Well one way to think about this is - how many people are needed for a self sustaining colony in the middle of a desert on the Earth if you first remove all the oxygen and nitrogen from the atmosphere, and most of the CO2 making it a near vacuum with hardly any cosmic radiation shielding, and reduce the temperature to that of Antarctica?
(And reduce the gravity to 40% of Earth normal - nobody knows if humans can survive that long term, might need to live in large centrifuges for health.)
Answer is - that we can't yet do it. No matter how many people, even a billion. Not if you want them to be able to continue to survive with no contact with the rest of the Earth, if that's what you have in mind.
(If you mean economically sustainable then the problem is that there is nothing on Mars that would pay back the high costs of sending it to Earth as far as we know with present or near future technology.)
They need to build houses to far higher level of engineering than a house on Earth - withstand tons per square meter outward pressure (Earth atmosphere is 10 tons per square meter) - only houses you can build are likely to be spheres or cylinders made of strong materials to withstand the pressure - like the ISS modules. And a window is a major technological challenge in vacuum conditions (most ISS modules don't have them).
And have to build spacesuits and spaceships - and environment control systems and computer chips ...
As well as the more mundane - make all your food, clothes, mini sewage treatment plant and complete recycling within the habitat,...
They might make all their own food - but at what cost? Would it cost a thousand dollars to grow a single tomato, say?
The closest to this was Biosphere 2 - which failed. Quite possibly a new Biosphere would succeed, here on Earth, building on the lessons learnt. But still, that's just a first step on a long road towards making a food and oxygen self sufficient greenhouse that would work in space conditions.
Also, Biosphere 2 would just explode from the internal pressure if you tried to put that building into the vacuum of space.
When we do have that technology, and if we are able to do it in space at a reasonable cost - then we will have solved just about all problems that we currently have on the Earth of a technological nature and will surely have no shortage of food.
Because if you can do it in space at a reasonable cost (say with 3D printers) - that means you can also grow food in a self sustaining way, at a low cost, in any desert on the Earth - in the same way - using machines built with 3D printers, just from the desert sands and minerals, and chemicals from the atmosphere.
It's all very well these ingenious ideas to develop a sustainable colony on Mars - but if they work so well - why aren't we already using the same techniques for sustainable colonies in our deserts? Far far easier here, with an Earth normal atmosphere (so buildings far easier to construct, same pressure inside and out) and with oxygen and nitrogen in the atmosphere already. And everything else Mars has of value for colonization, we have here in abundance in our most barren deserts.
If you can do that on Mars you can certainly do it on Earth.
It's future fantasy / science fiction at present - might be real some day but nowhere near that technology yet. Is true that technology can advance quickly - but we don't have that yet.
In any case - there's the problem that any surface mission to Mars currently has to keep Mars free from contamination by Earth lifeforms according to the Outer Space Treaty.
This has the potential to be the most astonishing and groundbreaking discovery in biology - possibly in the whole of science for the last century or so.
For that reason (contamination issues) I think first colonies - if we have colonies at all - will be elsewhere and most likely close to Earth for safety.
The first settlements - rather than colonies - if we have any will be big expensive things like the ISS,
I think most likely research stations than places that people live - unless you are a multi-billionaire - and even then - you'd need some way to earn a living in space - or support your settlement at the rate of billions of dollars a year from Earth.
Getting the mass to outer space is, I think, not even a tenth of the total problem though it is a big thing in itself. It's probably not even a hundredth of it.
When a typical house has to be engineered to hold in tons of atmosphere per square meter - and covered in meters thick layers of cosmic radiation shielding - and with a design life of just a few decades before the habitat has to be replaced because of the thermal stresses - and have continually maintained internal atmosphere requiring complex expensive equipment to keep it going (itself costing many millions of dollars per household to make) - how can it make sense to colonize space?
To make day to day living worthwhile in space - I think you need huge colonies able to create a large space for colonists to build houses that are reasonably low maintenance, and protected from the vacuum and thermal stresses of space so that they can use ordinary construction methods to build and maintain them..
One day, might be possible big Stanford Torus type colonies, or large underground caves - or the floating "Cloud Nine" type colonies of Venus.
But to make it worthwhile to build such a colony - you need some return to Earth.
The Stanford Torus in the 1970s was going to be financed by sale of power beamed to Earth from solar satellites built by the colonists.
(And reduce the gravity to 40% of Earth normal - nobody knows if humans can survive that long term, might need to live in large centrifuges for health.)
