Making a Greenhouse on Another World: Where Can We Paraterraform in Our Solar System?

There is no shortage of dreamers today who believe that humanity can, will, or must explore space and establish a human presence among the stars. For some, this is a matter of meeting our true destiny and finding ourselves out there in the Universe.

For others, its all about the desire for new frontiers, new horizons, and new challenges. By leaving Earth and migrating to other planets and celestial bodies, humanity would be returning to its roots, making a home out of new lands, like our ancestors did hundreds of thousands of years ago.

And for others still, it's a matter of survival. On the one hand, it makes sense not to keep all your eggs in the same basket. On the other, there is plenty of evidence to suggest that humans will not survive on Earth indefinitely.

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Whether it's the result of a cataclysmic event (like an asteroid impact), anthropogenic climate change, or our well-documented capacity to destroy ourselves, many believe that humanity will become extinct if it does not colonize space.

Of course, this presents some serious challenges. Right now, it is still costly to launch payloads and crews into space, not to mention send robotic probes to other planets. Sending human beings to colonize other planets would be even more expensive.

Beyond simply getting there, there are also many long-term issues that would need to be addressed. For instance, how are human beings expected to live indefinitely on worlds that are inhospitable to life as we know it?

Even if we could rely on advanced technology and be as self-sufficient as possible, it is very difficult to live in an environment that is constantly trying to kill you!

The Problem with Long-Term Habitation

This is where ecological engineering comes into play. The theory is that humans could alter the local environment on a planet or moon to create a hospitable atmosphere and life-cycle that would allow for long-term habitation.

This process, when conducted on a planetary scale, is termed "terraforming." However, such a process could take thousands of years and would require an unprecedented amount of resources, technological advances, labor, and a multi-generational commitment.

In addition, there are only certain places in the Solar System that can conceivably be terraformed. There really isn't any plausible means for terraforming any bodies in our Solar System.

But what about converting only a portion of a planet, moon or large asteroid? Instead of attempting to alter the ecology of an entire world, could we not simply alter a small corner of it, creating a garden and breathable atmosphere where there is only ice, rock, dust, and vacuum?

Would this be enough to establish long-term human settlements throughout the Solar System?

Definition

Also known as the "worldhouse" concept, the basic idea here is to build an enclosure around a certain part of a planet and alter the environment within. This concept was originally coined by British mathematician Richard L.S. Talyor in a 1992 study, "Paraterraforming – The worldhouse concept."

Using this method, sections of a planet that are otherwise inhospitable or cannot be terraformed as a whole could be made suitable for human habitation. It would be especially useful on planets or moons that had little to no atmosphere, and where much of the surface is subject to lethal levels of heat and radiation.

Some key examples include Mercury and the Moon, two celestial bodies that have very tenuous atmospheres and are bombarded by intense amounts of solar and cosmic radiation.

While these locations could not conceivably be made "green," enclosed colonies could be created in certain locations. These colonies could conceivably have enough resources at their disposal that thousands (or even hundreds of thousands) of people would be able to live there.

The Shell World Concept

Taking a larger view, there is also the concept of terraforming entire planets using the same basic idea. This idea was first proposed in 2009 by Kenneth Roy – an engineer with the US Department of Energy – in a paper published with the Journal of British Interplanetary Sciences.

Titled "Shell Worlds – An Approach To Terraforming Moons, Small Planets and Plutoids," this paper explored the theoretical possibility of using a large “shell” to encase a planet, keeping its atmosphere contained so that long-term changes can take root.

Shells could also be used to enclose an entire planet that has no atmosphere, which would allow engineers to slowly create one, through mining or pumping in atmospheric gases. The shell would ensure that the atmosphere would be retained until such time as the engineers completed the process.

However, this proposal is more in keeping with the concept of "megastructures" than paraterraforming. The number of materials, the technology, and the time such a feat of engineering would make it out of reach. 

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However, paraterraforming on a small scale, which would involve enclosing a patch of livable terrain the size of a city or a rural district, could be within the realm of possibility. While we should not expect anything like this to happen soon, it is something we can plan for the not-too-distant future.

So how would we go about doing this, you ask? Using current technology, or technologies that are expected to be available in the not-too-distant future, a number of options are available.

Paraterraforming Methods

When it comes to plans to colonize locations beyond Earth, the name of the game is sustainability and self-sufficiency. To achieve this, NASA and other space agencies are investigating a number of technologies and methods.

One of these is the technology known as additive manufacturing (e.g., 3D Printing). In recent years, this concept has been investigated as a way to possibly construct bases on the Moon, Mars, and beyond.

