Even when the question was recast as “What about life-ending impacts?”, it still wasn’t answered. This is pretty lame people.

Come on now. There’s an asteroid travelling at 10 km/second. How big does it get for us to start worrying? Now change the scenario and the asteroid is travelling at 100 km/second. How big does it take to worry us? What if we back it off to 1 km/second. Where’s the mega-disaster threshold?

Saying “it depends” is just a way of dodging the question. Of course it depends! Everything depends. I imagine it depends upon what the object is made of, but then again 10 billion tons of ice, or iron, it’s still 10 billion tons of trouble. I suspect that the angle of impact doesn’t matter much (glancing blow or straight on). I also suspect that land or sea impact doesn’t matter much, it’s bad either way.

I know that NASA’s Spaceguard Survey is looking for objects down to 1 km in diameter. What I’m not clear on is, was this threshold chosen for it’s technical achievability, for a need to achieve a cost objective, or is this an important danger threshold?

It fused with the earth. Both bodies melted and the iron and other heavy stuff from both sank down to form the core of the “new” earth. The surface ejecta became a ring around the planet that coalesced into the moon.

I’ve even heard some sources give a name to the this object – Theia, or something like that. It really isn’t accurate to call it an asteroid. It was a planet in its own right. (Though by the current definition, both earth and the impactor would have been only minor planets at that time, as the orbit had not yet been cleared!)

A 10 mile wide asteroid (like the one associated with the K-T imact…when did that become K-Pg?) apparently is enough to cause major extinctions but clearly leaves lots of survivors. Humans are pretty crafty and would likely be among them, but you could expect a huge population crash and probably the collapse of civilization. Is that Armageddon?

Anything below that could cause a bad tsunami, wipe out a city, cause years of climate change, but not exactly the end of the world.

And what about the Armageddon precursors that we always see in the Meteor/Impact style movies? Well, meteors large enough to cause local damage on the ground are pretty common, arriving every few centuries or so. (The last one was the 1908 Tunguska strike.) Figure the odds of seeing one during your lifetime are about one in ten.

But in the movies those little impacts always hit the Eiffel Tower, Wrigley Field, etc. Odds of that? Well, Earth’s surface area is 5×10**14 square meters. If you figure there are 500 major world landmarks and they average 10,000 square meters each, that’s 5×10**6 square meters of landmark area. Odds that a strike will hit a landmark: about one in 100 million. Odds it will happen in your lifetime: about one in a billion. Odds that any one person on Earth has had sex with Angelina Jolie (assuming she has had 100 partners–possible–and that she selected them completely at random–likely): about one in 100 million.

So these things are unlikely, but they *could* happen.