Anyone can claim anywhere in space, of course. You could just point to the Moon or Mars and say “I claim the Moon or Mars”. and various people have done essentially that - laying claim to the Moon, Sun, the asteroids, all extra terrestrial objects etc. But according to the Outer Space Treaty no signatory can uphold that claim. So there isn’t really much point in it.

You do however own your spaceship and habitat. And there is some precedent for a safety region around it, as the ISS has one. That’s one basis for establishing a form of ownership consistent with the OST.

There are other ideas to extend some form of ownership consistent with the Outer Space Treaty.

NO PROBLEM FOR A FEW THOUSAND IN SPACE

I don’t see it as much of a problem myself for as long as we have only a few hundred or a few thousand in space. To have a million or a billion people in space is way beyond our current situation. Mars, and the Moon are no “New World”. They are extremely hostile places. They aren’t places where you can set up home unless you have other reasons for being there. No air, no fertile land, no liquid water.

TERRAFORMING IS SCIENCE FICTION

The idea that you can terraform Mars is just science fiction; there are ideas for ways that it could be done, but we have never terraformed a planet and to do that would be a thousand year project just to get to the point where trees grow there, with no breathable oxygen yet. And to get that far would be a mega project involving giant mirrors the size of a planet in space, or greenhouse gas factories mining cubic kilometers of fluorite ore every century just to create the greenhouse gases to keep it warm, also probably involve redirecting, and smashing numerous comets into Mars to create an atmosphere etc.

Then, if somehow magically we could give Mars an Earth atmosphere it would be too cold, unless we had those planet scale mirrors or non stop greenhouse gas factories powered by 500 nuclear power stations running continuously. Even an Earth pressure CO2 atmosphere on Mars is not warm enough - it’s a bit of a mystery how it had seas in the past though it might be that they were liquid only at times when Mars’ orbit was much more eccentric than it is now, also with the poles so tilted it had equatorial instead of polar ice caps - and probably froze over every two years. At any rate a CO2 atmosphere on Mars right now would not keep it warm enough for an Earth like ecosystem, and to remove much of that carbon to make it into an oxygen atmosphere would make it even worse (and would also take 100,000 years if you did it with photosynthesis).

(N.B. I go into all these things in much more detail in the books linked to at the end of this answer, along with cites to find out more).

HUGE TIME SPEED UP

It would also be a huge acceleration in timescale over the way it worked on Earth, when our planet became hospitable, which took us millions of years starting from a much more promising start. And Mars is very different from Earth in many ways - and there is no guarantee even that we could do it for a second Earth identical to ours in all respects except no life, and only CO2 in the atmosphere. Who is to say that what we have is the expected end state? But on Mars with its such different orbit, temperature, geology, surface chemistry, … We get oops moments and big mistakes in projects as small as Biosphere II.

SURELY WE’D USE ALL THAT INGENUITY AND EFFORT FOR EARTH?

Why would anyone do that, as a thousand year project, when we have such a habitable Earth? What government on Earth would vote, year after year, for a thousand years, to spend all that money on this attempt to make Mars more habitable? There is no way it could be done without a huge amount of support from Earth. Why not use all that ingenuity and all that effort to protect and benefit Earth instead? Instead of planet sized mirrors to warm up Mars, planet sized regions of thin film mirrors focusing on smaller solar panels, to make solar power to give Earth endless free solar power from space? Instead of smashing comets into Mars, if we ever have that technology, use that ice for rocket fuel and settlements in the Earth Moon system. The ideas are great to study, and fun ideas, but I don’t see them as a near term practical future.

Also, our Earth is nowhere near over populated if you applied the sort of technology they would use for space colonization. The habitats they claim they would be able to build on Mars - if you used the same technology on Earth you could set up home in any desert anywhere, even cold dry Atacama desert, Gobi desert etc and be self sufficient. For much much less expense. You don’t have to make your own oxygen. You can maintain your habitat without using spacesuits. You don’t need protection from cosmic radiation. You don’t need to engineer it to hold in ten tons per square meter of atmospheric outwards pressure.

