Perhaps a better question would be what we can’t learn. The biggest thing we can’t learn is how to cope with different gravitational fields. One disadvantage of living at the bottom of a deep gravity well is that you are stuck with at least that much gravity for anything more than a short free fall. Maybe, maybe, in the future we will be able to manipulate gravitational effects, but write now it just science fiction.
Another issue that we will have to deal with in space colonies that we can’t fully duplicate here on Earth is near vacuum. A colony in orbit or on the moon will have to deal with an almost perfect vacuum, and despite the storms of Mars, the atmosphere there is pretty close at 0.01 atmospheres, give or take. What we can do here is start learning about how to deal with these issues, even if we can’t get the full effect. We could have a pressurized colony at altitudes where the natural pressure is lower, which would mean having it at high elevation. We can learn a lot by having a colony at an altitude where the pressure is at 0.7 atmospheres, but pressurized to 1 atmosphere.
One more issue that Earth protects us from is hard radiation, and any space colony will have to deal with that. This issue is fairly easy to plan for, so it isn’t as important that we practice for dealing with it. While there are doubtless other differences, such as how to deal with utilizing the very different “soils” of the moon or Mars, finding water, etc. That does leave an enormous number of issues that we can practice here.
While Elon Musk and others are working to get the cost of transporting material to orbit down, short of some unforeseen breakthrough, transportation costs are still going to be very high. While the transportation costs will likely be lower for any Earth-based colony, in the remote locations we are proposing the costs will vary from moderate to very expensive. We can start to learn about all of the logistics. What is needed and how much to get started. What will we be able to produce in the colony and what will need to be imported.
Earth-based colonies can test a variety of structures to see what works best. While there are doubtless experts who already have ideas, testing these in a variety of environments will be very useful. Do we need to set up the colony in an area of loose dust, that can be piled over the colony structures for shielding, or is it practical to dig tunnels. What will we need for each of these or other possibilities and what will the weight and cost be?
We can learn a great deal about supplying food for colonies. A colony will need to get to self-sufficient on food as quickly as possible. What are the best techniques? What foods should be grown? What do you start with and how much do you need to bring as a reserve? There is so much we don’t know yet. Again, while there are numerous experts, the best way to learn is to actually do this in a colony setting.
How about energy? Are we better off with solar or nuclear or some combination? What is the best combination of weight versus energy produced and which is better for long term maintenance and growth. Are there other options?
We already mentioned that we can partially test a pressurized colony on Earth, but it is more than just developing air tight structures. What do we need to do to ensure the proper flow and mixture of air for the colonists and for plants, etc? Do they need to be the same? Maybe we grow some crops with a different atmospheric mix. There are a lot of necessary skills, here and certainly a lot to learn.
How about all the thousands of things we take for granted every day, running water, waste handling, chemicals from soap to cleaners to lubricants, computers, and all of the other conveniences of a modern life. What do we really need and what is easy and what is hard? So much to learn!
What is the best size for a colony? Is a colony with 50 adults a good starting point, or is 100, 150 or 300 better? What skill sets do we need and who goes in first? What is the ramp up time? These question really need a lot of practice colonies to answer the questions.
How do we structure the political, social and economic systems of the colony? Are there criteria we need to use to select colonists? Again, these are questions that are best answered with actual practice. Another area on the social front is how to handle the political situation with Earth governments. There are some treaties in place, but how is it going to work in practice. A colony in Antarctica will face very similar issues to a space colony on this front.
These are all areas where we can learn a lot. Yes, we can study each of these areas individually, and there are doubtless experts in each of them, but doing it all at once and seeing how it synergizes is only possible with a colony. How do each of these things interact with each other.
One of the big arguments for doing this on earth based colonies first is that the failure mode is less harsh. If a colonist decides they can’t hack it, leaving is at worst difficult, not impossible. If a pressurized colony has a failure in space, fatalities are likely, but if you are pressurizing from 0.7 atmospheres to 1.0, no one is likely to die. If all of your plants die, no one is going to starve to death.
Another reason for doing this on Earth is training. Suppose we are going to set up a colony on Mars, and we selecting colonists, how about a resume that says that John has three years experience living in a colony of 150, that he was in charge of air handling for the colony and cross trained in several other critical skills. You just can’t get that without Earth-based colonies as training sources, and maybe, the entirety of a functioning Earth base colony moves in mass to a space based colony. They have already proved they can work together and be successful.
Perhaps the most powerful reason for doing Earth based colonies is that they build momentum. The problem with space colonies is that even for the richest men on the planet, or even governments, all they can do is work on building up the structures. For the average person, even dreaming about it is unrealistic. That means these energies are necessarily turned elsewhere. But with Earth-based colonies, there is an option that is in the reach of far more people, and as more people are able to start working on this, focusing their energies towards this goal, and taking practical steps towards it, it develops a momentum that we can’t achieve any other way.