<div style='font: 10pt verdana; text-align: left; padding: 0% 10% 0% 10%; '>The Earth gains about one ton of material a year from meteors - most are small, but they still deposit material. Some of this material contains iridium, which does not occur naturally on Earth. Hence, measuring the amount of iridium at a particular level in the ground is a good indication of the amount of meteor activity during that time. At the KT layer around the world, there is an extremely large amount of iridum in a relatively small amount of space. The amount is consistent with the amount of iridium that would be depositied by a 10km meteor. This theory is widely accepted - I've read it in a few places, and we went over it in my astrophysics class. If a 10km asteroid did not hit the Earth, there needs to be an explanation for where all the iridum came from.
Huge asteroids hitting the planet (like KT) are extremely rare, but they do happen. The moon has about 5 impact craters of 10km. Assuming the moon has been around for 3 billion years, that's about one impact every 10 million years. It is statistically likely that the Earth will be hit again - several more times, in fact - by a huge asteroid. It just might not be for another million years, or 10 million years.
Although there is an asteroid that might hit us sometime around 2040, I think. If it did, it would be the largest natural disaster ever recorded.
Oh, and for the record, it is possible to kill just about all life on Earth. If an asteroid of 100km size hit the Earth, it would cause "global sterilization." The impact would impart so much energy that the oceans would literaly boil.</div>
