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A New Threat from Space

On February 15, 2013, the world was awakened with the fear of a new space threat. The Chelyabinsk meteor entered Earth's atmosphere over Russia at about 0920 local time with an estimated speed of 18.6 km/s (66,960 km/h).
It quickly became a super-bright fireball over the southern Ural region. In fact, the light from the meteor was brighter than that of the sun. Observers felt intense heat from the fireball.
The Chelyabinsk meteor's shallow angle of entry almost allowed it to skip off the upper atmosphere and continue in heliocentric orbit. However, the angle of entry was just steep enough to allow atmospheric friction to slow its initial velocity below the earth escape speed of 11 km/s and below Earth orbital speed of 7.9 km/s.
This resulted in the object exploding in an air burst over Chelyabinsk Oblast, at a height of about 23.3 km.
The explosion generated a great deal of heat and a bright flash. In addition the event produced many small fragmentary meteorites and a powerful shock wave. Fortunately, the atmosphere absorbed most of the energy, estimated as the equivalent of approximately 440 kilotons of TNT. This is 20 to 30 times more energy than released at Hiroshima at the end on WW II.
This event demonstrated that a small asteroid could pose a bigger danger than previously thought. Astronomers have mapped about 1,000 such objects-between 10 meters and 50 meters in diameter in near-earth orbit, similar to the 20-meter-dimater asteroid that exploded over Chelyabinsk. However, astronomers think there are a million of these rocks that have not yet been detected.
Prior to the Chelyabinsk event experts thought near Earth asteroids had to be at least one kilometer in diameter to be a significant threat to Earth. The unexpected explosive force of the Chelyabinsk meteor has shifted more of the concern to smaller bodies with diameters between 10 and 50 meters.
This reassessment has increased the threat to human life from asteroids. Furthermore, small asteroids cannot be detected in advance of Earth encounter.

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