Actually - well first I think all of these are more likely settled than colonized - reason being that for foreseeable future Earth would be a better place to live so children would emigrate back to Earth - I think you will only get people living in any of these places out of a deep interest in the place itself.
They will be there similarly to the reasons that tourists or scientists visit Antarctica - visiting a harsh difficult place to live either because they are interested in it for its own sake, or for the adventure of going there. That is after the allure of space travel has worn off and it becomes as easy to travel into space as to travel to another country in a plane.
Then - I think that Venus also could be settled in the upper atmosphere. Cloud top layer where you can use lightweight construction methods that are impossible anywhere else in the solar system. Far more living area for less material and less cost. That is - assuming that most of the cost comes from need to transport materials there.
I think myself that Venus cloud tops may be the least expensive habitats to build per person and per living area in the solar system outside of Earth. You can send an inflatable habitat - and one that inflates to many times the size of the same weight of inflatable habitat anywhere else.
Eventually there's a chance of growing trees - trees are nearly all water / CO2 - and so capturing the Venus atmosphere and making it into building materials for new habitats - which could be almost totally self sufficient, and what they lack could be taken up from the surface of Venus by dredging eventually.
There's also the possibility of free habitats in space, and there are enough resources to build the equivalent of 1000 planet surfaces similar to the land area of the Earth, cosmic radiation shielding, just in the asteroid belt.
So - I think Venus atmosphere (depending on whether we find life there, if we find life already in its atmosphere, may need to rethink those plans - it is not impossible, that there is life there, are some ideas which could make it work). And free habitats in space. Moon probably first - well we have the ISS already sort of a habitat in space - but perhaps Moon first for a more sizeable there to stay for a while type settlement, though that also could be in space.
Then more settlements in space and in orbit around other planets. Once you can build self enclosed habitats in space - and once you have good high quality telerobotic avatars - you don't really need to visit planet surfaces. And in case of Mars as I've said in various other answers here, I'm sure that we will postpone landing on the surface until we understand it better, just as we don't send humans down to investigate the sub ice lakes in Antarctica like Lake Vostock because we want to study them in their original state not contaminated by surface microbes. So even more so, surely, for Mars.
So that's how I see it happening. First, the Moon, asteroids, comets, free habitats in space orbiting Earth, and spaceships and space stations eventually throughout the solar system but orbiting the planets and moons, with avatars on the surface. Won't get humans landing on Europa for instance - that would be silly I think, if you are searching for life and want to investigate an uncontaminated Europa. Titan ditto, or Triton, Encladus, all these places surely the humans would be in orbit either around the Moon or the host planet.
But eventually, once we understand the solar system better, we might then get a few settlements here and there.
At an early stage could be Moon, Poles of Mercury, Deimos and Phobos (pending what we discover about them first), the asteroid belt, comets.
Later on could be cloud tops of Venus (depending what we find out) - some people have suggested the atmosphere of Saturn in heated hydrogen hot air "cloud cities" (heating should be easy as a natural result of technology) - that would be a vast area. And could have lots of Stanford Torus and O'Neil type settlements in space.
And later - could get grand schemes to reshape planets. Terraform Mars. Move Venus and terraform it. Make some of the planets, say Mercury, into a prototype Dyson type sphere or swarm around the sun to capture its solar power (I suggest Mercury as perhaps least likely to destabilize our solar system if it is removed and already close to the sun so in an ideal place to catch huge amounts of solar power).
But then - what if some ET civilization spends centuries dismantling planets and building Dyson swarms just to have a technological breakthrough at the end that makes the whole thing unnecessary? E.g. some breakthrough in power generation that makes it possible to generates amounts of power similar to those produced by the entire sun, but just using fusion reactors or something else we can't imagine yet?
All those I think don't make any sense until we have a stable civilization with technology established for centuries as they have a commitment for the future for thousands of years to complete the project. I don't think we are ready yet, to take on projects that take 1000 years to complete or more, and have to be seen through to completion to work.
And - these grand scale planet reshaping projects - they may have major issues some of them. Some that we can foresee already. And probably many others that we can't yet foresee. So I think they are "for the future" and that we shouldn't start on them yet. There are plenty of smaller scale projects we can do. Where small scale here means habitats for up to say a a few thousand or a few million settlers at a time, completed in decades, rather than these grand schemes projected to house billions of people a thousand years from now.
We could easily do things to make these grand scale projects far harder to do than they would be otherwise by making mistakes in the early stage - start a century earlier but in so doing make the project require an extra thousand years at the end, for instance, or even impossible. And also they are after all projects for a future generation 1,000 years from now or more - so - with our technological civilization so young, how can we be sure what we will want and need, a thousand years into the future?