Another method that is considered a must for off-world settlement is known as In-Situ Resource Utilization (ISRU). This process entails the use of local resources to manufacture everything from building materials and energy, to breathable air and potable water.

"As human space exploration evolves toward longer journeys farther from our home planet, ISRU will become increasingly important. Resupply missions are expensive, and as astronaut crews become more independent of Earth, sustained exploration becomes more viable. For travel in space, as on Earth, we need practical and affordable ways to use resources along the way, rather than carrying everything we think will be needed. Future astronauts will require the ability to collect space-based resources and transform them into breathable air; water for drinking, hygiene, and plant growth; rocket propellants; building materials; and more. Mission capabilities and net value will multiply when useful products can be created from extraterrestrial resources."

It is theorized that by using 3D printing and ISRU, enclosed settlements could be built on-site without the need to import a lot of prefabricated parts or building materials. Once complete, they would also be able to achieve a degree of self-sufficiency, which could go a long way towards ensuring sustainability.

But as with all things in real estate, the biggest issue of all is the location. If we to build settlements on other planets, moons, and bodies, the bases will need to be accessible, have sufficient protection against radiation and extreme conditions, and not too far from sources of resources and energy.

One way to do this is to build these settlements in locations that afford natural protection against radiation and are also resource-rich. Several options exist, such as building settlements beneath the surface.

Another way to protect against hazards like radiation is to build enclosures out of radiation-resistant material. For example, the basic structure of a settlement could be fashioned out of locally-sourced regolith (loose deposits which cover solid rock).

Alternately, this could be done through a process known as "sintering," where regolith is bombarded with microwaves or lasers to create a molten ceramic. This could then be altered, using 3D printing robots, to form the settlement's foundation, outer walls, and superstructure.

There's also the possibility of using magnetic shielding. This concept was proposed by civil engineer Marco Peroni at the 2018 American Institute of Aeronautics and Astronautics (AIAA) SPACE and Astronautics Forum and Exposition.

Peroni's concept included a modular base architecture, where hexagonally-shaped units are grouped together in a spherical configuration beneath a torus-shaped apparatus. This apparatus would be made of high-voltage electric cables that generate an electromagnetic field to protect against radiation.

Based on simulations and test models, Peroni and his colleagues determined that the apparatus would be capable of generating an external magnetic field of 8 microteslas (0.08 gauss). Given that Earth's protective magnetic field ranges from 25 to 65 microteslas (0.25 to 0.65 gauss), this apparatus would need to be strengthened further in order to keep the inhabitants safe, but it is still in the early stages of development.

This proposal is similar in many ways to the Solenoid Moon-base concept that Peroni presented at the 2017 AIAA Space and Astronautics Forum and Exposition. This concept involved a lunar base consisting of transparent domes enclosed by a toroid-shaped structure of high-voltage cables.

In addition to shielding, artificial magnetic fields would also allow for habitats that provide views of the surrounding environment. This is key to preventing things like claustrophobia, isolation, and cabin fever that could inevitably result from subsurface enclosures or those with opaque walls.

There is also a significant amount of evidence that plants could be grown in lunar and Martian soil.

These include studies conducted by astronauts aboard the ISS, the NASA-funded Prototype Lunar/Mars Greenhouse Project (PLMGP) and the joint study between NASA, the University of Engineering and Technology in Lima, and the International Potato Center.

There have also been independent studies, like the one conducted by ecologists at the Wageningen University and Research Center. These experiments have shown that Earth plants can be grown using Martian and lunar regolith, assuming that adequate irrigation and organic nutrients are provided.

Another important aspect to consider is the fact that these settlements would have to be closed systems. Air, water, and other resources will need to be recycled with a high degree of efficiency.

This would lead to the creation of a microclimate where precipitation occurs, oxygen gas is produced, carbon dioxide is scrubbed from the air, and water is naturally recycled and filtered.

The rest could be handled by a combination of recycling systems. Organic waste and human waste could be composted and used as fertilizer, and other forms of waste could be recycled to create new tools and commodities.

So where exactly could these enclosed, microclimate colonies be created?

Inner Solar System

Like Earth, all of the planets of the inner Solar System are rocky and terrestrial. With the exception of Venus, these could all suffice as potential sites for future colonies. All are rich in minerals and potentially in water ice, and some even have organic molecules. They also have their fair share of hazards!

