.html
+====Other moons====
+Many to choose from.
+Enceladus around Saturn has H<sub>2</sub>O.
+Europa around Jupiter has lots of H<sub>2</sub>O and O<sub>2</sub>. Underneath the ice are liquid oceans which may harbour life.
+Titan around Saturn - Pressure of 1.45 atmospheres. Lots of atmospheric nitrogen, carbon (as methane) and H<sub>2</sub>O.
+====Asteroid Belt====
+=====Ceres=====
+(Water ice)
+== Habitat design ==
+There have been many studies over the past thirty years to understand what kind of habitats could be built and what size constraints there are based on current engineering knowledge. Surprisingly in the 1970s the answer was that using bridge and ship-building techniques it would possible to build cylindrical mega-structures up to 30 kilometres long and 6 kilometres in diameter, with a single one able to comfortably house several million people. These habitats would be assembled in zero gravity, which would greatly reduce the energy costs for such large-scale engineering. Materials like [[Advanced materials|carbon fiber]] have since allowed us to imagine safer, stronger, more efficient habitats. A more recent design for a space habitat is described [http://www.youtube.com/watch?v=fcC0ixQi4B8 here].
+Being [[Advanced automation|mega-scale engineering]] projects, it is not hard to see that similar techniques used in the habitat's construction could be used to make the interiors of these habitats be like beautiful places on Earth, such as rolling green English countryside. Some designs even have enough atmosphere inside them to make the sky appear blue.
-==Types of habitat==
 *Stanford torus
 *O'Neill cylinder
-*Crater bubble
-*Rotating moonbase for 1G
 *Hollowed out asteroid
-*Zero-G station
 *Rotating dumbell
+*Crater bubble
+*Rotating moonbase for 1g
+*Zero-G station
+*Underground habitations on the Moon or Mars (perhaps in lava tubes). These would offer protection from radiation.
+===Life support===
+To survive in space, a person needs a minimum of 1kg of oxygen, 0.5kg of dehydrated food and 3.5kg of water  per day <sup>[http://www.youtube.com/watch?v=_BRfvnCDOvE]</sup>. Shuttle flights simply bring all the food they need, and bring their waste back with them, but for permanent space habitats, it is necessary to find a way to produce food in space.
+[[Image:Biosphere.jpg|thumb|250px|{{wp|Biosphere_2|Biosphere 2}} was an attempt to make a closed ecological system on Earth. Mastering this is an important step towards building permanent space habitats.]]Modern space stations like the International Space Station use mechanical reactors to recycle the CO<sub>2</sub> and water that humans use. While this is quite an effective approach, it does generate waste that accumulates over time and requires shuttles to come every few months to remove waste and bring in new supplies (especially of hydrogen) which leaks out of the system over many iterations of the cycle. A major disadvantage of the systems is that they do not recycle solid waste like faeces. The ideal would be to create fully closed-loop systems for recycling water, air and waste.  The only way to do this is to incorporate plants, to move from a mechanical life-support system to a partly biological one.
+There have been several attempts on Earth to construct a {{wp|Closed_ecological_system|closed ecological system}} and we have learned a lot from these. If we can build a fully closed ecological system on Earth, there is no reason why we cannot do so in space.
+Space settlements will not have the easy access to the large store of resources we have on Earth. For this reason, they will have to be models of sustainable communities without pollution, waste or unsustainable harvesting of materials. The technologies of sustainable communities on Earth and sustainable communities in space must be considered together; they both require non-polluting industry, circular resource-chains and sophisticated [[Recycling|recycling]].
+[http://plantsrescue.com/category/chrysalidocarpus-lutescens/ Areca palm], {{wp|Sansevieria_trifasciata|Snake plant}} and {{wp|Epipremnum_aureum|money plant}} are exceptional for their ability to create clean, fresh air for people <sup>[http://www.ted.com/talks/kamal_meattle_on_how_to_grow_your_own_fresh_air.html]</sup>. A human can survive in a sealed environment with just four plants like these. The application of this to space habitats is obvious.
+Aeroponics, as discussed [[Food|here]], can be used to grow plants in space. An aeroponic greenhouse could easily provide all the air and food space-dwelling humans need. Conservatively, this would require 20m<sup>2</sup> per person, perhaps as little as 5 m<sup>2</sup>.
+{{wp|Biophilia|Biophilia}} is an important aspect of plant-based life-support systems. It is psychologically difficult to be surrounded by nothing but metal and machines for months or years on end, and psychological stability is a major issue on long-term space missions. Introducing a leafy, green, fresh stimulus to the space-dweller's sensorium is of immense benefit.
+===Material===
+See [[Resources in space]]. It would be far too inefficient to build these huge structures from material brought up from the surface of the Earth, so for orbiting colonies near Earth it has been proposed to use material mined from asteroids or the Moon.
+== Further information ==
+* {{wp|Gerard_O%27Neill|Gerard O'Neill}}
+* {{wp|O%27Neill_cylinder|O'Neill cylinder habitat}}
+* [[Resources in space]]
+* [http://www.spacehabs.com Space Habitats Gallery] - Great art of Mars habitats, asteroid mining equipment, orbital habitats and more.
+* Marshall T. Savage's [http://www.amazon.com/Millennial-Project-Colonizing-Galaxy-Eight/dp/0316771635/ref=sr_1_1?ie=UTF8&s=books&qid=1251836768&sr=8-1 The Millenial Project: Colonizing the Galaxy in eight easy steps] - employing known technology. (Forward by {{wp|Arthur_C._Clarke|Arthur C. Clarke}})
+{{br}}
+{{detailed tour|Colonising Space|AdCiv:About}}
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