This is not a threat we need to lose sleep over.

I did some articles on this years ago.

First of all, you would have to be a complete idiot to waste a resource like a nickel-iron asteroid on something like dropping it on someone. Mark it for mining later.

Second, you would have to find an asteroid that is already very close to the right trajectory, and put a "bump" propulsion unit on it. The further off it is, the more likely you will be able to do what you want with the tiny course corrections that such a propulsion unit could provide. You are not going to be able to drop a rock on Earth on short notice. In fact, it might take years.

It is unlikely that any propulsion package you could ship there from Earth would do the job unless you were SPECTACULARLY fortunate in locating an asteroid on almost the right trajectory. The ideal propulsion unit would be a mass driver that used nuclear or solar energy to chop bits off and fire them into space. NOT near-term technology.

Now, as to strategy: What use would you have for a Doomsday weapon that needed years to use? You would have to keep it a secret. ("what good is a Doomsday Weapon you keep secret?") If the bad guys can get a propulsion package to the target (ain't going to pull THAT off from a terrorist camp in Yemen), you can be damn sure the good guys will have that tech and probably more so, and could put their own package on the rock to screw yours over. If there is a detection system in place to find asteroids on a dangerous trajectory you might be found out anyway.

Short answer: As a terrorist technique, this sucks. If you've got the tech to detect the right rock and put a propulsion package on it, you sure as hell have the tech to drop a normal warhead on pretty much any city on Earth, shoot down the ISS, or do a lot of other damage without ever getting beyond the Moon's orbit.

An 800 meter nickel-iron asteroid contains metal exceeding the known world reserves of platinum, nickel, and cobalt. For that matter, it would contain about 8 percent of the known world reserves of gold alone. Now you've got a real reason for going out there.