Mercury:

It might surprise you to know that Mercury, the closest planet to our Sun and the second hottest (behind Venus) is actually a viable candidate for colonization. You see, while the planet receives an intense amount of heat and radiation from the Sun, a well-position colony would be able to avoid these and other hazards.

For instance, since Mercury has a tenuous exosphere, heat is not transferred from the Sun-facing side to the dark side. As a result, whatever side is experiencing daylight reaches temperatures as high as 427 °C (800 °F) while the night-side experiences extreme cold (-173 °C/-279 °F).

Also, Mercury experiences what is known as 3:2 orbital resonance. What this means is that the planet completes three rotations on its axis (each one takes 58.6 days) to rotate twice around the Sun (a single orbit takes 88 days). In short, Mercury experiences three sidereal days for every two years.

However, since the planet is traveling rapidly around the Sun and rotating slowly on its axis, the actual length of a full day - i.e., the time it takes for the Sun to return to the same place in the sky (aka. a solar day) - works out to roughly 176 Earth days.

In other words, a single day on Mercury lasts as long as two of its years. However, Mercury’s very low axial tilt (0.034°) means that the vast majority of sunlight it receives is absorbed around the equator. Meanwhile, its polar regions are permanently shaded and cold enough to contain water ice.

This was confirmed by NASA’s MESSENGER probe in 2012, which found evidence of water ice and organic molecules in the craters that dot the northern polar region. There is also speculation that the southern pole could contain ice in its permanently-shadowed cratered areas, perhaps as much as 100 billion to 1 trillion tons that would be up to 20 m ( ft) thick.

In these regions, domes could be built on the crater floors or covering an entire crater. Some possible candidates include the Kandinsky, Prokofiev, Tolkien and Tryggvadottir craters, all of which are thought to have supplies of water ice.

Sunlight could be harnessed by positioning mirrors on the craters' edges, to redirect it into the domed enclosures. Temperatures inside would gradually rise, water ice would melt, and soil could be made by combining the water and organic molecules with regolith from the crater floor.

Plants could also be grown to produce oxygen which, combined with nitrogen gas, would produce a breathable atmosphere. The region inside the biodome would become a livable environment with its own water cycle and the carbon cycle.

Alternately, oxygen gas could be created through chemical dissociation, where evaporated water ice is subjected to solar radiation to produce hydrogen gas (which could be vented or captured and stored for fuel) and oxygen gas.

Alternately, teams of engineers could pump the necessary gases into a domed enclosure until the atmospheric pressure inside reached 100 kilopascals (or 1 bar). The ice could then be harvested as needed or stored for drinking, sanitation, and irrigation.

The Moon:

As Earth’s closest celestial body, colonizing the Moon would be comparatively easy compared to other bodies. In many respects, it presents the same potential hazards as Mercury does, and the strategies for dealing with them are largely the same.

For starters, the Moon has an extremely tenuous atmosphere, one that is so thin that it can only be classified as an exosphere. The Moon is also rich in minerals and potential resources like Helium-3 and water ice but sparse in terms of volatile elements that are necessary for life (i.e., ammonia, methane, carbon dioxide, etc.)

In addition, the surface of the Moon experiences extreme ranges in temperature around the equatorial region. Depending on whether or not a portion of the surface is in direct sunlight, temperatures vary between a low of -173 °C (-280 °F) to a high of 127 °C (260 °F).

However, in the polar regions, temperatures go from a low of -123 °C (-189 °F) to a high of -43 °C (-45 °F). While this is still enough to make Antarctica seem balmy by comparison, it is a much narrower range.

In addition, like Mercury, the polar regions are permanently shaded and have access to supplies of water. This is particularly true of the South-Pole Aitken Basin, a cratered region where multiple orbiter missions have found evidence of water ice.

In locations like the famous Shackleton Crater, an enclosed microclimate could be created by building a dome and using solar mirrors to direct sunlight into it. A weather system could, therefore, be created, plants could then be grown, and a breathable atmosphere potentially created.

Mars:

Mars is another popular destination when it comes to human space exploration and settlement. Like the Moon, much of this has to do with its proximity to Earth and the similarities between it and our planet.

Every 26 months, Earth and Mars are at the closest point in their orbits with each other. This is known as an opposition, where Mars and the Sun appear on opposite sides of the sky. This creates regular “launch windows” to send colonists and supplies.

In addition, a Martian day lasts 24 hours and 39 minutes, which means that plants, animals and human colonists enjoy a diurnal cycle (day/night cycle) that is almost the same as Earth's. Mars' vertical axis is also tilted in a way that is very similar to Earth's - 25.19° vs. 23.5° - which results in seasonal changes over the course of an orbital period.

