More on capturing asteroids
It seems like I read something about this in the news a couple of weeks ago, but I can't find the reference now. Anyway, Centauri Dreams has a post reviewing a paper by Didier Massonnet and Benoit Meyssignac. A Novel Strategy for Asteroid Deflection
Essentially, we capture a near Earth asteroid and park it at L1 or L2. If the Earth is ever threatened by a cometary or asteroidal impact, then the captured asteroid can be maneuvered into the path of the oncoming object to either deflect it or break it up. This is one of those ideas that make perfect sense once you hear them, but it sometimes takes a rather insightful person to come up with it in the first place. It gives new meaning to the old expression "Fight fire with fire."
There are two potential drawbacks to this plan that I can see right away. The first is the propulsive requirements to first capture the shielding asteroid, and then to move it again into the path of the approaching object. Of course these requirements would likely depend on the mass and orbital elements of the shielding asteroid, but I'm sure some kind of lower bound could be computed to give us some idea of the magnitude of effort that would be involved in such a plan.
Assuming, however, that the asteroid can be suitably maneuvered, there is always the chance that introducing a second body of significant mass would just make matters worse. There is always a little bit of margin for error in orbital calculations, and that error gets more significant when massive bodies come in close contact. I'm sure that such a plan would not be put into effect unless it were clear that the intervention were necessary and the outcome, whatever it may be, preferable to doing nothing.
Anyway, the Centauri Dreams post goes on to talk about some alternative uses for a captured asteroid, namely fuel production. As long as you have a massive chunk of raw materials nearby, you might as well make some good use of it.
Personally, I think there are probably a great number of benefits that could be realized from having one or more asteroids in close proximity to the Earth, most of which we haven't even thought of yet. It will be very difficult to establish any kind of significant off-world human presence without having a large amount of material resources readily available. That is why the moon is such an important destination for the near-term future of manned space exploration. There are abundant natural resources available on the moon, the explotation of which will open up vast new possibilities for human activity off-Earth.