And with the self sufficient high tech ideas of growing crops for space habitats - to be efficient enough to be feasible in space - these are so efficient, you could feed the entire world from only 2.5% of the Sahara desert. Also our oceans are four times the surface area of the land, in effect giving us four new "ocean world" planets. If we use space habitat technology for the seas as well, we could feed the population of those four extra "ocean world" planets, with four times the population of Earth, from only 0.5% of the Pacific ocean. We are talking here about minimal impact sea steading, in tethered floating sea cities.

I go into it all in more detail in my books linked to at the end. The main problem at the moment is advancing desertification, and there are desert projects already to use sea water to reclaim deserts. These are already rather like "space habitats" on Earth but much simpler, in the way that they are rather self sufficient, contained, using local reasources only. The seasteading idea is also, minimal impact, not fishing the seas, solar power, only using sea water and sunlight as the main imports.

What scenario could lead to us setting up a thousand year project, hugely expensive, setting up huge mirrors and a thousand nuclear power plants on Mars in an attempt to make a pale imitation of Earth? When we could do so much here instead?

So I don’t see that happening any time soon. There are many issues with Mars anyway. I see the Moon as far more likely to succeed for small scale settlement. Mainly because it’s close enough to Earth so that there are possibilities of exports, and of putting some of our heavy technology on the Moon. But Mars is so far away- how could anything made there benefit Earth?

PARATERRAFORMING IS FEASIBLE - BUT WHY NOT ON THE MOON OR IN FREE SPACE

We could do paraterraforming on Mars much more easily - space colonies inside caves or in greenhouses covering large areas, and eventually possibly the whole planet. But if you are doing that - well you might as well do it on the Moon, at the lunar poles to start with or in the lunar caves - if you look at it in detail they have many advantages over Mars.

And paraterraforming anyway - that means building your own habitats. So - would need to be a lawyer to look into it in detail but I think it means according to the OST that if you can paraterraform like that - including making free space colonies from materials in the asteroid belt - well you made the whole thing yourself, even if you made it with local materials. So surely you’d own a paraterraformed area of the Moon.

IDEA YOU’D HAVE MILLIONS IN SPACE AND THEY WOULD ONLY BE THE “GOOD GUYS”

But - why this idea that we will have millions in space? And would it make us safer to have mllions of people with space technology?

This doesn’t just mean whoever you think are the “good guys” in space. Once you have millions, you are going to have the likes of North Korea, and ISIS in space with technology far more powerful than an ICBM. And what’s more, with space habitats as fragile as eggshells. What would happen to the ISS or any space habitat if someone just did a suicide bomber type run against it with a spaceship traveling at 39,897 km/h - the maximum speed anyone has traveled, during Apollo 10’s return from the Moon? Such speeds would be common place once you have millions in space. We don’t have “shields” as in Star Trek. There would be no defense at all for space habitats against anyone with malicious intentions and access to space technology.

Or if not, how do you make sure only the “good guys” get into space? I don’t see any need or point in rushing into space as fast as possible. And - I don’t buy Elon Musk’s argument that we have only a short window of opportunity to get into space. If that was the case, then the space colonies would be doomed also, because how could we have space colonies when Earth, the most habitable place by far in our entire solar system, is not working for us?

THREAT OF WAR CAN’T WORK AS A WAY OF ESTABLISHING TERRITORY IN SPACE

Territories on Earth are enforced on the country level by the threat of war. Even Buddhist countries have armies. But a war in space would lead to destruction of everyone, because the habitats are so fragile, like eggshells or bubbles, and the speeds of the spacecraft so high, tens of thousands of miles an hour.

Also, after a habitat is destroyed, there is nowhere else that you can go because you can't even breathe the air if you are forced out of your habitat even if you survive a direct hit in a spacesuit. An all out war in space would be over quickly, with no survivors on either side. So before we can have territories in space, if we do, we need another approach.

