Comet Elenin, originated in the Oort Cloud
Main page: Asteroids (theoretical models)
The Oort Cloud is the outermost region of the Solar System. We have never seen any object inside the Oort Cloud. Its existence is known only from theoretical models. All we know is the presence of some very long period comets that are supposed to have originated there.
Other stars might also have an Oort Cloud. Also, similar objects might exist and form on their own in the interstellar environment.
Theory[]
The Oort Cloud is located very far away from the Sun. There, temperatures are extremely low, so that all volatiles (including methane, nitrogen, hydrogen and perhaps even helium) are solid. Small grains of dust, ice and solidified gasses can, with the help of gravity, collide together and form tiny celestial bodies. This process can appear anywhere: at the border of a stellar system or in the interstellar environment. It only calls for a little gravity to exist. The mass of a small dust cloud can be enough if the particles are moving slowly enough. Also, the source of gravity can be a small asteroid ejected from its inner orbit.
As far, nobody has ever seen an object inside the Oort Cloud. There are a few highly elliptic celestial bodies (like Sedna) that at perihelion reach the inner Oort Cloud. We don't know for sure what they are. The fact that their orbits are so elongated made astronomers suppose that at some point the Solar System came very close to another star. If that is true, much (if not all) of the Oort Cloud was disrupted from orbit into interstellar environment. Another theory suggests the existence of one or more large planets far beyond the orbit of Neptune. Again, if this is true, their gravity pull might have cleared part of the Oort Cloud.
Observing an asteroid inside the Oort Cloud is almost impossible, because at that distance from the Sun, there is not enough light to make the object visible even with the highest telescopes available.
Oort Comets[]
What we do know is the existence of some very long period comets. Some of them might originate in the Kuiper Belt. Interactions with the four giant planets might have changed their orbits.
The objects that might have been created inside the Oort Cloud are long-period comets that usually disintegrate when reaching close to the Sun. Kuiper Belt comets are usually solid (see Kuiper Belt Asteroid Terraforming for a better explanation). Why Oort Cloud comets are more fragile is not clearly known. They must be made of soft materials, not linked together, like a pile of rubble. My personal explanation is that they never encountered any source of moderate heat to alter their structure and to make them more compact.
One of the most known comet originated in the Oort Cloud is C/2010 X1 (Elenin), also known as Comet Elenin (named after its discoverer). The comet has disintegrated after it was hit by a corona mass ejection. After that event, no outgassing could be seen, only the dust cloud. This means that the comet was held together by a very weak force. The event also gives us useful information, proving that Comet Elenin had some dust (solid material) among with many volatiles.
Environment[]
Solar magnetosphere does not reach the Oort Cloud. This means that a base is protected from the solar wind, but exposed to interstellar radiation. Also, the Oort asteroids are far too cold to sustain an internal dynamo. Temperatures are extremely low, so that no gas should sublimate. We might find solid hydrogen on the surface. Luminosity is very low. At the end of the Oort Cloud, she Sun shines just like the bright star Sirius. Even at the inner edge of the cloud, a human might not have enough light to see. So, there is no way that light can be used to illuminate or to grow plants.
Because solar gravity is very low, objects inside the Oort Cloud must be at high distance one to another. Otherwise, they would be attracted and form larger bodies, like planets. However, NASA does not rule out the possibility of an outer planet to exist. Speed of the Oort asteroids is very slow. Their movement might be chaotic and highly affected by passing stars.
Structure[]
Oort Cloud asteroids might be very fluffy. Their matter is held together by a weak gravity force. If there is enough mass and gravity is higher, they should be more compact. If the mass is even greater, they should be round. As said above, NASA does not rule out the possibility that a remote Oort planet exists.
Chemical compositions of Oort asteroids is unknown, but we can conclude that they contain high amounts of solid like methane, ammonia, nitrogen, hydrogen, carbon dioxide and monoxide, molecular oxygen, argon, xenon and maybe even helium. After a long exposure to the interstellar ionized radiations, Tholins are expected to form. Since tholins are organic molecules, they can be useful in some branches of chemical industry.
