Our planet gets hit with meteorites all the time, they pose very little effect and cause only minor damage from time to time, the reason for this is, our atmosphere creates a barrier of particles and as we know particles have mass, a fast moving object through mass causes friction, friction causes heat and the meteorite burns up on entry.
Comets, on the other hand, are much larger, and will not burn up on entry. A comet will enter our atmosphere at a tremendous velocity, and impact the Earth capable of annihilating life as we know it. Of course this is a worst case scenario, or better known as an extinction level event the end of mankind on earth.
What can we do? First we have to consider where it will impact. A land impact would cause a large blanket of dust blocking out the sun for perhaps a couple of years leaving us to try and survive a nuclear winter, below freezing temperatures, loss of wildlife, vegetation and clean drinking water not to mention the complete breakdown of social services like power and medical responses.
If it hits one of the oceans, it is still hitting land. It will flash boil millions of gallons of sea water and slam into the ocean bedrock. It will create massive tidal waves that can travel extremely fast, almost at the speed of sound and will travel inland for hundreds of miles before it ends.
It is the opinion of this writer, that our government won't tell us if such an event were about to occur. We have no current technology to stop such an event from happening, so why would they tell us? It would only cause chaos and panic.
It is also likely we won't even see it coming. Space is vast,and deep and we cannot monitor all of space. It would be like trying to find the head of a pin somewhere on earth with nothing more than a pair of binoculars.
If your interested in seeing this kind of event, Hollywood made some great movies on this type of disaster. One movie is called Deep Impact and the other is called Armageddon.
So, what can we do? prey it never happens because, your guess is as good as mine.
This has been a test of the emergency broadcast system. If this had been an actual emergency our government would have done nothing.