What we know about is not the problem.
I wonder how they figure that they have located and have good orbital tracks on 93% of the big rocks that are out there? Even if they have, shouldn't we be worried about the other 7% given the dreadful consequences of a collision? Oh, and what about them thar comits? Shouldn't we be worried about them too?
In truth, even if we identified something, comet or asteroid, that was about to collide with Earth, could we do anything about it? The safest procedure would be to have machines rendezvous with said menacing object with the intention of strapping engines on it or doing something else to alter its course. Such a plan is a lot like savings with compound interest. It is not how much you deposit every pay day, but when you started saving that matters. Remember, the sooner you start such a job, the less you will have to do.
Have we done any of the preliminary engineering to accomplish such a thing? No, we have not. Right now, we are too busy bickering over the scraps of this little mudball we were born on. I think that we are pushing our luck by ignoring space and its potential. The Friends of Ned Ludd are wont to tell us that we should mind our own business and leave celestial matters in the hands of God, but I don't believe in God. I don't think that God believes in God, let alone Ned Ludd and his stick your-head-in the-sand approach to living in a dangerous universe. Not only is the universe rife with hazards, it is also full of opportunities for the alert and imaginative.
Let's just hope that visionaries like Elon Musk have their day. Otherwise we will be reduced to a mindless rabble squabbling over whether Arabic or Mandarin should be the official language of government.