We can be competitive of course, for instance in the Olympic games, or in friendly scientific competition, without violence or warfare. So a future without any possibility of war doesn’t need to mean a future with no competition. It could have fierce competition. But how can there be territories in space without any possibility of war, once we have millions living there?

NO NEED FOR A “COLLISION INSURANCE POLICY” - NOTHING CAN MAKE EARTH LESS HABITABLE THAN MARS

As for the idea that we need two planets because of things that could happen to Earth - there is nothing that is remotely able to make Earth less habitable than Mars. That would mean removing our atmosphere, all of our oceans too, most of the ice. Even then it would be more habitable than Mars (in a better locale in the solar system, not as cold as Mars). It’s just not going to happen. Not for at least a few hundred million or billion years.

You often get people saying that we could get a collision with an asteroid to make Earth uninhabitable. Yes, there were such collisions in the early solar system and through to soon after the formation of the Moon, billions of years ago, the collisions that made the Hellas basin on Mars, the Aitken crater on the Moon and the Caloris Basin on Mercury. But we haven’t had impacts that large for well over three billion years. Modeling also supports this. Jupiter protects us from those huge 100 kilometer impactors. Deflects them to hit itself, the Sun, tear themselves to pieces through tidal interactions or ejects them from the solar system before they have a chance to hit the tiny target of Earth.

It’s true that such an impact could literally make Earth uninhabitable for us, for millions of years. But it ain’t going to happen. Jupiter doesn’t do such a good job of protecting us against the smaller up to 10 kilometers and a bit larger impacts - it still takes many of those also “for the team” but many get through. But none of those would make us extinct on Earth or make Earth anything like as uninhabitable as Mars. The dinosaurs didn’t have our technology, and even with the simplest of technology, millions of people would survive.

For detailed reasons of why we would survive a Chixculub impact see my Why Resilient Humans Would Survive Giant Asteroid Impact - Even With Over 90% Of Species Extinct

As for supernovae, and gamma ray bursts, well some of the experts are skeptical that they cause mass extinctions. If they do - then it’s indirectly through creating holes in the ozone layer. We are protected from the radiation beneath the equivalent of ten meters thickness of water in mass, by our atmosphere. UV gets through our atmosphere but it is just light and easily protected by a thin layer of any opaque material or by sun block cream. Again humans could survive easily, whatever the effects on other species. For more about all this see the section Earth best for a "backup" in http://robertinventor.com/bookle...

Indeed, however bad the disaster, it just needs a few people to have some idea of how to make a plane to survive, to know enough to avoid the whole thing of trying flapping flight for instance. It’s the same for many other things. We’d know so many things that could let us leap-frog centuries. It would also just need a few books to survive, and some examples of our technology to give a huge head start. And if we want insurance - well its knowledge, seed banks etc that we want to preserve, so a great place to preserve those in addition to Earth would be on the Moon. See my Backup on the Moon - seed banks, libraries, and a small colony

Also, just to reassure anyone who is scared of dinosaur era impacts - of course those are incredibly rare too. And we don’t risk anything like that anyway for the next century, because we already know the orbits of all the Near Earth 10 km plus asteroids and none are headed our way. As for the chance of a comet of that size, well it is a 1 in million chance for the 10 km asteroids of a hit during a century - so 1 in 146 million for a comet, so tiny that we can ignore it since currently only 1 in 146 of flybys is by a comet.

IT MAY BE A PROBLEM A BILLION YEARS FROM NOW - BUT IT’S TOO EARLY TO KNOW WHAT WILL BE NEEDED THEN

We can face this problem a billion years from now, and I think it is too early to think that whatever we do now will solve whatever problems future civilizations will have on Earth a billion years from now. Surely a healthy Earth will be more important to them than a probably failed attempt at terraforming Mars. In any case the Mars terraforming wouldn’t last that long. It would lose its atmosphere over such long timescales. It would then be a dead dry planet with almost no chance of re-terraforming. Unless there was some way to keep it self sustaining - but how do you deal with the atmosphere loss?