See also, my other answers here on this topic, plus
They will be there similarly to the reasons that tourists or scientists visit Antarctica - visiting a harsh difficult place to live either because they are interested in it for its own sake, or for the adventure of going there. That is after the allure of space travel has worn off and it becomes as easy to travel into space as to travel to another country in a plane.
Then - I think that Venus also could be settled in the upper atmosphere. Cloud top layer where you can use lightweight construction methods that are impossible anywhere else in the solar system. Far more living area for less material and less cost. That is - assuming that most of the cost comes from need to transport materials there.
I think myself that Venus cloud tops may be the least expensive habitats to build per person and per living area in the solar system outside of Earth. You can send an inflatable habitat - and one that inflates to many times the size of the same weight of inflatable habitat anywhere else.
Eventually there's a chance of growing trees - trees are nearly all water / CO2 - and so capturing the Venus atmosphere and making it into building materials for new habitats - which could be almost totally self sufficient, and what they lack could be taken up from the surface of Venus by dredging eventually.
There's also the possibility of free habitats in space, and there are enough resources to build the equivalent of 1000 planet surfaces similar to the land area of the Earth, cosmic radiation shielding, just in the asteroid belt.
So - I think Venus atmosphere (depending on whether we find life there, if we find life already in its atmosphere, may need to rethink those plans - it is not impossible, that there is life there, are some ideas which could make it work). And free habitats in space. Moon probably first - well we have the ISS already sort of a habitat in space - but perhaps Moon first for a more sizeable there to stay for a while type settlement, though that also could be in space.
Then more settlements in space and in orbit around other planets. Once you can build self enclosed habitats in space - and once you have good high quality telerobotic avatars - you don't really need to visit planet surfaces. And in case of Mars as I've said in various other answers here, I'm sure that we will postpone landing on the surface until we understand it better, just as we don't send humans down to investigate the sub ice lakes in Antarctica like Lake Vostock because we want to study them in their original state not contaminated by surface microbes. So even more so, surely, for Mars.
So that's how I see it happening. First, the Moon, asteroids, comets, free habitats in space orbiting Earth, and spaceships and space stations eventually throughout the solar system but orbiting the planets and moons, with avatars on the surface. Won't get humans landing on Europa for instance - that would be silly I think, if you are searching for life and want to investigate an uncontaminated Europa. Titan ditto, or Triton, Encladus, all these places surely the humans would be in orbit either around the Moon or the host planet.
But eventually, once we understand the solar system better, we might then get a few settlements here and there.
At an early stage could be Moon, Poles of Mercury, Deimos and Phobos (pending what we discover about them first), the asteroid belt, comets.
Later on could be cloud tops of Venus (depending what we find out) - some people have suggested the atmosphere of Saturn in heated hydrogen hot air "cloud cities" (heating should be easy as a natural result of technology) - that would be a vast area. And could have lots of Stanford Torus and O'Neil type settlements in space.
And later - could get grand schemes to reshape planets. Terraform Mars. Move Venus and terraform it. Make some of the planets, say Mercury, into a prototype Dyson type sphere or swarm around the sun to capture its solar power (I suggest Mercury as perhaps least likely to destabilize our solar system if it is removed and already close to the sun so in an ideal place to catch huge amounts of solar power).
But then - what if some ET civilization spends centuries dismantling planets and building Dyson swarms just to have a technological breakthrough at the end that makes the whole thing unnecessary? E.g. some breakthrough in power generation that makes it possible to generates amounts of power similar to those produced by the entire sun, but just using fusion reactors or something else we can't imagine yet?
All those I think don't make any sense until we have a stable civilization with technology established for centuries as they have a commitment for the future for thousands of years to complete the project. I don't think we are ready yet, to take on projects that take 1000 years to complete or more, and have to be seen through to completion to work.
And - these grand scale planet reshaping projects - they may have major issues some of them. Some that we can foresee already. And probably many others that we can't yet foresee. So I think they are "for the future" and that we shouldn't start on them yet. There are plenty of smaller scale projects we can do. Where small scale here means habitats for up to say a a few thousand or a few million settlers at a time, completed in decades, rather than these grand schemes projected to house billions of people a thousand years from now.
We could easily do things to make these grand scale projects far harder to do than they would be otherwise by making mistakes in the early stage - start a century earlier but in so doing make the project require an extra thousand years at the end, for instance, or even impossible. And also they are after all projects for a future generation 1,000 years from now or more - so - with our technological civilization so young, how can we be sure what we will want and need, a thousand years into the future?
See also, my other answers here on this topic, plus
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
4.8m answer views110.4k this month
Top Writer2017, 2016, and 2015
Published WriterHuffPost, Slate, and 4 moreHuffPost, Slate, Forbes, Newsweek, Business Insider, and Science 2.0