I think science fiction has a lot to do with this. The idea in the Mars trilogy for instance that Mars could be made habitable within a few centuries. The most optimistic projections by the Mars society require a thousand years of high technology (giant mirrors in space, greenhouse factories, etc) to get to the stage where humans would be able to survive without a spacesuit, using aqualung type air breathers - and trees grow on the surface but no animals or birds yet. And that's optimistic. After all on Earth it took millions of years - and Earth started off with far more water, and closer to the sun.
Also many ways it can go wrong. There's the idea that it doesn't matter, that Mars is a "tabula rasa" where you can start again. But it isn't really like that, because whatever you do to Mars will change it, for instance if you introduce life to Mars, then you now have microbes on Mars that weren't there before. The optimists assume that these microbes will be beneficial and aid terraforming - but what if they are not - or evolve to be harmful to humans and to counteract terraforming attempts, or they produce biproduct gases that are harmful to humans? How then could you ever rid Mars of these harmful microbes, if this happened?
Also I think not often realized - that these ideas to terraform Mars are temporary. Mars lost its atmosphere and oceans in the past, used to have quite extensive oceans and a thick atmosphere though never as much as Earth. We don't know quite how it lost them, though the lack of a magnetic field and lower gravity may be something to do with it. Whatever process caused this is likely to happen again. So, 500 million years from now, when some descendants of present day Earth life might possibly need Mars, it would have lost all its atmosphere and oceans yet again. Maybe this time in a state where it is far harder to recover them.
Also, 500 million years from now - that is long enough for humans to evolve again a second time from the very earliest multi-cellular lifeforms. Tiny creatures we can only see in a microscope may well be the ancestors of the intelligent beings that might be on Earth by then. So - is it perhaps a little early to think about what would be the best way to manage Mars to deal with the issues ancestors of some Earth lifeforms may face half a billion years into the future? We don't even know what kind of environment they would be adapted to or what kind of atmosphere they would like.
As for escaping present day disasters on Earth - well it is hard to come up with anything that would make Earth less habitable than Mars. I've not seen any suggestion except for the very unlikely possibility that Earth gets swallowed by a black hole (and if this was possible, then the sun is a far larger and more likely target than any of the planets - but stars don't blink out so encounters with black holes like this must be exceedingly rare, if they happen at all).
If the idea is to escape diseases - well the main advantage then is quarantine. But it would be easy to set up a quarantined colony on the Earth if this was a concern. And if Mars did ever get colonized there would be frequent traffic between the planets, probably voyages of days rather than months - and any diseases on Earth would soon spread to Mars just as they spread to other countries.
As for asteroid impacts - the chance of a large asteroid hitting the Earth before 2100, say, is less than 0.001%, and there is pretty much zero chance at all of an asteroid that destroys all human life on Earth including, e.g. people who are underground at the time or in submarines. There were impacts that large in the first few hundred million years of the solar system, but not any more - I mean an impact large enough to boil the oceans or remove the Earth's atmosphere.
The movie asteroids are scaled up enormously - the asteroids that we have evidence of from geological history are at most about 10 kms across, so small you wouldn't see them at all against an image of the entire Earth. They could cause global firestorms. They could put dust into the atmosphere and create an impact winter. But can't do anything that would make Earth anything like as uninhabitable as Mars.
And after the impact, the Earth would be the very best place to rebuild society - anyone on Mars would be best advised to come back to this planet where at least you can breath the atmosphere and have plenty of water.
So - I think myself it is based on science fiction, which is sometimes predictive, but it is not science fact, and often gets things wrong.
In my view, it is sad that he has fixed on colonizing Mars rather than - for instance - setting up a colony using Stanford Torus type habitats with materials from the asteroid belt - or a colony on the Moon - because Mars is one of the places in the solar system with most interest for the search for past life and present day life. This makes it a place that we should keep free of Earth life at least until we understand it better, otherwise we risk going there just to find life we brought to the planet myself.
