So, I was browsing the web today and I came across this site . (BTW, thanks Michael for the link). Check out the number of bodies that orbit in our solar system that are over 200 miles in diameter. 88 total (mind you this includes the 8 planets, our sun, and all moons so not all are available for use). So, I did some number crunching. Depending upon what these "Dwarf Planets" or "Trans-Neptunian-Objects" are made up of I see some potential for mineral exploitation.
Think about it.. Total volume of an object 250 miles in diameter (round number closest to the smallest TNO) is about 8.177 million cubic miles. Some of that is going to be 'useless' material (more on this later). But some of it might be metals or other (directly) useful elements.
How might this help? Well, imagine being able to capture one of these bodies. We'll say for argument's sake that we can a.) Get out there ourselves, and b.) Figure out a way to propel it back towards Earth. Now, capture it into a Lagrange point (probably L4 or L5) and hollow it out. Leave say a 2-5 mile crust. (Here's where we first start running into technological limitations. I'm honestly unsure of if we could do this. However, I think if we take this idea as a goal to work towards while developing the ability to get a planetoid back to Earth we could do this). Mine out the metals (using some for structural reinforcement of the planetoid cause we'll want the thing to be stable at the end), and remove "non-useful" material. Once the planetoid has been hollowed out you should have a volume of about 7.2 Million cubit miles on the inside of the planetoid that's hollow. That's an internal surface area of about 180,864 square miles of potentially usable space.
During the extraction process oxygen, and nitrogen (the two main gasses essential for human life) could potentially be recovered from the planetoid and used to create an internal atmosphere. (note current estimates of about of nitrogen in asteroids is zero so nitrogen may need to be salvaged from other gas giants/moons). The assumption is made that if humanity could bring a planetoid back to a L4 or L5 point to be processed like this that we could then also start it spinning in such a way that the interior surface would have close to 1G of centrifugal force at/near the equatorial belt. This internal "gravity" would fall off till it reached close to zero at the 'poles'. This would then trap the 'atmosphere' near to the interior surface of the planetoid and enable this surface to be used for habitat (more on this later).
Extraction would start at one or both of the poles and once it was complete airlocks of some sort would need to be established to ensure retention of atmosphere/water. The crust being left at 2-5 miles thick is to ensure that cosmic radiation does not penetrate into the interior of the planetoid. This ensures that long-term exposure to radiation will not be a problem for inhabitants of the planetoid.
Another assumption is made here. That, because of the amount of energy required to move/spin a planetoid with a diameter of more then 200 miles, humanity will also have figured out some sort of answer to their current power problems. This might include power sources such as fusion, or a better adaptation of solar power. Solar power will cause the usefulness of this plan (as will be expanded later) to be less trans-Earth, but could still prove of some use. Also, fission will be problematic due to limited amounts of fissionable materials on Earth (or other planets/moons).
If such a power source is available then the planetoid can either be powered and allowed to be used as either a giant research lab (with gravity ranging from 1g down to microgravity conditions in an enclosed are), or perhaps a giant agricultural habitat. Depending upon our ability to understand weather (because such a large area will have some form of weather) the interior of the crust might even be sculpted during the extraction process to allow for better growing conditions.
Another possibility depending upon technological conditions on Earth at this time, especially depending upon if nanotechnology has shown to have true promise and a break-through has occured, or if it has fallen by the way-side and been discounted as wild-eyed dreams. If nanotechnology, or some sort of successor technology has provided industry with a way to process material into its constituent elemental components then there might be no truly 'useless' material scavenged from the inside of this planetoid. Instead the material could be used for construction, atmosphere, or simply to create soil to be used in a agricultural habitat.
What could these be used for then? Imagine a planetoid sitting either in a Lagrange point with the moon, or some other location where it wouldn't cause tidal problems for Earth. Lets say that the planetoid is 400 miles across. And lets say that it's been determined that as the size of the planetoid goes up the crust as to be thicker to provide proper stability. so we've got a 10 mile crust. That's a radius (of the internal sphere) of 190 miles. Plugging that into the surface area of a sphere and we end up with 453,416 square miles. If say, 50% of that was used as farmland, and the rest used for habitations/maintenance area. That's 226,708 square miles of farm-land. That's 6% of the total land-mass of the USA (including Alaska), or 7% if you don't include Alaska. Not that much right? It would be like dedicating the area of equal to two states the same size as Montana solely to farming. Not talking about land devoted to raising cattle or other animals. Heck, have yourself another planetoid devoted to that. Then think of this. With a pre-sculpted surface designed for optimal growing conditions (and some sort of light/heat source at the center of the planetoid designed to have day/night cycles), and a 365 day/year growing season; how much food could be produced? Maybe enough that we wouldn't need to have farms on Earth's surface? Maybe we wouldn't have to clear prairies, or rainforests for farm-land. "Maybe" because of agricultural planetoids, and the minerals cleaned out of them we wouldn't need Earth for anything besides a preserve.
Think then of what it would mean for travel between planets. If we wanted to explore, terraform, colonize, whatever any of the planets.. Why not 'build' another habitat? Design it with future agricultural use in mind. But instead of using it for farmland right away, use it as a giant ship providing lifesupport and 'gravity' for the inhabitance during the voyage. Then convert it to agriculture once orbit has been achieved.
With 51 TNO's and 4 astroids known to be larger then 200 miles in diameter there are probably more of these bodies then we could use to ensure plenty of food/science research, or simply enough living space for everyone. I saw somewhere (and now can't remember where) that any space-going vehicle designed to provide close to 1G of centrifugal force to act as gravity would need to be at least 200 meters in diameter so that the speed at which it had to spin would be low enough to ensure that the passengers didn't get sick. There are probably a lot more bodies out there that are smaller then 200 miles and larger then 200 meters in diameter (yes, I know I just used imperial and scientific measurements in the same sentence. so sue me). Use the smaller ones for personal craft. In that case they don't have to be hollowed out in a sphere. Just make sure that there is a cylinder at the center of the rock that is at least 200 meters in diameter. Then spin the rock around the axis that the cylinder makes up. This will give living space all along the inside of the rock.
Instead of mining metals from Earth to build spaceships. We should look at using the resources already out there to provide the living space for humanity as we leave the cradle and head first to the rest of the solar system, and then to the stars. Earth is our home. Why bother pulling it down around us? Indeed, if we leave it alone the biologists will have a field day watching from orbit what happens to the rest of the species on the planet. Instead of killing off thousands of species in our lifetime as we attempt to create "more space" we could leave our planet a better place then we "found it" (or were born on) and give the rest of the animals on the planet a fair chance of their own without a super-predator at the top of the chain.