With all the evidence that Earth has been hit several times in the past by large asteroids causing mass extiction events. What is being done to defend the planet if this should become a problem again?

If I am not mistaken, there is only about 2-3 million a year being budgeted to observe the skies for intruding objects. That covers maybe 1-2% of the whole sky.... I really don't know if there is anything that CAN be done (save becoming a space-faring race and spreading our jea...... err genes around!).

A few articles on the subject:
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The best we can do is spy the rock on its way and watch it happen.

Unlike the movies which paint heroic scenes were Technology and man overcome all odds and at the last possible moment advert ELE, in reality we would be helpless and unable to change the course of events.

If, on the other hand we have years of warning (Unlikely considering the dangers are earth passing rocks which are usually spotted days in advance, or just after they pass) We may get our act together enough (as a species) to do something about it.

That is if we scrap the notion that everything has to be approved through a committee. If we can't our project to divert disaster will be held up until the rock smashes into the earth.

Sadly, though we are aware of the potential fo disaster, we are doing nearly nothing to catalog and watch the skies. We are doing absolutely nothing to come up with a defensive plan.

In the case of an incoming comet, we are screwed. Not enough time, or knowledge of what comets really are to do anything about it.

We have been warned, In Arizona USA there is a far sized crater to tell us that rocks tend to fall from the sky.

Each night we can spy a falling star which is a clear indicator that stuff is raining down upon us all the time.

The cometary impact into Jupiter was a glaring reminder of the probabilities of comets hitting planets.

We have need warned, it appears that we continue to ignore the threat.

Revelation8 And the second angel sounded, and as it were a great mountain burning with fire was cast into the sea: and the third part of the sea became blood;

David