But - he would need to show that his colony passes planetary protection standards for Mars, which I think is likely to be impossible. So I can't see it happening myself. Not in the near future at any rate until we know more about Mars.
In the long term, who knows what we might discover, and what our objectives might be. Large solar system, many places to explore, many potential places where humans could live if we do decide to colonize and it becomes practical. Or we might just stay on the Earth and never colonize anywhere else, just temporary settlement. I see no problem with that myself. There are many places on Earth that we don't colonize, including the sea bed, for instance, or Antarctica, or the tops of the highest mountains.
So why do we have to colonize space?
Though acting to protect the Earth from space hazards - e.g. mapping all the meteorites and asteroids in the solar system - and taking steps to prevent them from hitting Earth in event that we find some that do - that is all worth doing. Also mining and returning resources to Earth if done with care. And beaming solar power from space back to Earth. Many ways that space missions can benefit the Earth. And also exploration to find out more about the solar system - and exploration using telerobotics to explore places that humans can't visit either because they are too hazardous for humans - or because we risk contaminating them with Earth life.
For an example perhaps similar to Mars - nobody has any wish to colonize Lake Vostock, in Antarctica - the oxygen rich ice covered lake which may have been isolated from the surface for millions of years. We want to know what life has managed to survive there, without any contact with the surface, so the last thing we want to do is to send humans there in a sub. Is same with Mars, far more isolated, by the vacuum of space.
In my view, the last thing we ought to do right now is to send humans there unless we are sure they won't contaminate it. I think that if the time comes for Elon Musk's ideas to be practical, then - yes the colonization movement is strong in the US (not elsewhere so much). But people also feel strongly about environmental issues, and if they come to see Mars as an environmental issue, of humans spreading our problems to other planets rather than solutions, I think that will stop it in its tracks. And it is really hard anyway I think to see how it can be compatible with planetary protection.
For all this - I'm talking about - at present state of knowledge. It may well change as we find out more but based on our current understanding of Mars and Earth I can't see it happening.
For the terraforming Mars issues:
Trouble With Terraforming Mars
And for more context to all this, see some of my other answers here on Quora:
Mars and Space by Robert Walker on Lists of My Answers
Also many ways it can go wrong. There's the idea that it doesn't matter, that Mars is a "tabula rasa" where you can start again. But it isn't really like that, because whatever you do to Mars will change it, for instance if you introduce life to Mars, then you now have microbes on Mars that weren't there before. The optimists assume that these microbes will be beneficial and aid terraforming - but what if they are not - or evolve to be harmful to humans and to counteract terraforming attempts, or they produce biproduct gases that are harmful to humans? How then could you ever rid Mars of these harmful microbes, if this happened?
Also I think not often realized - that these ideas to terraform Mars are temporary. Mars lost its atmosphere and oceans in the past, used to have quite extensive oceans and a thick atmosphere though never as much as Earth. We don't know quite how it lost them, though the lack of a magnetic field and lower gravity may be something to do with it. Whatever process caused this is likely to happen again. So, 500 million years from now, when some descendants of present day Earth life might possibly need Mars, it would have lost all its atmosphere and oceans yet again. Maybe this time in a state where it is far harder to recover them.
Also, 500 million years from now - that is long enough for humans to evolve again a second time from the very earliest multi-cellular lifeforms. Tiny creatures we can only see in a microscope may well be the ancestors of the intelligent beings that might be on Earth by then. So - is it perhaps a little early to think about what would be the best way to manage Mars to deal with the issues ancestors of some Earth lifeforms may face half a billion years into the future? We don't even know what kind of environment they would be adapted to or what kind of atmosphere they would like.
As for escaping present day disasters on Earth - well it is hard to come up with anything that would make Earth less habitable than Mars. I've not seen any suggestion except for the very unlikely possibility that Earth gets swallowed by a black hole (and if this was possible, then the sun is a far larger and more likely target than any of the planets - but stars don't blink out so encounters with black holes like this must be exceedingly rare, if they happen at all).
