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. I wonder how close an asteroid can come before it's pulled in by the Earth's gravity.

I'm certainly no astrophysicist, but astronomy, along with cosmology and quantum physics, have long been a hobby of mine.

Generally speaking, anything with the velocity of a typical asteroid will blaze by an object the size of the earth in a matter of about fifteen to twenty minutes or so. Add to that the mass of such an object, which adds to it's momentum, and chances are rather slim. This rapidly changes if the object is close enough to interact with the Earth's atmosphere, which then imparts enormous drag and friction, and dramatically decreases the object's kenetic energy (not to mention creates quite a show).

Bottom line, unless something like this is coming straight at us to start with, chances of the Earth "snagging" it are slim. Gravity is a terrifically weak force, although, since it is only accumlative (unlike the other three fundamental forces of nature), it overall is the most pervasive and influential.

I think the biggest concern, if this or one like it were to hit the Earth, is how governments would react to the impact/explosion. For instance, if it would have hit the US, Russia, or China, particularly near a population center. This could very easily be mistaken for a blast from a nuclear weapon. SAC/NORAD would probably be able to figure out that it was a big rock from space, but what about some of the other members of the nuclear club? Russia's detection infrastructure is probably nowhere nearly as sophisticated or well maintained as ours is. I imagine China would have a hard time identifying where it came from as well. With the explosion yeilding a blast in the mega-ton range, an event like this could possibly cause a country to believe that they had just been nuked and then launch a retaliatory strike against whoever happens to be on their list of targets, which might include the US.