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This is my most recent draft. I hope it summarises my core argument first and has clearer writing about the other peripheral subjects later. Warning: it's fairly long, but the first 2 points are so worth it! Stop at Blue Mars if you wish as that's half way, and everything after there becomes a bit more speculative.



While I eventually would love to see *all* of the solar system settled and O'Neil colonies everywhere, I think we should start with Mars first. There are 3 main reasons: the City Size Bonus, Technology Bottlenecks, and the Myth of Self-Replication Mining ships.


1. City Size Bonus

One day we'll hopefully colonise both Mars and the Belt, but my first goal would be to shoot for Mars.  Why? The City Size Bonus. (I'm not worried about the moon at this point, because I don't see that as a 'home away from home', but rather a glorified gas station to get us to our new home). I've seen it described as city metabolisms, but I prefer to call it the City Size Bonus. There is a kind of "Moore's law" of efficiency for city sizes. If a city population doubles, you get an extra 30% for free. Bigger is better. It's real, measurable, and very, very powerful. It's like a kind of magic. For example, if you had 2 separate towns with 50,000 people in each, you get a total GDP output of the labor of about 100,000 people. But if we just combine all those people into the one town of 100,000 people — the same amount of people but just *together* — their output jumps 30%. It's as if that town actually had 130,000 people. This factor is *so* strong that we're not sure when it wears off. Some have reasons to believe it might finally cease at a city of 40 million!

Elon Musk wants a city of a million people on Mars, which is when they estimate they'd have a GDP large enough to start terraforming the place. So what's better? One city of a million people on Mars or maybe 40 separate O'Neil colonies? You can see where I'm going. Bigger is better. The Martian city gets the magic City Size Bonus of an extra 300,000 people for free! You did not have to lift them into space! The divided O'Neil colonies miss out. I think this calculation gets forgotten by the Belters.



Based on todays technology, Belters emphasise the energy costs of leaving the Martian gravity well, as if a little gravity is an absolute show stopper. But we know how to make rocket fuel on Mars, and Martian gravity is 1/3 of Earth's so it takes far less fuel to leave Mars than Earth. It also lets us build a space elevator out of today's materials. Also, why leave? Mars is a whole *planet* to play with. (More on that below). While Belters go on about gravity wells, I go on about building unnecessary (at this early stage) space industries and O'Neil colonies! 



Mars is ready. Space X can fly us there in big rockets that can carry 100 people and all their equipment . We'll take flat-pack habitats and unload and construct these on the surface — and even bury them in radiation protecting regolith with hand-shovels if necessary! A fleet of rockets will land on Mars, and unpack, and the rockets will refuel from in-situ resources, manufacturing their fuel out of the very Martian atmosphere itself. Mars has all the resources we need to build an enormous civilisation. The colonists enjoy working from the big rocket for 2 years. By then they'll have built their habitats, be growing their own food in greenhouses, and the Space X rocket will have refilled, ready to go home. Here's the key advantage with the Belters. After 2 years, the rockets are free to go home and get more people. The rockets can be ferries shuttling people and equipment out to Mars. But if we go the Belter route first, the Belters will require the rockets for *decades* until they build their first home! Building an O'Neil colony is much harder than building a 10km or 20km long aircraft carrier. We're building in space, in zero gravity, in hard vacuum, in radiation, and the miners can't just go home for holidays and a few good pizza's. It's all vastly more challenging! Instead, the rockets are trapped there for *decades* until their first home is built and the people can unload and settle in.


I'm quite sure our Martian city of a million people would be built before the first O'Neil colony was finished. Check it out. Each Space X rocket can carry 100 people to Mars, so that's 10,000 flights to Mars. That sounds like an awful lot. But it's not that many when you realise each rocket can do 25 trips between Earth and Mars, or basically 13 flights to Mars per ship! (12 flights there and back, with the final flight leaving the rocket on Mars to be salvaged by the needy locals!) So 10,000 flights divided by 13 trips there  = 770 ships. If a fleet of 770 ships took off they'd land their first 77,000 people on Mars and return to Earth 2 years later. If we're prepared to use some extra fuel, we could reload and refuel them quickly back at Earth and relaunch them back at Mars so we have the 770 ships travelling to Mars every 2 years. In this hypothetical back of the envelope exercise, you'd have a million people on Mars in just 26 years. OF COURSE it will not happen like that, as we'll start with one or two BFR's and only gradually scale up to a few dozen over the next few decades, unless the getting to Mars bug really takes off! This exercise is for illustration purposes only! IF the governments of the world decided to make a viable off-world civilisation quick-smart, (say in the event of an imminent Extinction Level Event?) then we would get the money. 770 ships and 26 years after the first fleet left, and we'd have a million people on Mars. And probably a good chunk of Earth's biosphere in farms and zoos up there as well. Now, don't forget that those million people would enjoy the City Size Bonus factor which adds the GDP output of another 300,000 people for free!


But what would the belt have? They wouldn't have 770 ships acting as ferries, but as mining ships. Stuck out there. With a maximum crew of 100 per ship, or 77,000 people, trapped there for decades waiting for the first O'Neil colony. What do you get for all that effort?  Would the first colony even house 100,000 people?  The Martian City Size Bonus alone is 3 times that, let alone that Mars could have a city of a *million* people from the same 770 rockets! O'Neil habitats are great, but in my view are an unnecessary bottleneck for our *first* off world civilisation.



