Shortly after sunrise on February 15, a rocky projectile entered the atmosphere over the Ural Mountains traveling at more than 18 km/sec. It 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 20m, 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 100m 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 optical 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.

The explosion also produced a shower of stony meteorites, of a type (ordinary chondrite) common among the asteroids. These meteorites were distributed over an impact region more than 100 km long.

Perhaps most surprising is what did not happen: There was no panic reaction that this was a nuclear attack. As long ago as 1981, Gene Shoemaker warned that an unexpected cosmic impact might be misinterpreted as an attack and trigger a nuclear exchange. This fear has hovered over all subsequent studies of the impact hazard. Perhaps it was a legitimate concern during the cold war, and maybe this is still a danger if a cosmic impact took place over the disputed territory of nuclear-armed antagonists such as India and Pakistan. Chelyabinsk Oblast includes many Russian defense facilities, some of which design and build nuclear weapons and rocket delivery systems. Those with long memories may remember that this area was the target of the 1960 U.S. U-2 flight of Gary Powers that was shot down by a Soviet missile. Yet apparently neither the Russian military nor the public associated the event of February 15 with a nuclear attack. For this we can all breathe easier.

Response from the scientific community was quick. Apparently Facebook provided the first information. Paul Chodas of the NASA/JPL NEO Program Office said that Feb 15 “was a most memorable day ever here in the NEO Program Office, and I doubt I’ll ever see another one like it.” Many residents of Chelyabinsk began posting YouTube videos within an hour after the event. In less than a day, the cause of the explosion was identified as the stratospheric disintegration of an incoming rocky object. The initial Russian news reports speculated that the asteroid was only a few meters in diameter and the energy was only a few tens of kilotons. However, analysis by Peter Brown (University of Western Ontario) of data from a worldwide network of atmospheric pressure sensors and seismic stations quickly established the energy of the explosion as between 300 and 500 kilotons. The response from the U.S orbital monitoring system was also remarkably fast. Three days after the event they released the exact time and location, with measurements of the bolide track and its altitude (about 23 km) and velocity (more than 18 km/s) at peak brightness.

A curious coincidence is that February 15 was also the date of the closest passage by Earth of a 30-40m asteroid 2012 DA14. It flew just 18,000 km above the Earth’s surface. Many press stories suggested that the two objects were traveling together in space or were somehow related. However, several lines of evidence indicate that the two events were not related. The orbits were very different, with DA14 in a highly evolved orbit near the orbit of the Earth, while the bolide came from the inner edge of the asteroid belt. Further, 2012 DA14 approached the Earth from an extremely southerly direction, while Chelyabinsk asteroid approached from the east.

A number of scientists as well as meteorite dealers are busy collecting recovered fragments. The proposed name is the Chebarkul meteorite, named for the lake near the center of the impact area. It will be interesting to see what these meteorites will tell us about the Chelyabinsk bolide.

A special session on the Chelyabinsk event and its implications for Planetary Defense will be held at 7:00 pm on Sunday April 14 at the High Country Conference Center in Flagstaff, the site of the 2013 IAA Planetary Defense Conference that begins the next day. David Morrison is the chief organizer of the special Chelyabinsk session, which is open to the public. Others on the Organizing Committee are Mark Boslough, Clark Chapman, and Alan Harris. Morrison invites individuals with short contributed papers and/or posters relevant to the topic to contact him at david.morrison@nasa.gov.

A hearing on the subject of planetary defense is planned in the U.S. House of Representatives on March 19.

A complete file of previous NEO News posts can be found at http://impact.arc.nasa.gov

Other Internet resources:
NY Times article from Feb 16

Early FAQ posted by the B612 Foundation

Early collection of Russian videos

Compilation of videos of the shock wave hitting Chelyabinsk

Posted by: Soderman/NLSI Staff
Source: NLSI

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