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Essentially, when one hemisphere is pointed towards the Sun, it experiences summer while the other experiences winter. However, since a Martian year lasts about 687 Earth days (668.6 Martian days), each season lasts about twice as long.

Mars also experiences temperature variations that are similar to Earth's, though they are significantly lower overall. The average surface temperature during the course of a year is -63 °C (-81 °F), ranging from a low of -143 °C (-225 °F) during the winter at the poles and a high of 35 °C (95 °F) along the equator during summer at midday.

However, due to its thin atmosphere, these warm surface temperatures do not reach much higher than ground-level. And at night, the temperature can reach as low as -73 °C (-99 °F). Still, since the variations are much less extreme around the mid-latitudes, this would likely be the best place to construct a settlement.

There's also an abundant supply of water ice on Mars, which is largely concentrated in the polar ice caps. However, various studies have suggested that significant amounts of water may also be locked away beneath the surface. This water could be extracted and used by colonists for everything from drinking and irrigation to sanitation.

Because of this, Mars is well-suited for ISRU. In his book, The Case for Mars, Robert Zubrin explained how air, water, and fuel could be manufactured on-site by future colonists using nothing but the elements available in the Martian soil and atmosphere.

Furthermore, experiments have been conducted that show how Martian soil can be baked into bricks that have considerable strength. These could be used to fabricate the habitats and structures the colonists would live in. Experiments have also shown that Earth plants can grow in Martian soil, which would produce oxygen and scrub carbon from the air.

Alas, there is still the issue of radiation. According to recent studies by the Mars Odyssey probe, residents on the Martian surface will experience levels of radiation that are 2 to 3 times higher than what astronauts experience on the International Space Station.

On Earth, people who live in developed nations are exposed to an average annual dose of 0.62 rads. And while studies have shown that a dose of up to 200 rads is not fatal, exposure to these levels of radiation can dramatically increase health risks (acute radiation sickness, cancer, DNA damage).

The surface of Mars, on the other hand, is exposed to an average of 22 millirads per day - which works out to 8000 millirads (8 rads) per year. That's almost 13 times the yearly dose our bodies are used to, and close to the recommended five-year limit for exposure. The long-effects of that remain unknown.

The Mars Odyssey also detected two solar proton events that caused radiation levels to peak at about 2,000 millirads in a day and a few other events that reached 100 millirads. On top of that, recent research conducted at the University of Nevada, Las Vegas (UNLV) has indicated that the threat posed by cosmic rays may double the risk of cancer.

For this reason, mission planners have explored the idea of building habitats either beneath the surface or creating habitats with thick ceramic outer shells from the local regolith. Once again, the idea of magnetic shielding could be used to allow a transparent shell and afford inhabitants the benefit of a view.

In fact, NASA has explored the idea of positioning a magnetic shield in orbit, around Mars, to provide the same protection as a magnetosphere. The proposal was presented by Dr. Jim Green, the Director of NASA’s Planetary Science Division, at the 2017 Planetary Science Vision 2050 Workshop.

Dr. Green claimed that this shield should be deployed at the Mars-Sun L1 Lagrange Point, where it would create an artificial magnetotail that would encompass all of Mars. This would not only shield life on the surface from harmful radiation but would also allow Mars' atmosphere to thicken (thus affording more protection).

With these measures in place, a colony could be protected from the elements, which includes Martian Dust Storms and radiation. Inside, human settlers would be able to grow plants in Martian soil, produce their own air, and effectively create a self-sustaining microclimate.

Such a base (or many like them) could begin the process of terraforming Mars. After creating microclimates in certain regions, they could begin extending them until they reach across the entire planet.

The Main Asteroid Belt

Interestingly enough, the Asteroid Belt is more than just a loose collection of millions of rocky objects. It is also home to the dwarf planet Ceres, which is the largest body in the Belt and accounts for about a third of the Main Belt's mass. 

Ceres measures roughly 946 km (588 mi) in diameter and has a surface area of 2,849,631 km² (1,100,250 mi²). Given its size and density, Ceres is believed to be differentiated that consists of a rocky core, a liquid ocean next to that, and a mantle and crust composed of ices.

Based on the evidence provided by the Keck telescope in 2002, the mantle is estimated to be 100 km (62 mi) thick and to contain up to 200 million km³ (48 million mi³) of water. That's equivalent to about 10% of what is in Earth’s oceans and is more than all the freshwater on Earth.

