Earth Impact Effect Program

Now this is an interesting tool. It calculates the effect of an asteroid or comet impact on the earth.

You can enter the data for the dinosaur killer a 15 km (9.3 mile) wide dense rock asteroid and see what happens if you drop it on the city nearest you.
This creates a massive 113 mile diameter crater, a magnitude 10.2 earthquake, and 66 mph winds. Then the ejecta starts to fall.

You could also try to see what happens if you drop the 20m iron asteroid that created Meteor Crater in Arizona, something that is much more likely. That would go off like a 1 Megaton nuke and create a 20 mph wind. This would make a big mess but is survivable outside of the blast radius.

If you think that governments of the world are doing something. Well, they are but not very much. The US only spends about $4 million a year on the search for near earth objects (NEO) with a goal of finding 90% of object 1000 m or larger by 2020. Assuming we can get a satellite or two up there to do more looking.

A 1000 m object could create a 15 mile diameter crater, a fireball 10 miles across, an 8.1 magnitude earthquake, drop 26 inches of ejecta (rock and ash) and winds of 586 mph 100 km away. That would doom any major metropolitan area but most of the earth would be fine. Think of it as a randomly appearing volcano.

But that is just lot of numbers. For something more visual here is a video I found on YouTube.