The Chelyabinsk Fireball: asteroid or comet?

Just after dawn on the morning of February 15th, 2013, inhabitants of the Russian city of Chelyabinsk were making their way to work when an enormous fireball streaked across the sky. Those who stopped to marvel at the cosmic visitor felt a pulse of heat as it suddenly exploded, before the associated shock wave shattered glass, threw people to the ground and set off alarms all over the city. Around 1,500 people were treated for cuts and bruises in local hospitals, while several more seriously injured were flown to Moscow by helicopter.

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Despite initial suspicion from local military authorities that the explosion (equivalent to 500 kilotons of TNT) had been the result of a nuclear attack, it soon became clear from the enduring white trail and trajectory that the devastation had been caused by an extraterrestrial object: the immediate assumption was that this was part of the asteroid 2012DA14 that passed close by the Earth sixteen hours later.

Local people, perhaps sensing the global interest this event would arouse, were soon out searching the snowy countryside for fragments of the asteroid. Within hours several dozen small, black-crusted stones were recovered from the region to the west of Chelyabinsk, while searchers reported a 6m hole in the thick ice of Lake Chebarkul. It was widely assumed that many kilos would be found, as was the case in 1949 when over 200kg of material was recovered following a meteorite fall at Kunashak, a town in the Chelyabinsk region.

Scientists examined video and photographs of the event and reached a number of
conclusions:
• The energy released as pressure and sound waves, heat and light totalled 2,100TJ (Equivalent to a 500KT nuclear explosion).
• The object had entered the atmosphere from the south east at an oblique angle at a velocity of 30km/s.
• The object’s plotted trajectory suggested it was a member of the Apollo group of near-Earth asteroids.
• The mass of the object on entry to the atmosphere had been 11,000 tons - the weight of the Eiffel Tower!
• Had the object entered the atmosphere vertically above a city, the results would have been catastrophic, with an order of damage to that produced by the Tunguska event in 1908.

These conclusions were generally accepted by the scientific community and a number of documentaries (of varying rigour!) were televised over the following week.
As a meteorite dealer and lecturer, I was, naturally keen to obtain some specimens for examination and resale: accordingly I quickly contacted several dealers in Russia who (as fellow members of the IMCA) I knew to be trust-worthy. Their responses surprised me: only a few kilos had been located, mostly in the form of centimetre-sized whole-stones. Given that the calculated mass was so high, this seemed strange: the low trajectory and explosion should have resulted in several thousand kilos of debris being retrieved. Furthermore, examination of the bed of Lake Chebarkul failed to produce any sign of whatever had made the hole in its icy crust.

I reflected on this data and phoned a few friends and colleagues to discuss the theory I was formulating: that, rather than an asteroid, the impactor had been a small cometary nucleus.

Putting all of this together, we reach an understanding that there are far more comets than rocky asteroids in the solar system, and that a collision with one of these unpredictable objects is more likely than with a rocky asteroid: such an event occurred in 1994, when comet Shoemaker Levy broke into fragments and crashed into the planet Jupiter.

Reflecting upon the Chelyabinsk event, it seems most likely to me that a small Oort Cloud object tumbled into the solar system millions of years ago, being captured into an erratic orbit with the Apollo ‘asteroids’. Its icy surface was encrusted with hundreds of kilos of solid material: since most of what was recovered at Chelyabinsk seems to be LL5 chondrite, we can postulate a collision with a smaller rocky body in the past.

Entering the Earth’s atmosphere, the small comet heated rapidly, generating vast quanties of super-heated steam. This was responsible for both the devastating explosion and the white vapour trail, both of which are uncharacteristic of the entry of a stony or nickel-iron meteorite into the atmosphere. The remnants from the comet’s surface formed a widely-scattered strewn field of a few hundred kilos of solid material.

It seems likely (to me!) that many - if not all - major impacts on the Earth are cometary: this would explain the lack of solid debris from events such as Tunguska and Chixculub.

David Bryant

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