Two asteroids in one day: Coincidence? NASA addresses meteors

February 16, 2013 - A small asteroid entered Earth's atmosphere early Friday, February 15, 2013 over Chelyabinsk, Russia at about 9:20 am local Russian time. Initial estimates, according to Bill Cooke, lead for the Meteoroid Environments Office at NASA's Marshall Space Flight Center, is that the asteroid was about 15 meters (50 feet) in diameter, with a weight of 7,000 metric tons. It hit the atmosphere at a shallow angle of about 20 degrees, at a speed of about 65,000 km/h (40,000 mph).


It traveled through the atmosphere for about 30 seconds before breaking apart and producing violent airburst 'explosion' about 20-14 km (12-15 miles) above Earth's surface, producing an energy shockwave equivalent to a 300 kilotons explosion. That energy propagated down through the atmosphere, atmosphere, stuck the city below - the Chelyabinsk region has a population of about 1 million - and windows were broken, walls collapsed and there were other reports of minor damage throughout the city.

The official impact time was 7:20:26 p.m. PST, or 10:20:26 p.m. EST on Feb. 14 (3:20:26 UTC on Feb. 15).

Cooke said that at this time, the known damage is not due to fragments of the bolide striking the ground but only from the airburst. "There are undoubtedly fragments on the ground, but at the current time no pieces have been recovered that we can verify with any certainty," Cooke said during a media teleconference today.

A meteorite flashes across the sky over Chelyabinsk, Russia, taken from a dashboard camera.

He added that the space rock appears to be "an asteroid in nature" - likely a rocky asteroid since it broke apart in the atmosphere. It wasn't detected by telescopes searching for asteroids because of its small size, but also because "it came out of the daylight side of our planet - was in the daylight sky and as a result was not detected by any earth based telescopes. #RussianMeteor was not detected from Earth because it came from the daylight side (i.e the Sun-facing side of Earth).

The most intensively studied impact phenomenon, impact cratering, is of limited importance, due to the rarity and large mean time between events for crater-forming impacts. Almost all events causing property damage and lethality are due to bodies less than 100 meters in diameter, almost all of which, except for the very largest and strongest, are fated to explode in the atmosphere. ... Since explosions greater than 1 gigaton TNT are rare on this short of a time scale, we are forced to conclude that the complex behavior of smaller bodies is closely relevant to the threat actually experienced by contemporary civilization. [...]

The large majority of lethal events (not of the number of fatalities) are caused by bodies that are so small, so faint, and so numerous that the cost of the effort required to find, track, predict, and intercept them exceeds the cost of the damage incurred by ignoring them. ~ John S. Lewis, Professor of Planetary Sciences at the Lunar and Planetary Laboratory, Co-director of the NASA/University of Arizona Space Engineering Research Center, and Commissioner of the Arizona State Space Commission in: Comet and Asteroid Impact Hazards on a Populated Earth, 2000; Academic Press)


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