As paradoxical as it may sound, our solar system is of little interest to humans. The inner planets are a combination of inconvenient conditions and the absence of any resources interesting to mankind, while the outer ones (from Jupiter onwards) are not suitable for colonization at all, since they are gas giants. Their satellites could be of particular interest, but alas, their great remoteness makes them also unattractive.
However, humanity will still have to settle in the vastness of our system in order to survive elementarily, since the Earth's resources are not infinite. For this, all suitable space objects will have to be terraformed in the end, that is, to create conditions on them similar to those on a global scale. This is necessary, since the use of small bases for resettlement of the inhabitants of the Earth there is impractical.
Perhaps, in a few thousand years, mankind will be able to master the "disassembly into atoms" of gas giants, such as Jupiter and Saturn, in order to then make huge orbital stations or even entire planets from their substance. However, in the near future, we will have to confine ourselves to simpler engineering methods, similar to those that we use in everyday life on Earth.
If we consider the inner planets satellites: Mercury, Werner, Mars and the Moon, then, most likely, Mars will be colonized first. This has a very simple explanation. The moon, despite its proximity, is a lifeless and poor object, that is, it will not be of any interest to earthlings. Mercury and Venus, due to the conditions on their surface, consisting in a huge temperature (and on Venus also in pressure), may not be colonized even in the next millennia. But Mars … Mars is surprisingly ideal not just for colonization by earthlings, it is suitable for its complete transformation into a kind of Earth. What makes it so special?
First, it can have a thick, earth-like atmosphere. The first space velocity on Mars is 3.6 km / s; this means that the gravity of Mars is able to keep the atmosphere close to itself, preventing it from flying into space (for gases of the earth's atmosphere, the speed of movement is about 2.5 km / s). Second, water has been found on Mars; huge amounts of water ice are found not only in the polar glaciers of Mars, but also under its sandy surface. Water is the basis of our life, so if it is in large quantities on Mars, the chances of its colonization increase significantly. Thirdly, the structure of the soil of Mars is similar to the terrestrial volcanic sands, that is, at least, it is neutral to the flora of our planet; therefore, if nutrients are added to this soil, then it will be possible to grow potatoes on Mars. Well, and a little twist on the cake:a day on Mars, called "sol", has a duration of only twenty minutes less than on Earth, which will provide a certain convenience to the people living there.
However, all this may happen someday. At present, Mars is a very unattractive sight. The average temperature on the planet reaches -60 ° C, the atmospheric pressure is 100-200 times less than that of the earth, and the most common gas is carbon dioxide. And, nevertheless, these are the best conditions for colonization that humanity has. A separate problem is the lack of a magnetic field on Mars, which is the reason for the high level of radiation on the planet's surface; The optimal solution to this problem is to cover the module blocks in which the colonists will live with a layer of Martian soil.
How will this process take place? Most likely, colonization will begin with the construction of a completely autonomous base for the people living there. Even taking into account all the favorable factors, the flight to Mars from Earth by current means takes from 2 to 4 months with a favorable arrangement of the planets, which happens every 2 years. Thus, it is necessary from the beginning to count on the fact that help from the Earth in the event of an emergency situation may come with a serious delay and one must, at least, be ready to wait out a sufficiently long time in an autonomous mode.
The main task of the colony on Mars will be its constant growth, and it will be necessary to localize the extraction of natural resources and the production of station modules from them as much as possible in order to be independent of the supply of raw materials from the Earth.
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A separate problem will be the cultivation of food for the ever-growing population of the colony. Since it will be impossible to organize a cycle of continuous circulation of nutrients (similar to those that exist in the ecosystems of the Earth) in the early stages, some inconveniences in the nutrition of the colonists are not excluded. Therefore, options for feeding with freeze-dried food are quite possible; the use of an "amino acid cocktail" or even industrial food is not excluded. The latter is now receiving a lot of attention in China and Japan, so anecdotal "plastic porridge" may become a very real thing for the astronauts of the future. If it is possible to implement a completely autonomous power system in the colony, this stage of colonization can be considered completed.
When the population of Mars exceeds the number necessary to organize large-scale industrial production, the second stage begins: the construction of complexes for enriching the atmosphere of Mars with oxygen and nitrogen. Thus, the conditions necessary for the existence of mankind without any protective means will be achieved; taking into account the remoteness of Mars from the Sun, just one layer of the atmosphere will be enough to protect the surface from radiation.
Futurologists allot about a hundred years to the first stage of the colonization of Mars, and about a thousand years to the second. In the historical framework, this is, of course, a trifle, but will we have enough time? The fact is that if humanity continues to grow and develop at a similar pace, in 200 years we will face an unmerciful end in the form of death from hunger. And we can say that at present humanity is awaiting a serious test of survival: will it be able to competently dispose of the rest of its resources in order to finally get out of its cradle-Earth?