Chelyabinsk airburst

6 Sep 2015

An article in Physics Today (Sept, 2014, page 32) by David King and Mark Boslough, describes what happened when the Chelyabinsk meteor exploded in an airburst event over Russia in 2013. The article can be acessed in full at http://scitation.aip.org/content/aip/magazine/physicstoday/article/67/9/... (or http://dx.doi.org/10.1063/PT.3.2515). Mark Boslough has been criticised by some commenters over at http://cosmictusk.com for his treatment of the Younger Dryas impact theory. He is a sceptic - which didn't go down well when he insisted in treating it as an impact event involving a single object when the current theory is that it involved multiple debris from a passing comet. He has the same propensity here as the authors only discuss the Chelyabinsk meteor as if it was the only cosmic body in the vicinity when it is common knowledge a much bigger object sailed past the Earth on the same day (or very close to when the meteor became entangled in the atmosphere of the Earth). Presumably this is because no link between the two objects has been established by NASA and it is much easier to concentrate on the meteor itself, and how it impacted with the atmosphere from solely a logistical vantage point.

The meteor came in at a low trajectory. As such it would have travelled a long distance across the upper atmosphere before exploding over the town of Chelyabinsk. Reports at the time suggested it came across South America and it is calculated that it may have entered the atmosphere as far south as Antarctica. The trajectory is therefore extremely slanted and King and Boslough acknowledge this. However, they do not appear to mention the point of entry - or even the possible point of entry. They assume it was above Russia - or so it would seem. They do mention it was torn apart at a high altitude and this suggests it was small, about 20m across. The debris from the event is rock, light coloured, and was full of jet black streaks, or veins. These are said to indicate shock waves, or melt, and it is assumed by the pair that at some time in the past the meteor had been part of a collision event, being one piece of a former larger body. They then spend some time on speculating it was a former occupant of the asteroid belt - as chondritic asteroids occur in that part of space. At once they are speculating about a prehistoric collision event - one that took place millions, if not billions of years ago - which isn't very helpful as far as Chelyabinsk is concerned.

So we have a fragment of something else, a bigger object. It just so happens that there are lots of asteroids and extinct comets in the asteroid belt - or thereabouts. What about the bigger object that went sailing past?

It may also be worth while finding out if there is any connection between the jet black veins of the meteor and electrical discharging - or something along those lines.

King and Boslough go on to establish a connection, via composition, with Itokawa, a Near Earth Object that the Japanese, in 2010, sampled. An image of Itokawa shows a body that looks something like Comet Churyumov-Gerasimenko in outline, which even has boulders on the surface - which presumably are capable of flying off in the right circumstances.

Various computer simulations have been done and these are said to show the parent body had a varied collision history over the last 4.5 billion years - so we can take it that the computer was fed limited information which confirms the hypothesis they started out with. The problem here is that at the same time as discussing the Chelyabinsk meteor they are also making a name for themselves in saying it was once a part of a big object that is no longer with us - or travelling around the solar system. The break up of this object, they claim, pushed the meteor into an eccentric orbit, an earth crossing trajectory. This brings us back  to the unmentioned object that also passed close to Earth almost at the same time that the meteor exploded in the atmosphere. Was this also on an eccentric orbit - and if so why was it not in equilibrium with the rest of the cosmic objects in the solar system - or perhaps it was and the meteor was not in resonant orbit. If so what was the meteor in resonant orbit with - the bigger NEO that went sailing past the Earth?

Basically, that is the most one can glean by reading between the lines of the various Chelyabinsk reports, which is where we have to leave the speculation - as King and Boslough clearly have their own line of reasoning (and it doesn't involve the other NEO).

Once they get on to the subject of what was seen by people on the surface of the Earth it all becomes more clear cut and less a matter of guesswork. As such it is an extremely useful article to have in your library as this subject will continue to be on peoples minds for some time yet. The meteor exploded high in the atmosphere, as a result of its line of trajectory, with pieces coming down with the largest lump ending up at the bottom of a frozen lake (rescued 8 months later). They then go on to say that much of their understanding of airburst damage, including the concept of an optimum height airburst, for which damage is maximised during an explosion, actually comes from literature on nuclear weapons effects - which is a remarkable admission (but understandable). A lower altitude airburst would concentrate its energy into a smaller area. A high altitude airburst, in contrast, diverges more, or spreads out before reaching the ground. It affects a larger area but it is weaker. This is an interesting observation when we look at the possibility Tall el-Hammam may have been destroyed by an airburst event, as suggested by Phil Silvia and Steven Collins.

They do mention an asteroid or comet fragment entering at a more direct angle (a steep fall rather than a shallow trajectory) and suggest the Tunguska explosion in 1908 shows evidence of a more abrupt explosion (going by the shock patterns on the ground).

The comet 'Shoemaker-Levy 9' famously came to grief in the atmosphere of Jupiter, back in 1994. It yielded an insight into hypervelocity airburst physics. Large spirals of air and comet debris were injected above the cloud tops at an altitude of around 300km before raining back down on to Jupiter's atmosphere. When the comet entered it broke up and ablated in much the same process as occurred with the Chelyabinsk meteor. Because the meteor was on a shallow trajectory such a plume did not form but the wake of the meteor split into contra-rotating vortices containing asteroid vapour that condensed to make them visible, and these were caught on videos from mobile phones.

The wake approximated a 'very hot cylinder of expanding gas' and the contra core  ran faster than the edges which caused the contra rotation. A computer simulation of the meteor's wake was done at the Sandia National Laboratories and it was shown temperatures were extremely high - several hundred kelvin to a few thousand kelvin. There is an online YouTube video of the simulation. Is this the origin of the 'fire from heaven' as described in various ancient sources - including the Biblical account of the destruction of Sodom.