Has a Meteoroid Comet or Asteroid ever hit the Earth?

The Earth has been hit by rocks from space many times. We are certain of a fair number of impact craters around the world, but which ones were meteors and which were comets? And does it really make a difference?

There have been a number of planetary extinction events, things that killed 75% or more of all life on Earth. Until 30 years ago, no one believed that a large meteor, asteroid or comet could have caused them. Then Luis Alvarez made a persuasive case that an impact near the Yucatan peninsula in Mexico had caused such global devastation 65 million years ago that it killed off, among others, the dinosaurs.

252 million years ago, a similar but bigger impact at the end of the Permian era probably killed off 95% of all life on earth.

And just this week (January 2, 2009), scientists have announced that an impact in northern Canada 12,900 years ago may have killed off the mammoths, mastodons, and saber-tooth tigers - and all the humans in North America - by melting a Canadian glacier and flooding the Mississippi River with icy water that then disrupted the Gulf Stream and caused the "Younger Dryas" mini-ice age. Beyond those, there is the Meteor Crater in Arizona, impact craters in France, and countless more. Install Google Earth, and start looking for circular hills; you'll find impact craters all over.