Jay Manifold has some updates on our prospects for getting hit by asteroids.

Someone once put it this way: "The Dinosaurs became extinct because they didn’t have a space program. Any questions?” A statement you just got to love! It was even speculated (by a Planetary Society Writer) that Asteroids are environmental factors that 'weed-out' those races incapable of mounting a defense; thus, implying an evolutionary ingredient that sort of helps lead (ultimately) to advanced races.

I, myself, look at all those exoplanets being found in abundance and question why most of their orbits are so elliptical. So many planets and brown dwarfs are being found that it might be common for some of them to go drifting through (or by) a solar system - only to disrupt the hell out of everything. One team (noting the direction and consistency of comets falling into our inner solar system, speculated about a Jovian-sized Planet X ‘in our own solar system’ - not one just beyond the orbit of Pluto (which is 39 AU) but one 50,000 AU away - but still bound to our star by gravity.

I've read SF stories that included that notion -- Lucifer's Hammer by Niven and Pournelle, I believe, for one. The unseen planet's gravitational influence is what caused the title comet to head for the inner solar system.

The hypothetical planet (or brown dwarf in some versions) is usually nicknamed "Nemesis."

The notion of periodic showers of comets is intriguing, but problematic; see http://www.ucmp.berkeley.edu/education/events/cowen3a.html. I think the risk we have to manage is geopolitical, that is, the misinterpretation of a kiloton-range impact as a nuclear strike. Shameless plug -- see the "Asteroid Warning System" posts permalinked in the left sidebar of my blog.

One wonders what life will be like when someone designs a rocket that attaches to and aims asteroids. The upside is that we'll have a vehicle that can deflect 'em away from Earth. The downside is that we'll have a vehicle that can deflect 'em toward Earth - kinda like The Moon Is A Harsh Mistress without the fixed-location linear accelerators.

My biggest fear is even a relatively small strike to an ocean. A six mile rock may not sound like a lot, but it would stand a mile out of the water an the ocean's deepest part. The waves from such an impact could reach 700mph and travel as much as 300 miles inland around the entire basin (..at least that's what I'm told happened 65m years ago).

Something like that happened in Niven and Pournelle's Lucifer's Hammer. If I had the movie rights, the surfer dude who catches the Los Angeles tsunami would survive, and several scenes after the drowning of LA he'd be seen hitchhiking with surfboard under arm outside of Vegas.