By Bruce Tabor
On 15 February, the sky over Russia was lit up by a great ball of fire – the Chelyabinsk meteor. NASA’s infrasound data can tell us a lot about it. But amazingly, so can amateur sleuths using YouTube, Google Earth, and some trigonometry.
The Chelyabinsk meteor entered the atmosphere, and exploded at high altitude near the Russian city of Chelyabinsk at 9:20 am local time Friday 15 February. Normally we rely on national space-science agencies to reconstruct these events but with some data from the web and some high school physics, you too could try your hand.
Stefan Geens of Ogle Earth was inspired to use maths to find out about the meteor using footage from car dash-board and building security cameras in Russia, which have proliferated as a way of fighting crime.
He made some assumptions about straight lines and constant speeds, and got some videos of the meteor over Revolution Square, Chelyabinsk. Then he used the distance between light posts to do some trigonometry.
To use this method you need some information. The explosion occurred at an elevation of 40 degrees almost due south of Chelyabinsk’s Revolution Square. The meteor was travelling a little south of due west.
You can get more information about distances using the gap between the flash of light and the sound of the explosion in the security camera footage. Time-stamped surveillance suggests a delay about 2.5 minutes until the shock wave reached the city. Assuming an average speed of sound of say 300 metres per second, you can calculate a distance.
Using two sides and one angle, you’re ready to do some trigonometry. The meteor exploded 45 km away at a height of about 35,000 metres. That’s three times higher than commercial airlines fly.
Most meteors start out their lives as asteroids, but when these rocks enter the atmosphere at high speed they change their name to meteor. Asteroids move through space on their own paths, but if they pass very close to us they can be effected by Earth’s gravity. Some of them enter our atmosphere and become meteors.
It’s friction with the atmosphere that makes them burn up as their kinetic energy gets converted to other forms like heat, light and sound (great balls of fire!).
So how much kinetic energy did the Chelyabinsk meteor have? NASA used infrasound data to find out. They estimate that the meteor had a diameter of 17 metres, a mass of 10 000 tonnes and entered the Earth’s atmosphere at nearly 18 kilometres per second.
Kinetic energy increases with the square of speed, so the astronomical velocity of the meteor meant that it had a lot of energy. And within a fraction of a second this energy of about 2 petajoules – that is 2 with 15 zeros – was converted into heat, light, and a blast wave.
There was 50 times more energy released by the Chelyabinsk meteor than would be released by an explosion of the same mass of TNT. That’s 30 times the Hiroshima blast and the largest energy release from a meteor since 1908 (when the Tunguska event released the equivalent of 10-15 megatons of TNT). Fortunately this blast occurred high in the atmosphere, which is why the damage on the ground was mostly limited to shattered windows.
All I can say is – goodness, gracious, great balls of fire!
This article celebrates 2013, the year of Maths of Planet Earth. The article was written by Bruce Tabor and edited by Arwen Cross. Thanks to John Sarkissian for proofreading for us.