If the idea is to escape diseases - well the main advantage then is quarantine. But it would be easy to set up a quarantined colony on the Earth if this was a concern. And if Mars did ever get colonized there would be frequent traffic between the planets, probably voyages of days rather than months - and any diseases on Earth would soon spread to Mars just as they spread to other countries.
As for asteroid impacts - the chance of a large asteroid hitting the Earth before 2100, say, is less than 0.001%, and there is pretty much zero chance at all of an asteroid that destroys all human life on Earth including, e.g. people who are underground at the time or in submarines. There were impacts that large in the first few hundred million years of the solar system, but not any more - I mean an impact large enough to boil the oceans or remove the Earth's atmosphere.
The movie asteroids are scaled up enormously - the asteroids that we have evidence of from geological history are at most about 10 kms across, so small you wouldn't see them at all against an image of the entire Earth. They could cause global firestorms. They could put dust into the atmosphere and create an impact winter. But can't do anything that would make Earth anything like as uninhabitable as Mars.
And after the impact, the Earth would be the very best place to rebuild society - anyone on Mars would be best advised to come back to this planet where at least you can breath the atmosphere and have plenty of water.
So - I think myself it is based on science fiction, which is sometimes predictive, but it is not science fact, and often gets things wrong.
In my view, it is sad that he has fixed on colonizing Mars rather than - for instance - setting up a colony using Stanford Torus type habitats with materials from the asteroid belt - or a colony on the Moon - because Mars is one of the places in the solar system with most interest for the search for past life and present day life. This makes it a place that we should keep free of Earth life at least until we understand it better, otherwise we risk going there just to find life we brought to the planet myself.
But - he would need to show that his colony passes planetary protection standards for Mars, which I think is likely to be impossible. So I can't see it happening myself. Not in the near future at any rate until we know more about Mars.
In the long term, who knows what we might discover, and what our objectives might be. Large solar system, many places to explore, many potential places where humans could live if we do decide to colonize and it becomes practical. Or we might just stay on the Earth and never colonize anywhere else, just temporary settlement. I see no problem with that myself. There are many places on Earth that we don't colonize, including the sea bed, for instance, or Antarctica, or the tops of the highest mountains.
So why do we have to colonize space?
Though acting to protect the Earth from space hazards - e.g. mapping all the meteorites and asteroids in the solar system - and taking steps to prevent them from hitting Earth in event that we find some that do - that is all worth doing. Also mining and returning resources to Earth if done with care. And beaming solar power from space back to Earth. Many ways that space missions can benefit the Earth. And also exploration to find out more about the solar system - and exploration using telerobotics to explore places that humans can't visit either because they are too hazardous for humans - or because we risk contaminating them with Earth life.
For an example perhaps similar to Mars - nobody has any wish to colonize Lake Vostock, in Antarctica - the oxygen rich ice covered lake which may have been isolated from the surface for millions of years. We want to know what life has managed to survive there, without any contact with the surface, so the last thing we want to do is to send humans there in a sub. Is same with Mars, far more isolated, by the vacuum of space.
In my view, the last thing we ought to do right now is to send humans there unless we are sure they won't contaminate it. I think that if the time comes for Elon Musk's ideas to be practical, then - yes the colonization movement is strong in the US (not elsewhere so much). But people also feel strongly about environmental issues, and if they come to see Mars as an environmental issue, of humans spreading our problems to other planets rather than solutions, I think that will stop it in its tracks. And it is really hard anyway I think to see how it can be compatible with planetary protection.
For all this - I'm talking about - at present state of knowledge. It may well change as we find out more but based on our current understanding of Mars and Earth I can't see it happening.
For the terraforming Mars issues:
Trouble With Terraforming Mars
And for more context to all this, see some of my other answers here on Quora:
Mars and Space by Robert Walker on Lists of My Answers
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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