In one week, "an office-building-size asteroid will speed past Earth faster than a bullet and closer than some communications satellites."

"The good news: There’s no chance of an impact. At its closest, asteroid 2012 DA14 will pass about 17,000 miles above Earth."

Wow. I had no idea communications satellites were that far away!

But anyway, an office building, eh? Which office building? Are we talking Pentagon-size? The Pentagon, you know, is the world's largest office building. Obviously, this asteroid isn't Pentagon-sized or the WaPo would have said an asteroid the size of the Pentagon, which would have sounded much more amusing (and scary) than "office-building-size asteroid," which leads me to picture an ordinary place downtown in my own city.

So, I'm picturing it, this office-building-size asteroid. What happens if it hits?

Meteor Crater in Arizona — nearly a mile wide — was... probably gouged by an office-building-size space rock about 50,000 years ago.

How big must we go before we're talking wiping-out-the-dinosaurs apocalypse? Apparently, that would be about 5 miles wide. And, no, the Pentagon is not 5 miles wide. It's 921 feet along each outer side facade.

ADDED: A reader writes:

While the physics that I do for a living doesn't have a whole lot to do with planets or asteroids, I do enjoy the stuff and am fairly well versed in it. Since you were wondering on the blog today what a 150 foot asteroid would do, it's about half the length of the asteroid that did the Tunguska event. Remember that mass goes more or less like length-cubed, so the 150 foot asteroid would be about 1/8th the weight/power.

Tunguska had the power of 1000 Hiroshima bombs. So 1/8th of that still would more or less destroy a city if it landed on it. Most likely, of course, any asteroid would hit the ocean. But that would still cause a massive tsunami like we haven't seen in recorded history.

I don't know if you've heard of the Apophis asteroid, but it's more than 3x the length of the Tunguska asteroid and it's going to come very close to Earth in 2029, and then again in 2036 (though it will not hit either year, it's still been mixed in with a lot of silly 2012-style apocalypse myths). Scientists have already accomplished the task of landing a spacecraft on another asteroid, so there is a lot of talk of attaching a beacon to Apophis in 2029 so we could follow it on its path. Would be really fascinating science.