Russian Meteor

Once again we are reminded that we live in a cosmic shooting gallery. With the unannounced arrival of a meteorite over the city of Chelyabinsk in Russia, and the unrelated near miss by asteroid 2012DA14 we can see how delicate the balance of our existence is hear on this planet, and how easily it could be wiped out. The Meteor of 15 February is now known to be the largest recorded object to have encountered the Earth since the 1908 Tunguska event, and it is the only such event known to have resulted in a large number of casualties. The object could be classified as a bolide, 'a large fireball which explodes or disintegrates' and the event has been referred to as an air burst, seeing as how it exploded during its passage through the atmosphere with spectacular and damaging results.

There are a number of estimates on the size and mass of the meteor, but it seems in general it was around 15 metres in size and had a mass, on entering the atmosphere, of some 7000 metric tons. It was travelling at approximately 18 km/second. If you do the maths this gives it a Kinetic energy of 1.134 x 10exp15 Joules. That's equivalent to the explosive power of approximately 300 Kilotons of TNT. In times like this we always seem to compare that with nuclear weapons and so this is around twenty times more than the nuclear weapons dropped by the USA on Japan during WWII. It was fortunate then (or maybe not for those injured) that the meteor exploded some 30 to 50 km above the Earth's surface. Another saving grace was the fact that it's trajectory was at a fairly shallow angle, meaning it travelled through a lot of atmosphere but exploded relatively high. Computer simulations of airburts show that the most dangerous are those that come in heading straight down. This is due to the fact that the most damaging shock waves from such explosions travel straight foreword in front of the trajectory.

For a more global comparison, the Chicxulub impact, which caused the last mass extinction here on Earth some 65 million years ago, was estimated to be equal to 100 million megatons of TNT. However lets not get complacent. This meteor was small, it exploded high in the atmosphere over an area of relatively low population density. It came in at a fortuitous angle and, spectacular as it was, caused only a moderate amount of damage. Many people were unfortunately injured, mostly by flying glass but it could have been a lot worse. The worrying points are we didn't see it coming, we are unprepared for such events and they will happen again. That is inevitable, it's just a matter of time and luck.