In this scenario the Belt now has ferried the rockets back and forth to load 100,000 people on the first O'Neil colony. The rockets have additional people ferrying around the Belt still using the original 770 rockets which will survive a lot longer, given they don't suffer the same reentry stresses of the Earth to Mars route. Now consider this. To truly grow exponentially, the Belters will need to grow their capital infrastructure exponentially. That means rockets, and lots of them! But we've already seen that the first O'Neil colony required an enormous volume of rockets working as space-mining vehicles, and for a much smaller final colony size as the result. Yet it takes a large population to run a space industry.  Australia has a population of 23 million and until 2017 had no space agency or industry. It wasn't considered economically viable, and so we just bought payload space with others.



Meanwhile the same 770 rockets would have moved a million people to Mars, and they have an entire planet just outside their front door! Where the Belters are required to keep a fleet of high-tech rockets running to mine the asteroids, the Martians can just walk outside and jump in their fleet of trucks. In other words, being in space is its own kind of bottleneck. Sure you're free of gravity, but that also means you're also free of trucks and trains and eventually bicycles! You need rockets to do anything. That might not be as big a problem in centuries to come, but it's quite the technological bottleneck now. I'm just not seeing the 'exponential growth' the Belters rave about, not in the early days at least. Once Mars has a viable city of a million people, there's nothing to stop Mars growing quickly to a world full of *billions* of people and unique ecosystems etc without building a single new rocket! Of course eventually they'll have their own space industry, and might become the gateway to the Belt. But I'm focussing on the true exponential growth potential of developing a whole new planet at the start.



Blue Mars is a really important step along the way to terraforming Mars. It's *not* the finished product with a breathable atmosphere a millennia from now, but it is a vastly safer place to live than today's Mars and much *sooner*. Zubrin says our city of a million settlers could create Blue Mars in just 100 years, pumping out super-greenhouse gases 17,000 times more powerful than CO2 to cook the planet! This would give it atmosphere and rain!


ATMOSPHERE: All the dry ice on Mars has melted, giving Mars a third our air pressure in beautiful radiation-stopping, heat-trapping CO2 that has turned the skies blue. It protects Martians from hard vacuum and space radiation. Belters will *always* require a steel O'Neil cylinder with meters of rock spun up to protect them from the vacuum and radiation of space. But on Blue Mars, we will be able to go outside wearing normal clothes with just a breather mask. That frees Martians from wearing cumbersome space suits that are more like wearing heavy sensory deprivation tanks than clothes. Future Martians will be able to ride bicycles to work!


OCEANS AND RAIN! Blue Mars is hot enough to melt water, creating oceans. It will kick start the hydrological cycle, creating rain on Mars! This will wash poisons out of the regolith (accumulating in the ocean) and allow us to slowly convert the regolith into soil. We'll grow fungi and plants and trees and eventually start farming on the surface. All that within about 100 years after building our first city of a million people. And eventually we'll find (or engineer?) phytoplankton that can detox the Martian oceans, adding life to the oceans of another world. One day we'll go fishing on the surface of another planet.


PANAMA CANAL TO THE BELT! Mars creates the opportunity and the demand for a resource gathering industry in the Belt, and a space elevator (or tether to Phobos) could provide a lot of the thrust to get there. Indeed, a Phobos tether could even provide much of the momentum to get all the way back to Earth! It's been called the Panama Canal of the inner solar system. "With judicious use of tethers and orbital momentum, rhinoceros sized delta V budgets are shrunk to hamster sized delta V budgets. No bucky tubes needed, ordinary materials like Kevlar can do the job."



Some Belters visualise gigantic AI driven self-replicating Von Neumann factory ships munching their way through the Asteroid Belt. These giant self-reps would gradually beget children who then beget more grandchildren ships on an exponential pathway towards AI demigods just raining gifts of fuel and manufactured goods down towards Earth, and parking O'Neil habitats in orbit for us to populate. 


But we're a long way from building a true Von Neumann rocket, precisely because I think we're a long way from true AI! I love automation. I love the safety of robot-cars and cheaper industrial goods and services we enjoy because of today's robotics. But we can't build self-rep ships yet because we do not see them here on earth yet! Think about it a bit. A computer chip requires thousands of people involved in the mining and refining of the raw materials, the engineering specialists to prepare them into the right components, and the super-high tech clean room to actually manufacture the chips. The kind of AI driven manufacturing process that could build the next computer and AI is the kind of AI revolution that would bring on a full scale technological singularity. Now, while the geeky kid in me longs for this — with demigod-like AI driving robots around in a post-scarcity economy — it might not happen. It might not even be possible! We have had decades of futurists promising it *will* happen, but we just don't know that it *can* happen. Instead we seem to be on a sliding scale towards it.  I celebrate every aching little advance, like the fact that 3d printing is becoming cheaper, but it isn't a true self-rep technology. Not by a long shot! It just makes off-world habitats run by people more viable, reducing the sheer amount of tools required in space. It does not eliminate other tools, or people for that matter!  It means *people* can 3d print that hammer or spanner or widget they need for their other mining, refining, habitat building, farming, or medical tasks. A 3d printer builds the widgets we need in a still very recognisably human economy. 3d printers mean we get to take less stuff with us as we head out into the solar system, but we still need to be there. Us and all our biological and psychological needs. We're on a long and slow scale towards a true self-rep spaceship, but until we get there, the only truly self-replicating 'machines' we have are vast off-world colonies that can reproduce themselves with daughter colonies. This takes us back to the debate above — where to go first? I may not be right about all this, but I hope I've generated some thinking about where we might build first. The City Size Bonus, O'Neil Bottlenecks, advantages of Blue Mars and myth of Self-Replicating Ships all means I'm a fan of Mars! But I reserve the right to change my mind if our robot-overlords arrive earlier than anticipated! ;-)

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