Internal paraterraforming is an unconventional way of inhabiting a celestial body. Instead of building an environment on the surface, all can be constructed inside. In case of a planet, this method has limited advances, since large caverns will collapse under rock weight. For smaller bodies, like asteroids, it is more easy to build. It is possible to extract entire core of an asteroid and colonize the cavern.
Advantages and challenges Edit
Building below surface has some major advantages. First of all, the colony will be safe from radiations (solar, radiation belts, cosmic rays). Second, the temperature will not vary as much as at the surface. Without an atmosphere, temperature dramatically drops below -200 C on night time. This method can be used even further away in Kuiper Belt. A third advantage is that, unlike classic Paraterraforming, the colony will be safe from meteors.
The colony needs to be made in caverns, large enough to host it and with walls and roof secure enough to prevent it from collapsing. The cavern can be natural, artificial or both. After digging, builders will need to secure the walls (if needed) and to cover them with a material similar with cement (to protect people from falling rocks and to keep direct contact with the rock; oxygen can be corrosive for many rocks). Icy asteroids would melt at room temperature, so a thermal insulation will be required. That insulation will not require high costs. A wall of ice of 50 cm thick will separate the extreme -220 C outside from a -30 C inside the wall. Then, a second layer is needed, to keep average temperature at +20 C inside the colony.
One major problem would be energy production, just like in any other human colony. If luminosity is strong enough, solar panels can be placed on the surface. There is even a more advanced technology available. Some solar panels can act like lens and mirrors and can force light to follow optic cables. A filter is required, to absorb ultraviolet. Then, through a network of glass fibers, light can be sent directly to the colony. If there is not enough luminosity, the only source of energy will be a nuclear power plant. A third source of energy might be available. Some planets or moons might have their core heated enough to feed a geothermal power plant. And if the core is hot, a colony on a frozen planet can be built deeper beneath the crust.
Some small asteroids are found to be a pile of rubble (space debris held together by a weak gravity). In this case, at least in theory, it is possible to build a dome in the center of that asteroid, while vehicles will move like worms from the dome to the surface.
Costs and maintenance Edit
Building a cavern is more expensive then working on the surface. So, for short term, there will be higher costs. But on long term, there will be more advantages. Maintenance costs will be smaller. Keeping temperature variations in line will not be hard. The fact that you need to bring in all the light from outside, will be more expensive. There will be almost no costs with radiation shields and meteor damages. The only exposed parts are the solar panels.
Since temperature will not change much, air will trap moisture until it will be saturated. No significant winds will form. So, air must be recycled. At some place, air needs to be cooled, to extract moisture, then reheated and recirculated. Water must also have its own cycle. So, the cave will need to have an air recycling plant. Another version is a long and wide tunnel, with one end cold (5 C) and the other one hot (35 C). If there is an altitude difference between the two ends and the cold end is higher, condensing water will naturally flow through the tunnel, creating a river.
Life in the underground Edit
Depending on available size, you would feel like in a room, a warehouse or like in a closed stadium. Larger caverns could still exist, but they will need central pillars or walls to support the weight. Very long caverns should be more useful. They could host entire cities. however, it is more likely that homes will be dug inside the rock. Building inside the cavern would be more expensive. The surrounding walls might host homes for the entire population. One major difference is that, when you look up, you will not see the sky, but the roof of your cavern, even if it can be painted in blue and illuminated by a fake sun.
Population density is limited to each cavern's resources, but global population is not limited. New caverns can be created, making room for new settlers.