Answer is - that we can't yet do it. No matter how many people, even a billion. Not if you want them to be able to continue to survive with no contact with the rest of the Earth, if that's what you have in mind.
(If you mean economically sustainable then the problem is that there is nothing on Mars that would pay back the high costs of sending it to Earth as far as we know with present or near future technology.)
SO MUCH MORE IS NEEDED FOR A TYPICAL "HOME" IN SPACE
They need to build houses to far higher level of engineering than a house on Earth - withstand tons per square meter outward pressure (Earth atmosphere is 10 tons per square meter) - only houses you can build are likely to be spheres or cylinders made of strong materials to withstand the pressure - like the ISS modules. And a window is a major technological challenge in vacuum conditions (most ISS modules don't have them).
And have to build spacesuits and spaceships - and environment control systems and computer chips ...
As well as the more mundane - make all your food, clothes, mini sewage treatment plant and complete recycling within the habitat,...
They might make all their own food - but at what cost? Would it cost a thousand dollars to grow a single tomato, say?
The closest to this was Biosphere 2 - which failed. Quite possibly a new Biosphere would succeed, here on Earth, building on the lessons learnt. But still, that's just a first step on a long road towards making a food and oxygen self sufficient greenhouse that would work in space conditions.
Also, Biosphere 2 would just explode from the internal pressure if you tried to put that building into the vacuum of space.
A FUTURE SCI FI / FANTASY IDEA AT PRESENT - IF IT WORKED - NEARLY ALL OUR CURRENT TECHNOLOGICAL PROBLEMS HERE ON EARTH WOULD BE SOLVED
When we do have that technology, and if we are able to do it in space at a reasonable cost - then we will have solved just about all problems that we currently have on the Earth of a technological nature and will surely have no shortage of food.
Because if you can do it in space at a reasonable cost (say with 3D printers) - that means you can also grow food in a self sustaining way, at a low cost, in any desert on the Earth - in the same way - using machines built with 3D printers, just from the desert sands and minerals, and chemicals from the atmosphere.
It's all very well these ingenious ideas to develop a sustainable colony on Mars - but if they work so well - why aren't we already using the same techniques for sustainable colonies in our deserts? Far far easier here, with an Earth normal atmosphere (so buildings far easier to construct, same pressure inside and out) and with oxygen and nitrogen in the atmosphere already. And everything else Mars has of value for colonization, we have here in abundance in our most barren deserts.
If you can do that on Mars you can certainly do it on Earth.
It's future fantasy / science fiction at present - might be real some day but nowhere near that technology yet. Is true that technology can advance quickly - but we don't have that yet.
CONTAMINATION BY EARTH LIFE
In any case - there's the problem that any surface mission to Mars currently has to keep Mars free from contamination by Earth lifeforms according to the Outer Space Treaty.
This has the potential to be the most astonishing and groundbreaking discovery in biology - possibly in the whole of science for the last century or so.
For that reason (contamination issues) I think first colonies - if we have colonies at all - will be elsewhere and most likely close to Earth for safety.
GETTING THE MASS THERE AS A TINY PART OF THE TOTAL PROBLEM
The first settlements - rather than colonies - if we have any will be big expensive things like the ISS,
I think most likely research stations than places that people live - unless you are a multi-billionaire - and even then - you'd need some way to earn a living in space - or support your settlement at the rate of billions of dollars a year from Earth.
Getting the mass to outer space is, I think, not even a tenth of the total problem though it is a big thing in itself. It's probably not even a hundredth of it.
WHEN TYPICAL HOUSE IS A HUGELY EXPENSIVE ISS TYPE CYLINDRICAL HABITAT WITH DESIGN LIFE OF A FEW DECADES...
When a typical house has to be engineered to hold in tons of atmosphere per square meter - and covered in meters thick layers of cosmic radiation shielding - and with a design life of just a few decades before the habitat has to be replaced because of the thermal stresses - and have continually maintained internal atmosphere requiring complex expensive equipment to keep it going (itself costing many millions of dollars per household to make) - how can it make sense to colonize space?
To make day to day living worthwhile in space - I think you need huge colonies able to create a large space for colonists to build houses that are reasonably low maintenance, and protected from the vacuum and thermal stresses of space so that they can use ordinary construction methods to build and maintain them..
One day, might be possible big Stanford Torus type colonies, or large underground caves - or the floating "Cloud Nine" type colonies of Venus.
But to make it worthwhile to build such a colony - you need some return to Earth.
The Stanford Torus in the 1970s was going to be financed by sale of power beamed to Earth from solar satellites built by the colonists.
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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