Wednesday, November 6, 2013

Warning Shot: The Cheliabinsk Impactor

By Milton Garces, 6 November 2013
@isoundhunter

The Cheliabinsk (Russian) Meteor startled the world on 15 February 2013 and reminded us how quickly the assumption of continuity can be disrupted. The United Nations promptly formed an Action Team on Near-Earth Objects (1) and constructed a plan for the formation of an International Asteroid Warning Network, which was approved in November 2013 by the UN General Assembly.

That was quick. I suppose a ½ Megaton (Mt) fireball (1 Mt nuclear equivalent) blasting above a (once secret) Soviet nuclear weapons complex in the Ural Mountains could be misinterpreted (Figure 1).


Figure 1. Mayak nuclear complex in Northern Chelyabinsk (http://en.wikipedia.org/wiki/Mayak).

Yet it was a rather fortuitous event in many ways. First, although many were unfortunately hurt, nobody was killed. As the UN wisely surmised, this was a warning shot and we should be better prepared next time. Second, it happened over a well-populated area where car-mounted cameras are ubiquitous, yielding an abundance of time-stamped video data. Third, we were all looking to the skies in anticipation of a completely unrelated Near-Earth Object (NEO), which most of us have already forgotten about. And fourth, this is an era of real-time geophysical monitoring, with global network of infrasound sensors ideally designed for the capture of massive airbursts.

Astronomers and geoscientists were quick to respond. Dash-cam videos, preliminary observations, results and analyses were distributed within hours of the event by citizens, individual researchers, NASA, IRIS, and the CTBTO (amongst others). The first analyses were out before the infrasound reached Antarctica (2), and numerous papers have already been published on this event.

Two letters in 06 November 2013 electronic edition of Nature revisit in detail the trajectory and explosive yield estimates for the Cheliabinsk Meteor. And this is where I do a full disclosure: I am a co-author on one of these Nature letters (3), but don’t let that lead you into thinking I know all about it. I’m number 16 in the 33-author list, and my minor contribution to this paper was the estimation of the infrasonic energy recorded at the nearest station in Kazakhstan, which at best confirmed other estimates. And I did not contribute at all to the Letter on the trajectory (4). But I have had the pleasure of learning from my colleagues and to revisit my old Space Science roots. And I also have an unfair advantage, as I have a bit of a head start.

But this is a fleeting advantage. The Cheliabinsk event is so important, it will take years for some of us to complete our studies. It is so big, and rocked Earth’s atmosphere so hard, that there is a special Natural Hazards session (5) on the Chelyabinsk Meteor at the Fall 2013 American Geophysical Union meeting in San Francisco (which I am co-chairing, since I’m in disclosure mode). I’m obviously invested in this event, and I anticipate it will influence my R&D agenda for the next 5-10 years. Here I concentrate on the Nature Letters because I know how much work has gone into them and that many of us are keen to use the chronology and trajectory results in Borovicka et al. (2013).

Let’s do a quick review the physics of an asteroid airburst. Let’s consider largish solid space rocks with substantial penetration depth, as the smaller and softer rocks usually burn up high. Since Earth is essentially plowing into most of these asteroids, they are coming into the atmosphere hot and fast, usually at hypersonic speeds. Thus their Mach cones are more cylindrical than conical, and as they rip into the atmosphere they generate intense low-frequency sound (infrasound). Their entry trajectories are often steep relative to the ground, but not always, as in the case of Chelyabinsk. The acoustic shock wave (airbust) impact on the ground will depend on the asteroid kinetic energy and the height of energy release. Their explosive airbust magnitude is often estimated by assuming all the kinetic energy is converted to explosive energy in Joules, which can be expressed in Megatons (Mt) of TNT equivalent by using 1 Mt ~ 4.2 x 10^15 J.




Figure 2. Left: Type 1 airburst (Tunguska type). Right: Type 2 airburst (Libyan Desert Type, Bucharest 5). From Boslough (2013a).

Boslough (2013a) proposed an airburst scale for hazard assessment and early warning of asteroid impactors based on a 1-5 rating, as in hurricane scales. Since this scale was first proposed at the 2011 Planetary Defense Conference (7) in Romania, it is referred to as the Bucharest scale.

Proposed Bucharest Airburst Warning Scale (Boslough, 2013a)

“1. High-altitude airburst with no possible damage. Bright light in sky followed by sonic boom. No recommended action.
2. High-altitude airburst with minor damage. Possible hazard from broken windows and dust from sonic boom shaking of structures. Recommended action: avoid standing near windows and anticipate respiratory hazard from dust in buildings. 2008 TC3 would have probably been this class.
3. High-altitude airburst with major damage. Possible hazard from many broken windows and unsecured structures like trailers blowing down due to blast wave. Recommended action: take cover in basements or strong structures. Consider leaving area.
4. Low-altitude airburst with heavy blast damage: Tunguska- class event. Structures within blast zone destroyed. Recommended action: evacuate blast zone and take cover outside that zone.
5. Low-altitude airburst with heavy thermal damage: Libyan Desert Glass class event. Fireball zone surrounded by blast zone. Everything within fireball zone incinerated, everything within blast zone blown down. Recommended action: evacuate fireball and blast zones, and take cover outside those zones.”

