Saturday, March 16, 2013
What Happened Over Russia
On Feb 15th the explosion of a rocky chunk from space high over a Russian town named Chelyabinsk seized the attention of the world and its news media. Now, scientists from around the world have had time to study the reports and measurements from this event and piece together what actually happened. My colleague and textbook co-author David Morrison, one of the world's experts on asteroids and impacts, has posted a very clear description of this event on his Impacts Web Site.
I will quote the first section of it, but encourage you to read the whole thing and watch the remarkable videos he recommends.
"Shortly after sunrise on February 15, a rocky projectile entered the atmosphere over the Ural Mountains travelling at more than 18 km/sec [40,000 mph], and exploded with the energy of half a megaton.
The Chelyabinsk bolide [the term astronomers use to describe such a chunk entering our atmosphere, which gets hot and bright] was about 20 meters in diameter, or half the diameter of the famous Tunguska impact of 1908, which flattened a thousand square miles of Siberian forest. The bolide left a trail of smoky condensation across the sky as it vaporized in the atmosphere.
Its terminal explosion, at an altitude of 23 km, released energy of about half a megaton, equivalent to a couple dozen Hiroshima-sized atom bombs. When it exploded, the bolide was for a few seconds brighter than the Sun. About two minutes later the shock wave reached the ground in Chelyabinsk, breaking windows and injuring about 1500 people from flying glass. With a diameter of 20 meters, the Chelyabinsk impactor was smaller than most asteroids that have been detected by the telescopes of the NASA Spaceguard Survey, which focuses on finding asteroids of about 100 meters or larger.
Furthermore, since it approached the Earth from very near the direction of the Sun, it could not have been seen by any ground-based [visible-light] telescope of any size. It therefore struck without warning, although the atmospheric explosion was measured by down-looking surveillance satellites. The Chelyabinsk bolide had about a tenth of the energy, and exploded more than twice as high, as Tunguska, and the blast energy was directed more sideways that downward. These factors resulted, thankfully, in much less damage on the ground."
For more, see David Morrison's NASA Impacts site at:http://impact.arc.nasa.gov/news_detail.cfm?ID=186