It's been four days since an asteroid tore a hole in the sky over Chelyabinsk, scaring the crap out of the city's inhabitants and pretty much everyone else around the globe. Details are finally starting to emerge about this jaw-dropping incident, so we've prepared a round-up of the preliminary findings.

The asteroid made its low angle descent into Earth's atmosphere on February 15 at 03:20 GMT, and was the largest in more than a century. In fact, events of this magnitude are believed to occur only once about every 100 years or more.


Based on extensive video evidence, the Chelyabinsk asteroid flew in at a shallow angle of 20° above the horizontal (NASA called it a "grazing impact through the atmosphere"). When it reached Earth, it was moving at about 11 miles per second, or 40,000 miles per hour (18 km/s, 64,000 km/h). It streaked across the Russian sky moving from the northeast to the southwest.

Image at left shows the meteor's vapor trail — click to enlarge (via ESA).

Peter Brown of Western University has calculated that the asteroid was about 56 feet across (17 meters), which is roughly the size of a school bus. For comparison, asteroid 2012 DA14 (which paid us a visit later that same day) measured 150 feet across (45 meters), and the Tunguska asteroid of 1908 is estimated to have been about 330 feet across (100 meters).


Brown also estimates that the Chelyabinsk asteroid weighed about 7,000 to 10,000 tons when it entered our atmosphere. Given what we know about other near-Earth objects (NEOs), it was likely comprised of densely packed iron and nickel. It may have also contained carbon dioxide or water, which would have accentuated the impact of the sonic boom.


Image at left shows the estimated orbit of the object around the sun (via Peter Brown).

So, given its size and speed, along with the measurement of low-frequency sound waves detected by a global network (the infrasound frequency of .4Hz to 20Hz could be heard halfway round the world), the asteroid unleashed a torrent of energy equivalent to nearly 500 kilotons of TNT. That's about 30 times the energy released by the Hiroshima atomic bomb. The bolide was about nine to 12 miles (15-20 km) above the ground when it disintegrated — immediately above the city of Chelyabinsk.

The resulting sonic boom let loose a shockwave that shattered windows across an extensive area, injuring over 1,200 people. Windows tend to break when air pressure exceeds about five times normal, but based on the evidence, it's likely that the air pressure in Chelyabinsk reached about 10-20 times normal.


And indeed, most of the damage was caused by the airburst, and not by falling fragments. Much of the object burned up on entry as the tremendous air friction ripped it apart; the asteroid would have experienced an incredible rate of deceleration. At one point, the meteor was brighter than the sun. But prior to disintegrating, air pressure accumulated in front of it, eventually releasing as a sonic boom.

The entire event, from atmospheric entry to the meteor's disintegration, took 32.5 seconds.


According to astronomers working for NASA and the ESA, the Chelyabinsk asteroid had no connection to DA14, noting that the trajectory, the location of entry, and the time delay all indicate no relation.

Sources: NASA, European Space Agency, B612 Foundation, iopblog.

Images: Discover Magazine, Mirror.