h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This entry quotes the cratering rate as "4.7 objects create craters above 20 kilometres in diameter per km2 per billion years" and goes on to imply that this is not a very high rate.

But the surface are of the earth is about 500 million km2, which is half a billion km2. So that means that there are more than 2 asteroid impacts causing a crater of more than 20 km every year! What am I missing here?

Either I'm severely off the rails or this entry is. It says the chances of me dying of an asteroid impact is 1 in 20,000. This means that given the population of the earth is about 6,000,000,000, a total of three hundred thousand people living today will die of asteroid impact.

???

You're not off the rails... hopefully someone will drop by with the right stats.

Well spotted, my friend

I've found the answer!

http://www.spaceguardindia.com/Threat.html - Table 1.

The table is labelled "Causes of *Unnatural Death* in the *USA*" [emphasis added], and gives the probability of unnatural death by asteroid impact in the USA as 1 in 25,000.

RhoMuNuQ

Ah America! That explains it, then! If you're an American, the probability of being abducted by aliens is 1 in 100, so I've heard.

You're right. That's stupid. Can't believe I missed that.

I think the "per km2" shouldn't be there. 5 per billion years sounds more plausible. It doesn't make sense anyway to have any number of craters above 20km in diameter per km...

TK[1]

I think the scenario which is in most of your minds at the moment is the one where a single rock flies down and smashes through one unfortunate individual's skull, and that this will happen to roughly 300000 people alive today.

A better mental picture would be one of a huge rock thundering through the sky and taking out a sizeable portion of any random country, including around 300000 of its occupants.

The figure is only a very rough one, and is calculated from estimates of the liklihood of a large impact (Anything from something that would wipe out Leicester up to an ELE) occurring within any random individual's lifetime and the liklihood that it will happen within a certain distance from them (i.e. a couple of 10s of miles for the Leicester scenario, or anywhere on the planet for the ELE).

I'd go through the maths, but I really don't know the actual figures used- all I know is that that figure is the one quoted by NASAs NEO/impacts website (I forget the URL) for someone on this planet and I stand by it- if anyone else wants to calculate their own interpretation of the statistic by all means go ahead, I don't have any objections to other estimates being included in this entry.

TK[1]