As shown by Comet Elenin, there are compounds that on Earth will remain solid, like dust and salts. We must also not forget that probably the most abundant substance in the Oort Cloud remains water ice.
Technical challenges[]
Building a base in the Oort Cloud is a real challenge.
First of all, the base will be completely isolated from the inner Solar System. Because radio waves travel with the speed of light, you will have to wait weeks or months (or even two years, at the outer border of the cloud) to receive an answer from Earth. Sending a manned spacecraft so far is, with current technology, not possible.
Setting a base on a large asteroid, with enough gravity to make it compact, will be the best solution. From there, the settlers will only see the Sun as a bright star. The base will need a central, powerful generator, to provide all heat and energy. To generate food and to recycle air, plants will need to be illuminated.
Building a base on a small, fluffy asteroid, will be harder. There is no solid structure to anchor the building. The asteroid body will not provide shelter for internal paraterraforming. In fact, it could simply disintegrate. Heat from human activities can transform the asteroid into a comet.
On a world made of ices, there are not many metals. Settlers will, at first, have to take from home all what they need. All building parts, all base modules, will have to be brought from the inner Solar System. The existing tholins might prove useful. Organic materials can be converted into plastic, rubber, fibers and building materials. Still, metals will be needed in many activities.
The colony[]
Now, let's imagine a colony on an Oort Cloud asteroid. The asteroid is fluffy and constructors have built everything through it. At the surface, in two different places, are the two nuclear generators that use deuterium for fusion and produce all heat and electricity needed. Then, in another place, there is a base, with platforms for spaceships, also equipped with high-gain antennas. The base uses water to produce hydrogen and oxygen, to refill the space vehicles. Because ground is made of a dust more diffuse then any dust on Earth, the base is strongly anchored to the ground.
Inside the ground, covered with layers of tholins, there are 4 independent greenhouses. They produce all the food and air the colony needs. Each one has rooms for plants and a central room where bacteria degrades organic residual matter into mineral compounds needed for plant life. Almost all buildings inside are made of plastic created from tholins.
The residential area consists of individual apartments, merged into 3 compact blocks. All walls, all furniture and almost all objects are made from tholins. The blocks are covered with a thick layer of tholin polymers, covered again with ice. The tenuous amounts of metals found in the asteroids are used only where nothing else can be used. Light is sent through fiber glass to each place where it is needed. Also, heat radiates as infrared light and is distributed through fiber glass.
Somewhere inside the asteroid, there is a special chamber with back-up products: food, fresh water, medical equipment, oxygen and a small back-up generator.
There is no cement and there are no stones. Since the ground is so fluffy and unstable, all structures will have to be well anchored. How? The most common and cheapest material is water ice. Only inside inhabited places, temperature will be higher then water melting point. In all other parts (including walls), it will never be hotter then -200 degrees Celsius.
Transportation[]
Asteroids inside the Oort Cloud must be at high distance one to another, probably higher then the distance between Earth and Saturn. If they were closer, their gravity would have forced them to collide. Also, if they were in a larger amount, their gravity perturbations would have made the cloud detectable. In such conditions, colonies will be far away one from another. If someone will create a secret base on an asteroid there, it could remain undetected forever.
Given the high distances, transportation will be difficult. Sending cargo from one asteroid to another will be possible with current technology and with little fuel consumption, but the time needed will be high. When arriving, the spaceship will need to slow down. The weak gravity of the target asteroid will not be enough to capture it into orbit.
However, because of the high distance between asteroids or the distance towards the inner Solar System, there is a far more efficient way to send cargo and passengers. A ion engine has a very low acceleration rate. The ion engine used by Dawn generated a thrust of 10 km/s and worked a few years. A ion engine might accelerate a spaceship for half of its journey, then decelerate it. Still, with current technology, a journey between two colonies will take years if not decades. Sending cargo towards the inner Solar System and back, with a continuously working ion engine, will take centuries.
In the end[]
Colonies inside the Oort Cloud will be the most remote places of the Solar System. Without metals, but with volatiles and organic materials, mankind will have to re-invent itself and design new ways of living, like it never did before.