You know, instead of arguing over whether or not a comet impact occurred, or whether it might have caused some geological damage or climate change, we could all just look with our own eyes at what a comet can do:

Shoemaker-Levy 9 Impacts Jupiter - July 1994

Before impact it broke into 20 smaller pieces, delivering the impact energy of approx. 6 million Megatonnes of TNT ---- each! That's 6 trillion tonnes of TNT times 20. The results shown vividly in photos are 20 Earth-sized bruises on the biggest planet in the solar system.

This is what it might look like if just one of those 20 fragments had hit the Earth... but I'm inclined to think it would be worse. Let's face it, Jupiter could take the hit. Earth? Probably not.

So, if you're arguing about geological evidence to justify NEO studies, it's kindof moot. There's at least one very good example with photos to show. Non-periodic comets are probably the most dangerous objects out there. Why?

1. We don't know they're coming;
2. They're on million-year orbits, so by the time they pass Pluto on their way to Earth they are already essentially at perihelion and are travelling at maximum velocity;
3. They're big. Really big.
4. We're sitting ducks.