NEED TO KEEP EARTH IN GOOD SHAPE

It’s obvious I think that we have to do everything we can to keep Earth in the best shape possible. Based on that, with a healthy Earth then we can have a healthy space industry and settlement too. And one of the things we can do in space is to do things to help Earth. Like solar power from space, scientific discoveries to help Earth, mining in space may help us also.

I think space is still of great interest. There is much we can learn, and often our robotic explorers will be an important part of that. And there is the possibility of commercial exports to Earth especially from the Moon. And increase in scientific understanding. And we could have thousands living on the Moon in the vast lunar caves. It’s closer and safer. Or at the lunar poles.

If you had a large colony in a cave - well - I think that would count as a space habitat so basically they could own their cave. That’s a vast area, they can be kilometers in diameter we think - from radar data - and over 100 km long.

MAIN CHALLENGE NOT LEGAL BUT SOCIAL

I think the main challenge here before we have lots of people in space is not so much the legal one of how you make it legally possible, I think the Outer Space Treaty is an excellent starting point because it gives a framework that could lead to a peaceful way ahead, which is essential in space. And there are many ideas of ways it can be extended to deal with various future needs.

The problem is a social one . How can you have large numbers of people in space and still have a peaceful stable prosperous society? I think if we manage to sort that out, then the legal issues are not likely to be a major problem. But before that can be possible, I think we need to find out to do it here on Earth, probably. Otherwise we just export our problems. You can start small scale in space and maybe have a peaceful society but as we are now, if you expand hugely and rapidly we will just have the same problems we have on Earth written out in large in space, but much more high tech with people with spacecraft able to travel at kilometers per second. I just don’t think we are ready for millions in space. But I don’t think it is impossible. Just not yet. But we are ready for a few hundred or thousand, even maybe tens of thousands.

NEAR FUTURE LIKE ANTARCTICA

I see the near future as more like Antarctica. The space stations like Antarctic bases. And then probably tourists visiting as well, like tourists visiting Antarctica.

Like this

That is hugely more habitable than anywhere in space. If you found somewhere like that in space you’d call it a “second Earth” what with the atmosphere and abundant ice for water.

How Antarctic bases went from wooden huts to sci-fi chic - BBC News

And it’s so much easier dressing up for Antarctica than putting on a spacesuit.

That’s the equipment you need for the Antarctic interior, where temperatures are - 10 C to -20 C even in summer and below -60 C in winter. For the coastal areas where tourist cruise ships visit in summer it’s often as warm as 0 C. To kit yourself out like that would cost thousands of dollars. But to kit yourself out with a spacesuit would cost an estimated $2 million dollars (you can’t actually buy one “off the shelf” at present). That’s just the cost of the components and of building it, a highly skilled activity that takes a long time as it is, essentially, a miniature spaceship. It doesn’t include the development cost. Tourists on the Moon would surely not bring their own spacesuits any time soon.

So long as we have activities in space rather like activities in Antarctica, along with maybe some commercial activities as well unlike Antarctica, I think the OST is just fine and we can work within it. If we ever have to go beyond it, that will be very difficult since so many have signed it, and other attempts at international space law (such as the Moon treaty) have shown how difficult that is to achieve - but I don’t think we need to do that at present.

There are also major planetary protection issues with sending humans to Mars, I think. Though the Mars moons Phobos and Deimos don’t have that problem, or probably don’t, and the Moon certainly doesn’t. Why send humans as quickly as possible to the one place in the solar system that we can mess up most easily by introducing Earth life, when we have no previous experience at all of any settlement in space anywhere? (I don’t think the ISS really counts as space settlement). And what we could discover on Mars if we don’t mess it up could be one of the biggest discoveries in biology ever - maybe just microbes, but extra terrestrial microbes based on a completely different biology, not using DNA, like a whole other world inside their cells.

For more on all this, you may like to check out my:

MOON FIRST Why Humans on Mars Right Now are Bad for Science.President Obama, if you love science, Please protect Mars life from contamination from Earth (I should change this title now that he is about to reach the end of his second term)

Case For Moon First

If Humans Touch Mars - Like the Lascaux Story - Another Tale of Human Missteps?