You're talking about two very different scenarios. Metals would be brought back to Earth to improve life here.
Carbon, Hydrogen, Oxygen and Nitrogen would be essential to establishing habitats in space but close to useless if brought back to Earth.
Carbon, Hydrogen, Oxygen and Nitrogen would be essential to establishing habitats in space but close to useless if brought back to Earth.
The Moon and Mars are relatively deep gravity wells. The economics of extraction in a deep well are totally different from those on light platforms. For an asteroid, over the long run, transport costs are minimal. (You can move mass very slowly and thus efficiently when you have that much of it.)
I think this solves the transport problem for space manufacturing. I know the math can sorta work if you use volatiles for transportation (e.g. rocket) but it seems so wasteful to me. Two problems that bug me are: (1) How do you capture volatiles that will be released when you disturb the surface? (Have to do this early before you have a storage tank factory running) and (2] How do you debug problems in the system with 10 min+ latency. Is it good enough to have a manned replica factory in cislunar space? Do you send people? If so do you just go “all in” in building a large habitat in the long term?
Unlike on Earth, where various processes have concentrated the precious metal into some deposits where they are much more abundant than on average, e.g. in alluvial deposits in river beds or in magmatic sulfide deposits, on such an asteroid the precious metals are dissolved in iron and spread uniformly through all the asteroid.
Therefore even if the total amount of precious metals is very great, a huge quantity of energy is required for its extraction. For instance, in order to produce one kilogram of gold more than four hundred tons of iron will have to be processed (e.g. by first producing hundreds of tons of some acid used to dissolve the iron and the other non-precious metals).
So it is very unlikely that we will see any such mining operation soon. There are other metals much easier to mine in such a place, e.g. iron, nickel, cobalt, germanium, chromium, gallium, manganese (but which are not worthwhile to be brought back to Earth).
Perhaps an icy asteroid could help, if it was parked in orbit and slowly dropped rain on the deserts.
But then the sea levels would rise, and the water vapor would act as a greenhouse gas.
My understanding is that almost all the water in small bodies inside Jupiter’s orbit is in Ceres and the amount of water on Ceres is similar to what is on the Earth’s oceans. I think it is better to send people there and grow a population organically as you could reassemble Ceres into more living space than Mars and get people ready for colonization of generic outer solar system and interstellar objects.
>> Could I cool down the Earth by capturing a comet and dropping it in the ocean, like an ice cube in a glass of water?
> No. In fact, it's honestly sort of impressive to find a solution that would actively make the problem worse in so many different ways.
The short answer is, the falling ice adds about 100x as much heat from the difference in potential energy than it cools.