Thanks to Russian drivers and amateur cameramen, a few weeks ago we had the privilege of witnessing a rare even - a falling fireball. Unlike most fireballs, this one caused damage: its soundwave blasted the window panes of thousands of homes in Chelyabinsk region in Russia and harmed more than 1,000 people. But just how rare are these kind of events? And what is the probability of a more dangerous or deathly collision?
In fact this was not the first time a similar event happened in Russia in (relatively) recent times. In 1908 another fireball fell on the Tunguska region and its blast tumbled trees in an area of 2,150 square kilometers. Many people would think this is a curious coincidence. An astonished visitor (@Oldtymer) of the Guardian Science blog on the Russian meteorite summed it up like this: "Why does God always seem to chuck his fireballs against the Russians? Does He have something against them?"
Actually, meteorites keep falling all over the Earth all the time, only we don't usually notice because most of them are too small or there is nobody around to see them. Just take a look at the Meteoritical Bulletin Database maintained by the Meteoritical Society. The database collects meteorite data found in different publications. It contains information on metorite name, type, mass, location and date where it was observed or found. You can search the database to see the meteorites fallen or found (not necessarily observed while falling) in your country and even map them. For example, if you have Google Earth installed on your computer you can see Russian meteorites locations clicking here (download the kml file and then open it on Google Earth).
The Guardian also published their own map based on this database. As you can see, the distribution of meteorite findings and observed falls is not uniform over the world...for example, meteorite locations appear all over the place in central Europe and the US, but rather sparsely in Russia. Or in Canada.
This is caused by observation bias: more meteorites are found where there are more people looking for them, obviously. And of course, that's why there are so few dots in the middle of the sea. Another different kind of bias is related with where they fall. Meteorites are more easily found in flat regions with low rainfall, such as deserts like the Sahara or ice deserts like the Antarctica, where meteorites have a contrasting appearance with the typical soil of the region. In Chelyabinsk, for example, meteorite hunters had a relatively easy job searching for dark stones in the white background of the iced lake Chebarkul or the snow.
Once we take into account these sources of bias (as well as some others) and the finding efficiency (the probability of finding a fallen meteorite), we can use databases as the one above to estimate how many metorites hit the Earth each year. According to this classical article by David W. Hughes, each year 90 meteorites fall in a surface of one million square kilometers. That means that about 1500 meteorites fall each year in Russia, with a surface of 17 milion square kilometers, about 20 meteorites in the UK and almost 900 in the US. So that now it seems clear which is the anser to old Oldtymer. As another visitor (Hydroxl) of the Guardian Science blog put it, "Russia is the biggest country (ie meteor target) in the world so it's just the statistical odds."
Now that we know how many meteorites hit the Earth each year, we need something else in order to estimate their risk. We also need to know their size. Even though about 150,000 meteorites will have fallen on Russia in the last 100 years or so since the Tunguska event, only two (that we know of) were big enough to cause real damage. And that will be the subject of the second part of this article.