Harvesting Asteroids

What is the carrying capacity of the asteroid belt? By this, I mean that irradiated vacuum-packed collection of rocks and gravel equalling 3 percent of the Earth's mass that orbits mostly between Mars and Jupiter?

It is probably in the trillions.

It has the capacity to be used to generate orbiting communities with a land surface area greater than that of 10,000 climate-controlled earths.  Growing areas can be made perfect for growing crops - with perfect temperatures, sunlight, and nutrients - all free of potential disease or pests.

It begins by taking an asteroid and using its material to build a shell in space. For simplicity, imagine a tin can kilometers in diameter and kilometers long.

Much of the mass for such a structure is its 1 meter thick shell - used to shield those inside from cosmic rays. This shell can be made of anything. There is no need to look for any type of special construction materials. All that us needed us mass - enough mass to stop most of the cosmic radiation that hits it.

This mass does not need to be set spinning. It simply floats in space. In fact, you would not want it to spin. Spinning that much mass will only create forces that will try to tear the cylinder apart. 

It's occupants will likely want an artificial gravity, however. That can be simulated by setting the structures inside spinning - like a roller coaster car on a track on the inside of the can. Made large enough, nobody will perceive the motion.

Physics being what it is, a spinning interior will transmit angular momentum to the exterior over time. This problem can be solved by having sections in the living area turning in opposite directions. For a 12 kilometer city, one could have the first three kilometers and last three kilometers spinning one direction, and the middle six spinning the other direction.

Constructed in this way, the material in the asteroid belt contains enough material to create the surface area equivalent of over 10,000 earths.

Each square kilometer of surface area on the Earth is supported by 2000 cubic kilometers of material beneath it. In space, a city constructed as described will require less than 0.002 cubic kilometers of material per square kilometer of (interior) surface area. Undoubtedly, much of that material will go to things other than creating surface area. Half? 90 percent? Using just 1 percent of this material to create surface area means the equivalent of 300 earths.

Furthermore, the earth is made up largely of oceans, mountains, deserts, jungles, and other inhospitable places. The inside of a space city is built the way it's inhabitants want it to be built. The temperature, humidity, are all determined by public will.

This will also be true of the farming pods. There will be no drought. No plague of locusts. No"growing season" bounded by harsh winters where the land must sit idol. Not only are we talking about the surface area of 300 earths. We are talking about earths that are made up, 100 percent, of the best land in the best climate for living or growing food.

Consequently, we are talking about earth-equivalent of land areas that can hold far more than the 7billion people of Earth itself.

So . . . . 

300 earth-equivalent land area * 20 billion people per earth-equivalent (which is far less crowded than our current land area on Earth) = 6 trillion people.

Which is the carrying capacity of a collection of airless, radiated rocks in space.

And any asteroid turned into space cities is an asteroid that is not going to hit earth.

The value of a space program is found in the desires that it fulfills - the propositions P that are true in the program itself and the effects of that program. 300 earth-equivalents of living space is available as a consequence of the space program. It can be used to fulfill a lot of desires.