This is a common topic in science fiction, but migration to other planets in our solar system will actually be much more difficult and more serious than you might imagine. The great Russian scientist, much ahead of his time, Konstantin Eduardovich Tsiolkovsky, well expressed the idea of the need to populate other parts of our galaxy: "The Earth is the cradle of mankind, but you cannot stay in the cradle forever." This idea has become a part of science fiction and should soon become the meaning of the future of humanity. Travel to the stars is often referred to as the fate of humanity, a measure of success as a species. But for a hundred years since such a future was proposed to us, we realized that settling in the galaxy in general may turn out to be an overwhelming task for humanity.
The problem that unites all the other problems associated with this idea is the enormous size of the universe, which people did not realize when they thought to go to the stars. Tau Ceti, one of the stars closest to us, is 12 light years away - 100 billion times farther from Earth than our moon. A large quantitative difference turns into a qualitative one; we simply cannot send people to such a huge distance in a spaceship, because the spaceship will be too poor an environment to support the centuries-old life of people on the ship. Instead of a spaceship, we would like to create something like an ark for space travel that is large enough to support a community of people, plants and animals in a completely closed ecological system.
At the same time, it should be small enough to accelerate to a relatively high speed, thereby reducing the time of exposure to cosmic radiation on travelers and possible breakdowns in the ark. It all leads to the fact that the larger the ark, the better, but again, the larger it is, the more fuel it must carry to get to its destination. And if you do it less, there will be problems with metabolic flow and ecological balance. Island biogeography points to the problems that can arise from miniaturization, but the isolation of the space ark will be much stronger than the isolation of any of the islands on Earth. Design imperatives big and small overlap, killing any intermediate or foreseeable project.
The biological problems that can arise from the dramatic miniaturization, simplification and isolation of the Ark, regardless of its size, must include possible impact on our microbiomes. We are not autonomous units; eighty percent of the DNA in our bodies is not human DNA, but a vast variety of small creatures. And this variety of living creatures performs the function of dynamically maintaining our health, relying on a complex system that includes processes on the Earth's surface, gravity, magnetic fields, chemical composition, atmosphere, insolation and bacterial background. Traveling to the stars means getting rid of all these variables and trying to replace them with artificial ones. We do not know what parameters will be impossible to replace, since it is incredibly difficult to simulate this whole picture. Any space ark will begin with an experiment in the laboratory, in which experimental animals will participate. The first generation of people on the ship will be there of their own accord, but their descendants are no longer there. And generations of descendants will be born in tiny rooms, trillions of times smaller than the Earth, and they will have absolutely no chance of escape.
In this radically reduced environment, strict rules will have to be followed to avoid jeopardizing the experiment. Reproduction will no longer be a choice, as the population in the ark will have to be kept at minimum and maximum values. Many works will be required for the ark to work, so the work itself will no longer be a choice. Severe restrictions can lead to norms of behavior. The position on the ship will resemble a totalitarian state.
Of course, in the field of sociology and psychology, it is difficult to make predictions, since a person gets used to everything. But as history has shown, people react poorly to rigid state and social systems. Combine these social constraints with permanent isolation, expulsion from your home planet, and possible health problems, and the likelihood of psychological and mental difficulties is very high. It is difficult to imagine how such a society will be stable.
Yet in the nature of man lies inventiveness and adaptability. It is quite possible that all the stated problems will be solved, and people in the confined space of the ark will successfully reach the nearest planetary system. Then their problems will only begin.
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Any planetary body that travelers try to populate will be either alive or dead. If there is life on it, contact with alien biology can be fatal, otherwise it will require careful research. On the other hand, if the planetary body is inert, the newcomers will have to terraform it using local resources and the energy they bring with them. Everything will swing extremely slowly, perhaps over the centuries, and all this time people will have to live in an ark or its equivalent on the surface of an alien planet.
It is also possible that the newcomers will not be able to tell if the planet is alive or dead, as we are now with Mars. They will still face the problem, but they will not know which of the two solutions will have bad consequences, which will slow down the process of solving the problem.
In conclusion, we can say that interstellar travel will present extremely difficult problems to solve, and arriving in another star system - a different set of problems. Together, these problems can be completely solvable, but with great difficulty, which significantly reduces the colonists' chances of success. The inevitable uncertainties point to the need for a strong ethical base before starting such a project. To begin with, we should create and demonstrate a sustainable human civilization on Earth, the achievement of which will allow us to learn how to build a viable ecosystem of the ark. Then we will have to roll the ark around our Sun for many years, studying possible breakdowns or the stability of the ship as a whole, until we are convinced that it will survive. And third,we'll have to do a fair amount of robotic missions to nearby planetary systems to see if it is theoretically possible to establish a colony on them.
Until all these steps are completed, humans will not be able to successfully travel and populate another star system. The very preparation for this is a project for many centuries, and the first step that will lead us to the creation of a stable and long-term civilization depends on its success. But if we do not achieve stability on our own planet, there is and cannot be any planet B.