Because of this, a colony on Ceres would present all kinds of benefits and opportunities for growth. This is partly because of the way it would make the Main Asteroid Belt and its abundant resources accessible. There are also the resources available on Ceres itself, which could facilitate paraterraforming.

For instance, Ceres has some impressive craters, the largest of which include the Occator, Kerwan, and Yalode craters. Within these, domes could be built, and water could be harvested from the local ice, with silicate minerals used to pave the crater floor.

The locally-harvested ice could be used for irrigation, but also for producing oxygen gas. Since Ceres is thought to have large deposits of ammonia-rich clay soils, ammonia could also be harvested. Since ammonia is largely made up of nitrogen, it could be processed to create nitrogen gas (an important buffer gas in our atmosphere).

Light could be provided by a series of orbital mirrors that would focus and direct sunlight into the dome, providing a sense of a diurnal cycle and also allowing for plants to grow.

The Moons of Jupiter

The idea of colonizing Jupiter's moons has been floated many times ever since the Pioneer 10 and 11 and Voyager 1 and 2 probes passed through the system. Since then, it has been discovered that three of its four largest satellites (Europa, Ganymede, and Callisto could all have interior oceans.

What's more, multiple surveys of Europa and Ganymede have shown that their oceans could be warm enough to support life. For this reason, many are anxious to send robotic missions to look for signs of this possible life, and eventually crewed missions that could establish outposts.

For instance, in 1994, the private venture known as the Artemis Project was established with the intent of colonizing the Moon. They also drew up plans for a colony on Europa, which called for structures to be built out of ice on the surface (modeled on the igloo).

The authors also recommended creating long-term habitats within “air pockets” contained in the ice sheet. Given the presence of abundant water ice and volatiles like methane and ammonia, based on the surface could leverage these resources to create bases with mini-climates. 

A base on one or more of the Galilean moons was also advocated by Dr. Zubin in his 1999 book, Entering Space: Creating a Spacefaring Civilization (1999). These bases could help facilitate atmospheric mining among the outer planets – i.e., Jupiter and Saturn – to obtain Helium-3 fuel.

NASA also produced a study in 2003 that advocated for the creation of a base on Callisto, which they believed could be done by 2045. Titled "Revolutionary Concepts for Human Outer Planet Exploration" (HOPE), the plan called for the use of nuclear rockets to transport all the materials and robots needed to build a base there.

The destination was selected because of its distance from Jupiter, which means that it is exposed to far less radiation than its counterparts. It was stressed that a base there would be able to harvest water ice to create rocket fuel, making Callisto a resupply base for all future missions in the Jovian system.

Radiation is of particular concern when considering the moons of Jupiter. Owing to Jupiter's powerful magnetosphere and the existence of a belt of high-energy radiation, the moons of Io, Europa and Ganymede are subjected to varying amounts of harmful rays.

Io, which orbits within the high-energy radiation belt, receives about 3,600 rads of ionizing radiation per day - enough to kill very rapidly. Combined with its volcanic activity, soft mantle, and subsurface lava flows, Io is not a good place to live!

The surface of Europa gets about 540 rads per day, which still falls well within the lethal range. On Ganymede, things are a bit better due to its greater distance and the fact that Ganymede has a magnetic field, making it the only body in the Solar System (other than the gas giants) to have one. But it still gets 8 rads per day, more than a year's worth of radiation here on Earth.

Only Callisto falls into the safe range, receiving only 10 millirads from Jupiter a day. Of course, this gets worse when you add solar radiation and cosmic rays, but the fact remains, Callisto is the safest place to colonize in the Jovian system.

So while settlements could be built on Ganymede and Europa, both locations would require significant radiation shielding and settlements may only be possible beneath the icy surface. On Callisto, a surface environment could possibly be created, similar to what could be built on Ceres.

This would include a domed enclosure within one or more of Callisto's many, many impact craters. Candidates include the Valhalla, Asgard, and Adlinda ringed-craters, which measure 3800 km (2360 mi), 1600 km (995 mi), and 1000 km (660 mi) in diameter, respectively.

There are also craters like Heimdall and Loftn, which measure 210 km (130 mi) and 200 km (124 mi) in diameter, respectively. In any or all of these, domed structures could be erected that would span from rim to rim or along the crater floor.

Using silicate minerals harvested from Jupiter's Trojan and Greek asteroids, the soil could be created on the colony floor. Using locally-harvested water ice, ammonia, methane, and orbital mirrors, a microclimate could be created.

