There are over 600 known asteroids that are closer to the Earth than the Moon is.
Admittedly, that statement depends on a play on words. In terms of physical distance (kilometers), that statement is false.
However, there is another measure of distance in space known as "delta-v". It’sliteral meaning is "change in velocity" – the amount of speed that one must either add to or take away from one object in space in order to get it to rendezvous with another object in space. It represents how long and how hard (and how much fuel must be used) in order to get two things to meet up.
What we are interested in here is the delta-v between a clump of material that can be used for building and the construction site in Earth orbit. I have written about building three industrial sites in geosynchronous orbit. However, I have not said anything about how we are going to get the material to these places in order to do the constructing.
We certainly are not going to be using local materials. Local materials in geosynchronous orbit are relatively scarce. We are going to have to import our material from someplace else.
Importing building materials is no great threat to construction. With very few exceptions, every building that you see as you look around was made from materials that were imported from someplace else – wood from a distant forest, concrete and stone from a quarry, and aluminum and copper from mines. Importing material to a point in space so that it can be used in construction is, in itself, a rather common idea.
It is still the case that we will want to keep construction costs down. So, there is an economic incentive to look for the cheapest material to transport.
The Moon has two disadvantages.
First, it is at the bottom of a gravity well. This is reflected by the delta-v that is required to get something from the moon into space. Even at one-sixth gravity, it requires a lot of heavy lifting.
On the other hand, lifting material from an asteroid would involve very little lifting at all. Just a slight nudge, and it will start to drift away from the asteroid. Simply build some spring-loaded legs that will cause the miner to leap away from the asteroid when it is full of material, and the bucket ship will drift away. Toss some of the material it collected back at the asteroid at a high enough velocity and it will start to drift into an orbit where it will rendezvous with earth.
Another advantage that asteroids have is that many of them are not even solid. These asteroids are made up of piles of sand and gravel all embraced in a group hug – like pouring a bag full of pellets into a bowl. Mining the asteroid consists only of picking up some of this gravel and sand and putting it in a bucket. When ready, the bucket will head on back to Earth to deliver its load at the refining station in geosynchronous orbit.
I mentioned that geosynchronous orbit could become the location for industry in space. Recall that three stations could handle all of the work being done by communication and earth monitoring satellites that currently exist. Also recall that geosynchronous orbit will not require as much expense to keep the satellite’s orbit from decaying. It would be a good place for a solar power station, which means a good place with enough energy to melt and refine the materials that come from the asteroids.
That material then can go into building the power plant, building the refining station, and eventually building larger space stations capable of holding hundreds, then thousands, then tens of thousands of people.
Another issue that will likely turn out to be significant is that, in space, people can 'choose their gravity level'. I have spoken about a rotating baton in space. It has a module on each end and a tube connecting it, and it twirls in space like a baton.
The length of the tunnel, the distance from the center, and the speed of rotation all determine the level of gravity being approximated. The inhabitants can effectively 'dial up' the level of gravity that suits their needs the best. They can even make individual choices to live or work at 'levels' between the tube and the modules at the ends of these twirling batons.
On the moon (and on Mars) an inhabitant only has the gravity that nature provides – 1/6th or 1/3rd Earth gravity respectively. If there are health risks or other costs associated with this level of gravity, the inhabitants are still stuck with it, just as we on Earth are stuck with living at normal Earth gravity.
So, there is reason to suspect that our attention should not be so solidly fixed on the moon. We should, instead, be looking at the 600+ asteroids that are “closer” than the moon – in terms of delta-v. These bodies are so small we can set a space baton next to it and give the miners all of the luxuries of dial-up simulated gravity, while scooping up the material that can then be turned into space cities.
Each asteroid itself would become its own city. It would take centuries to harvest the material in even one asteroid with a radius of 1 kilometer. This is more than enough time to build a space city right next to the asteroid, made largely out of asteroid materials. Hopefully, its founders will have the insight to ask the question, “What do we do when the rock is gone?” But even here, as on Earth, there are countless possibilities. The future, in space, is wide open.