Visitor from space

On the morning of Friday 15 February, residents in the Russian town Chelyabinsk, near the southern end of the Ural mountains, saw a bright fireball flash of light followed by a shock wave that broke the windows of thousands of buildings as a superbolide, entering the Earth’s atmosphere at high speed and a shallow angle, exploded in the atmosphere. About 1,500 people were injured, mostly from indirect effects (i.e. they were not hit by the meteoroid itself) including broken glass caused by the shockwave.  Luckily no one was killed. The impact was the most powerful since the Tunguska event of 1908. (The Tunguska event was a powerful explosion caused by a large meteoroid, perhaps 100 m in size, that exploded about 5 km above the Earth’s surface over the Podkamennaya Tunguska River in Siberia. The explosion flattened millions of trees over an area of 2,000 square kilometres and was felt hundreds of kilometres away.)

Chelyabinsk meteor flash, 15 February 2013. (Credit: Marat Ahmetvaleev)

Using information from dozens of cameras across the region, plus low frequency infrasound detectors operated by the Comprehensive Test Ban Treaty Organization (who monitor nuclear explosions) as well as U.S. Government sensors, scientists have been able to piece together information about the amount of energy released from the explosion. With this information the size and trajectory of the meteoroid can be estimated.

The Chelyabinsk meteoroid is thought to have exploded about 15 to 25 km above the Earth’s surface, causing a bright fireball flash, numerous small meteorite fragments, and a powerful shock wave.  The energy released from the explosion was equivalent to about 440 kilotons of TNT (which is over 20 times the energy released from the atomic bomb dropped on Hiroshima).  The metoroid is estimated to have had an initial mass of about 11,000 tones and diameter of about 20 metres. Recovered meteorite fragements indicate that the Chelyabinsk meteorite is a silicate-rich ordinary chondrite.

Chelyabinsk meteorite fragments collected by an expedition of Chelyabinsk State University. (Credit: Alexander Sapozhnikov)

Using information from a camera in Revolution Square in Chelyabinsk, eyewitness accounts from the nearby town Korkin, and the assumed impact site at Lake Chebarkul, Jorge Zuluaga and Ignacio Ferrin have determined a preliminary reconstruction of the orbit of the Chelyabinsk meteoroid.  They used information from the three sites to triangulate the atmospheric orbit of the meteoroid, and then, with the help of JPL ephemeris data to provide the extact position of the Solar System’s nine planets and the Earth’s moon at the time of the impact, they used the N-body integration software Mercury (Chambers 2008) to integrate the Solar System backwards for 4 years to try to determine the orbit of the meteoroid.  Given the uncertaintinties in the observational data, they provide a series of orbits and present a “best fit” model, and suggest that the meteoroid come from a family of asteroids that cross Earth’s orbit called Apollo asteroids.

Proposed orvbit of Chelyabinsk meteoroid. (Credit: NASA/MSFC/Meteroid Environment Office)

One of the major uncertainties is in assuming that Lake Chebarkul as the landing site. The frozen lake’s surface was found to host a 6m-wide home the morning after the impact, so likely marks the landing spot of a large chunk of the meteorite. However, after the explosion, the meteoroid fragments aquire different velocities and as a result are spread over a wide area.  The author stress that their results are, at this stage, a preliminary reconstruction of the orbit.

A hole in the ice of Chebarkul Lake. (Credit: The Asahi Shimbun via Getty Images)

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