Thanks to the security necessities of driving in the lawless chaos of Russian roads, the world was treated to dozens of awesome Hollywood-quality dash-cam videos of a substantially-sized asteroid suicide-diving into Earth's atmosphere over the Ural city of Chelyabinsk.  But aside from the "Wow" factor, a number of fascinating details have now emerged and were posted on the Scientific American website and others.

1.  Detection.

One of the most interesting tidbits is that meteor explosions are typically detected around the world by a network of 337 low-frequency sonic sensors as part of the Comprehensive Nuclear Test Ban Treaty.  These sensors were put in place to detect nuclear explosions, and are more than capable of detecting - and distinguishing - shockwaves caused by meteor airbursts.  So the global governments monitoring this sensor network knew about the Chelyabinsk explosion well before the first Tweet or dash-cam video was posted to the internet.  That is (slightly) reassuring.

2.  Size and mass of the object.

Before exploding, the meteor was about 15 meters (50 feet) in diameter - the size of a large house - and weighed roughly 7,000 metric tons (15.4 million pounds), which is about 34 times the Statue of Liberty.  Now, various sources report that the Russian Academy of Science has offered a drastically different estimate (10 metric tons), but that seems to have been the very first preliminary estimate.  

3.  Composition.

This is not yet known to any degree of certainty, but the fact that it fragmented before reaching the ground is indicative of a stony rather than metallic asteroid.

4.  Power of the explosion.

The meteor expert interviewed by Scientific American places the power of the airburst at around 300 kilotons TNT equivalent - about 20 times the power of the Hiroshima bombing (~15 kt).  Given how high in the atmosphere the explosion occurred, and the angle of entry, this would seem reasonably consistent with the relatively modest damage that occurred.  It is the most powerful airburst recorded since the Tunguska event in 1908, which also occurred in Russia.

5.  Speed of entry.

It was traveling at 65,000 km/h (40,000 mph), which we are told is a typical speed, and lasted only 30 seconds in the atmosphere.  This is why hoax claims to the effect that the meteor was shot down by Russian missiles are easily dismissed - no existing weapons system is capable of detecting an atmospheric entry, launching a missile, and delivering the missile to a high altitude in front of an object traveling that fast within that short a period of time.  And even if they could, it's doubtful that available conventional warheads would make a dent in an object that massive.  There has no been no motivation to develop such a defensive system because ICBM payloads move much more slowly and are much less massive.

6.  Altitude of the explosion.

Apparently the meteor burst between 50-20 km high (160,000 ft - 66,000 ft), which is roughly where weather balloons fly and between twice and six times the height of Mt. Everest.  There's not much air up there, but to an object traveling that fast, even tenuous atmosphere is basically solid.  The smoke trail you see in some of the videos may seem little different from something a low-flying plane that exploded might leave behind, but in fact it is HUGE.  The fact that it looks so big from so high in altitude means that it's probably several miles wide.

7.  Shallow entry.

It came in at a very shallow angle, giving it a lot of time to bleed off energy and reach lower altitudes than it might otherwise have.  I'm not sure whether this was a good thing or a bad thing (the SciAm article isn't clear on that): If it had come in straight down, it would have exploded much higher up, but the explosion could have been more energetic and the shockwave directed right at Chelyabinsk.  On the other hand, traveling to lower altitudes meant the ultimate shockwave originated closer to the ground.  My intuitive sense is that a steeper angle would have been Bad (in the Ghostbusters sense).

8.  No relationship to 2012 DA14.

Although there were a number of visible meteor entries in the two days prior to the Chelyabinsk explosion, it's entirely possible that these were part of a swarm that was not collectively associated with 2012 DA14, so it would remain only a single coincidence rather than several.  The trajectory of the Chelyabinsk object was such that association with 2012 DA14 is practically impossible - their inclinations were radically different, and as I explain in part of my solar system diary on Ceres, changing orbital inclination is very difficult.  Here is an explanatory graphic making the rounds - sorry for just posting a link, but Daily Kos is having a technical issue reading image embeds for the moment [the technical issue has apparently been resolved]:


As you can see, the orbital planes of the two objects are practically at right-angles to each other, which basically never happens with objects that are related to each other.  I should also note the 2 meter size shown in the graphic is outdated.

Sun Feb 17, 2013 at 8:30 AM PT: Trivia factoid: Apparently the name of the city is pronounced with emphasis on the "ya" - ChelYAbinsk.  I'd been thinking of it as CHELyabinsk.

Sun Feb 17, 2013 at 10:58 AM PT: Reuters is reporting that NASA now estimates the mass of the object at 10,000 tons, its size at 55 feet, and the power of the explosion at 500 kilotons.

Originally posted to Troubadour on Sat Feb 16, 2013 at 02:30 PM PST.

Also republished by Astro Kos and SciTech.

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