Let's be sure we do the math right, eh?

Why was Leif Erickson's kin able to grow crops for 200 years in Greenland? Today it is still far too cold...

Count me out on this idea, mon.

I'm still waiting for my Al Gore promised global warming to kick into high gear. I have a few palm trees I'd like to plant outside year-round as they are getting too tall to be in the house.

We're going to need all of that carbon when we colonize Mars.

Why go across the solar system to get asteroids when we have plenty of dust already here? We also have no shortage of iron. besides, I think we already put too much pollution into the air as it is, we don't need more.

There is no such thing as a dust mine. To get the kind of fine powder we need, the kind that stays suspended in the atmosphere for years, we either have to start with something tiny and build it up, or start with something big and grind it down.

Starting tiny and working up- scientists have proposed burning sulfur to generate aerosolized SO2. Downside is that SO2 reacts with water to make sulfuric acid. Acid rain, yuck.

Starting big and grinding down has problems, too. It takes a lot of energy to grind a rock (or whatever) down to a super fine powder. A lot of energy. Then lifting that quantity of material (tons and tons and tons) up into the stratosphere is a major undertaking as well.

In contrast, I am proposing that we launch just a few tons into space. Then, because we would choose a "near miss" asteroid for the source of our meteorites, it doesn't take much energy to redirect them to collide with the planet. No energy intensive processing is required, since the collision with the atmosphere provides the energy to turn the rocks into powder. Likely, we won't even have to exert any energy to dig the rocks out of the "ground": near-earth asteroids are anticipated to be loosely bound together "rubble piles".

You don't need a mine to get dust. Many countries in the Middle East are literally buried in the stuff. The quantity you would need to have any effect would require either a really big asteroid or a heck of a lot of small ones. A big one would be too dangerous, we're already trying to figure out how to avoid them. Many small ones would be very expensive and would still pose a safety hazard.

I don't really know if this would work or not, but I wouldn't want us to try it.

Dust:

On earth, the super fine dust that we need has a very strong tendency to get blown or washed away. It ends up in the ocean where it dissolves quickly because it has such a high surface area. You can't just go and dig the stuff up. Maybe "dust" isn't the right word. Think "cigarette smoke." Bigger particles wouldn't linger in the stratosphere long enough.

The calcium and magnesium silicate minerals that we would like to use are certainly only available on earth as solid rock. If they were already powdered, they would already have reacted with CO2 from the air.

Danger:

The idea is not to throw a whole asteroid at the earth. The idea is to throw chunks of an asteroid at the earth. Chunks that are small enough to burn up completely in the atmosphere, well before hitting the ground.

Cost:

The cost comes from launching the mass driver. After that, everything is basically free. We would choose a "near miss" asteroid so we don't need too much energy to redirect the chunks of rock. The little energy that is required would come from solar panels. It would take a few years to redirect a significant mass of meteors with our little mass driver, but orbital dynamics are so certain that we can act that far in advance and still be confident that the pass will be a completion.

The Sun-Earth L4 and L5 Lagrange points might be a good place to look for asteroid raw materials. Fight global warming and prevent potential catastrophic asteroid impacts.