The question of exploring and conquering other planets has been a rather frequent topic in discussions about the future, especially because that could allow humankind to survive in case of a planetary catastrophe. But what part of humanity would be saved in that case?
Indeed, in the event of the destruction of planet Earth many of our artefacts and achievements would not be saved, at least considering our actual or foreseeable means of transportation. Simply put, while it could be relatively easy to send the works of Picasso in a spaceship to another planet, the same cannot be said about the pyramids, Spanish castles or even entire museums. In the same token, not all of humanity would be saved.
Those actually living in other planets would almost certainly survive as representatives of our species, save the case that the destruction of Earth would lead them into some sort of collective suicide. Of course, animal and vegetal species, aside from those transported to other planets, would also not be saved. Thus, from a brief analysis of this issue it becomes apparent that saving humankind or life on Earth would be most likely a partial achievement.
So, considering that - as we have just seen - it would be almost impossible to save everyone and everything, the nature of things makes it inevitable to consider some sort of selection process. Who and what are going to be saved, and why?
One of the factors that will certainly have influence over the results would be the urgency of the matter: if a catastrophe of a global scale occurs with no warning, then only those outside the planet and its neighbourhood would be saved. If the thing happens only with a short warning, then those who would likely survive are those owning vehicles capable of taking them out of the disaster area, as well as those able to pay for the ride, be it as passengers or as owners of their own, newly-purchased ships. So far, the situation doesn't seem to be much different from what we have already seen on Earth's history.
But what would happen if news of the impending, inevitable and terminal disaster reach the wide public long before it happens? Certainly, some types of global catastrophes could then be avoided, or plans to save everybody could be put in place, at leas in some cases such as an incoming asteroid. In such a case, perhaps something could be used to destroy or deviate it. However, in some other scenarios it could well be impossible to do anything to avoid the final result even with adequate warning for a lack of sufficient technology, like in the case of an approaching black hole or the explosion of a supernova. What then, after people come to grips with the fact that most of us will be doomed?
The most immediate consequences:
Panic: Irrationally, many individuals would try to seek shelter even when there is no chance to survive. That is, what could produce to hid beneath a table when the problem is a black hole that comes to gravitationally eat the entire planet?
The fall of law and order: Why would people continue to respect the state and its laws when there is no future? Why pay taxes? Then, how would the state act? It is likely that this would lead to violent, repetitive rebellion, repression and ultimately, dictatorship in order to keep control of the situation.
The disappearance of all means of salvation: those in possession of any sort of "lifeboat" - in this hypothetical case, spaceships capable of taking anyone out of the planet and onto the next viable stellar body - would just jump inside and leave, use their ships for a profit, hide them in fear of having their machines seized by the authorities or people; or perhaps they will lose their ships at the hands of others who would then behave regarding the ships as their former owners did.
The struggle for resources
The means required to carry all the people in the planet will not exist even within a far more advance human society, plan and simple. We don't have transportation means to move all the people of the planet at the same time right now, and that's for a simple reason: it is not economically feasible. No body needs trains for all, aircraft for all, ferries for all and even bikes for all. It makes no economic sense to build even the simplest transportation infrastructure to cover all the bases, and likely the same principles will be applied in the future in the case of even interstellar spacecraft.
Moreover: As more capabilities are required of any transportation technology, the costs related to manufacture, acquisition and operation increase. This is why even kids can buy a bicycle but only their parents in practical terms can buy a family car, provided that they are at least middle class folks. Then, fewer people can buy a sailboat and even fewer can own an airplane or helicopter. Who would be in a position to own a spaceship?
This means that only the rich and powerful could save themselves, be it because they had spaceships long before the disaster happens, or because they could buy or force violently their way out of the problem. The masses in general would only be able to compete with the rich and powerful in such a survival race taking advantage of their intrinsic capability to raise money collectively or thanks to the use of force, taking advantage of larger numbers.
Then we should make a conceptual but very significant difference between what means to save the whole of humanity and humankind as a species, because in order to ensure that there will be humans after such a catastrophe, only a limited number of survivors would be necessary. Plus, taking into account that there would also be a need to save other plant and animal species in order to save Earth as a whole, a point would be reached in which it would become more important to use the available cargo volume I the existing ships to carry plants and animals rather than people. And to this we should add that while it would be in principle desirable to save everyone, a new host planet would probably not be in a condition that would allow for the sudden invasion of billions of humans and provide them with food and shelter by extraction of its natural resources plus what could be eventually be carried in their lifeboats. IF all the people were transported in such a way, at the same time and to the same place, instead of being a salvation, the destiny planet would be just another place to die because people will starve, die of exposure, or fight for scarce resources.
Triage: How to decide whom to save
People would react if confronted with a planetary-grade catastrophe in a way probably very similar to what history teaches us about past apocalyptic events: It is well know that Pompeii was largely evacuated before it was actually covered by ashes of the then erupting Vesubius. Since people in that town could see the volcanic material falling unto them they became aware of the danger and left the area. Meanwhile, the citizens of Herculaneum just went about their usual chores because no ash was falling from the sky where the town was located. Almost nobody evacuated because they felt no danger or they simply denied the situation; however, during the night a piroclastic cloud simply covered and killed them all. It wasn't mass suicide but simply ignorance and denial, and it is likely that in the event of a global catastrophe a certain number of people will simply abandon themselves to die.
An orderly evacuation and the rescue of anything of value for the continuity of humankind and Earth as a whole would likely be left in the hands of governments, and these will likely work in unison in order to coordinate efforts and avoid redundancies. Such an evacuation would likely take place along a variable number of private efforts, but it would be stricter because some sort of triage would be required to determine more or less objectively who would be in a condition to board a ship to leave the planet for good.
Certain kinds of people would almost certainly be rejected:
Old people and terminally-ill patients.
People with no abilities or useful skills.
Men and women incapable of procreating.
Criminals, prison inmates and mentally-ill patients.
It is easy to see according to this hypothetical scenario that the rich and powerful would be at an advantage, no matter from where they get their power or money: some would be members of the political caste, while others would be millionaire businesspeople or just VIPs with deep pockets. Common people could eventually try some actions to save themselves taking advantage of their larger numbers. However, while a large mass of people can raise even large sums of money, put pressure on politicians or even build some on their own, it is doubtful that the capital that they would eventually collect it could be used to build a sufficient number of ships or to purchase all the places necessary to evacuate everybody. Plus, forceful attempts to seize existing ships could eventually be stopped with the use of weapons. Once again, the common Joe would be doomed.