The Moons of Saturn

Then, there are the moons of the Saturn system. In advocating for the colonization of the outer Solar System, Zubrin claimed that Saturn, Uranus, and Neptune could be made into the "Persian Gulf of the Solar System" because of their rich resource base.

Zubrin identified Saturn as the most important of these because of its relative proximity to Earth, low radiation, and an excellent system of moons. For one, the system is one of the largest sources of deuterium and helium-3, which could be used as fuel sources for fusion reactors in the future. 

Saturn’s moons are also exposed to considerably lower amounts of radiation than Jupiter's system of satellites. This is because Saturn’s radiation belts are significantly weaker than Jupiter’s – 0.2 gauss (20 microteslas) compared to 4.28 gauss (428 microteslas).

This field extends from Saturn’s center to a distance of about 362,000 km (225,000 mi) from its atmosphere. This makes it much tighter to the planet than Jupiter’s radiation belt, which reaches distances of about 3 million km.

Titan was also designated as a good location for a human settlement because it is the only celestial body other than Earth to have a dense nitrogen atmosphere. There's also the large amounts of liquid and atmospheric methane and other hydrocarbons that moon boasts.

Another possible location is Enceladus, which periodically experiences plume activity around its southern polar region. In March of 2006, the Cassini-Huygens mission obtained possible evidence of liquid water on Enceladus, which was confirmed by NASA in 2014.

This water emerges from jets that are likely connected to an interior ocean that is just than tens of meters below the surface in some locations. This would make collecting water considerably easier than on a moon-like Europa, where water would have to be harvested from solid ice.

Data obtained by Cassini also suggested the presence of volatile and organic molecules in the interior, bolstering the case for life inside Enceladus. Density readings also suggest that beneath its outer layer of ice lies a core of silicate rock and metal.

These resources would be invaluable when it comes to creating a colony, especially if paraterraforming were involved. Much the same is true of Titan, which has abundant water ice in its mantle, as well as plenty of volatiles like ammonia and (in particular) methane.

Thanks to the Cassini-Huygens mission, astronomers have learned that Titan has methane lakes on its surface and a methane cycle that closely resembles Earth's hydrological cycle. Surveys of the moon also found that it has an environment that is rich in organic chemistry and prebiotic conditions.

Titan also orbits safely beyond the reach of Saturn's radiation belt, and its thick atmosphere may be enough to afford protection from cosmic rays. While Enceladus has a very tenuous atmosphere and orbits within Saturn's radiation belt, the low-levels (compared to Jupiter) mean that they could be mitigated.

In short, on both Titan and Enceladus (and possibly other moons within the system), self-contained colonies with mini-climates could be built that take advantage of this natural resource base. Water harvested from the icy surface could also be converted into fuel, making the Saturn system a stopover point for exploratory missions to Uranus, Neptune and beyond.

Along with the rich supply of deuterium and helium-3 from Saturn's atmosphere, the resources of the Saturn system could also be a major source of exports. In this way, a colonizing of the Saturn system could fuel Earth’s economy, and facilitate exploration deeper into the outer Solar System.

Looking Beyond

When it comes right down to it, there is no limit to where human beings could conceivably colonize in our Solar System. In addition to all the aforementioned examples, people could create habitats out of hollowed-out asteroids, on the moons of Uranus and Neptune, on Pluto and Charon, and even in the Kuiper Belt.

The farther we get from the Sun, the more heavily we are going to have to rely on technology to produce air and food. For example, in the outer Solar System and Kuiper Belt, settlers will probably have to rely on things like UV lighting to grow plants and process volatiles into breathable gases.

But even though increasingly artificial means might have to come into play, the name of the game remains the same. Through the creation and maintenance of natural environments, humanity could extend its presence further throughout space.

In the end, the limits are really only those imposed by our imaginations, finances, and the state of our technology. And considering that advances are being made all the time, the latter limitation probably won't remain an issue for long!

Further Reading:

• Paraterraforming - The Worldhouse Concept
• Paul Glister - Terraforming: Enter the ‘Shell World’
• Lunar Bases and Space Activities of the 21st Century: Chp. 11 Mars
• NASA - Science in Orbit: The Shuttle & Spacelab Experience, 1981-1986
• Space.com - Incredible Technology: How to Use 'Shells' to Terraform a Planet
• JBIS - "Shell Worlds - An Approach To Terraforming Moons, Small Planets and Plutoids"
• Lunar Bases and Space Activities of the 21st Century: Chp. 2 Lunar Base Concepts (LPI, 1985)