I apply this scale to a couple of meteors and a couple of atmospheric nuclear tests. The Bucharest scale does not apply to nukes, but it is useful to have a reference in Mt from better-calibrated point-source nuclear detonations. But even for man-made explosives, yield estimates can be easily off by a factor of two due to incomplete detonation or focusing, and in this narrative I will not belabor the difference between tonnes and tons or high vs nuclear explosive yields. Brown et al. (2013) provide the estimated yield of 0.5 Mt for the Chelyabinsk meteor using various methods, and below is a list of historical airburst events with increasing order of magnitude yields.

Nuke: Hiroshima  (1945), 0.6 km detonation height, Bucharest 5, Yield of 0.01 Mt

Meteor: Chelyabinsk Meteor (2013), 30 km detonation height, Bucharest 3, Yield of 0.5 Mt
Meteor: Tunguska Meteor (1908), 10 km detonation height, Bucharest 4, Yield of 5 Mt.

                                                      (Boslough, personal communication)
Nuke: Tsar Bomba (1961), 4 km detonation height, Bucharest 5, Yield of 50 Mt.

The lower detonation height of the Hiroshima explosion may be the cause of a higher Bucharest scale, and the sheer brutal massiveness of the Tsar Bomba guarantees a high Bucharest scale rating. The impactor of the postulated Chicxulub (Yucatan) Mass Extinction Event (66 mya) had an estimated diameter of 10 km and a yield of ~10^8 Mt (8). The Bucharest scale is not applicable, and neither should the TNT yield equivalence for an event of this scale. The ~1023 Joules of destructive energy released during this ground impact event has not been witnessed by humanity, and it is statistically unlikely that we will for a long while. The odds appear to be in our favor: the probability of Earth getting hit by an impactor with a diameter greater than ½ km is much less than one-in-a-million per decade (9). However, it is difficult to build reliable statistical models with small sample populations, and sometimes we just need to collect more and better data before we can make accurate statistical predictions.

This is one of the points of the Nature Letter by P. Brown et al. (3), who inferred a Chelyabinsk asteroid diameter of ~20m with (4) and used a combination of seismic, infrasound, satellite, and video observations to derive the 0.5 +/- 0.1 Mt yield estimate. He also inferred an absolute astronomical magnitude of -28 for its brightest stage, which may be dim 10 parsecs away but was 30 times brighter than the Sun to those poor souls directly below it. The peak energy deposition was at a height of ~30 km, where it radiated at a rate of ~80 kt/km (~3.4 x 10^14 J/km) of altitude. A better experimental scenario could not have been designed for an infrasound calibration explosion, as the peak airburst energy was deposited in the midst of the stratospheric waveguide, ensuring circumglobal propagation of infrasonic signals.

Based on historical studies, the chances of getting another Chely-sized or larger event in the next 20 years is ~13%. Which is fairly high, essentially an 8-bullet Russian roulette over the next couple of decades. More interesting is that asteroid impactors in the 10-50 m diameter range, as inferred from infrasound and satellite observations, may be more abundant than expected from the statistical models derived from telescopic and lunar cratering surveys. More impact data from such big meteors would be needed to demonstrate that this trend is real. Then again, if it is, we should be seeing and hearing more of these Megaton-yield asteroids over the next decades.

The Letter by Borovicka et al. (4) is delightful in its use of YouTube videos to carefully constrain the entry, fragmentation, and afterglow of the Chelyabinsk asteroid. Their forensic analyses provide a careful chronology of when and how the rocks fragmented, and where they landed. It is and excellent example of the effective use of existing ubiquitous sensing technology, in this case from the dash-cams and other video recording equipment popular in that region.


Figure 3. “Extended Data Figure 3: Deviation of fragment F1 from the main trajectory. Frame from video15. The time is counted from 3:20:20UT. The labelled marks identify points on the main trajectory at the given altitude (in kilometres). E represents the endpoint of the main trajectory.” From (4). http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12671.html




Figure 4. Ground projection of the terminal part of the bolide trajectory and meteorite-strewn field. Main trajectory (thick red line) and trajectory of fragment F1 (thin orange line) as plotted on Google Earth. The marks denote altitudes in kilometres. The predicted impact positions of 11 observed fragments (F1–F4, F6, F7 and F11–F15).” From (4). http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12671.html

This work provides a valuable chronology and includes the “dark flight” transect, the section of the trajectory after luminous flight ceases. The remaining luminous dust trail provides a Mach cylinder radius estimate of ~ 1km above 40 km, when a dense rock of ~20 m diameter and a mass of at least 10^6 kg slammed into the atmosphere at a speed of ~19 km/s. By the time it reached the height of 22 km, the main body had been reduced to 10^4 kg, and below 17 km it had broken up into numerous pieces (Fn), the largest fragment (F1) was estimated at 4.5 x 10^2 kg, which was fairly close to the actual 6 x 10^2 kg rock fished out of the lake. The Borovicka et al. (4) reference will be of value to anybody interested in reconstructing or refining the Chely entry trajectory at high spatial and temporal resolutions.

As previously noted, we have a ~13% chances of hearing about another Chely-type event in the next two decades (3), but the odds could be higher. Surprising high-yield asteroid airbursts and associated geopolitical misunderstandings could be avoided with increased vigilance. The recent UN Asteroid Warning initiative (1) builds on a Planetary Defense community that is ready to head off to space to nudge or nuke incoming asteroids (7). Boslough (9) recently published a decision-making system to assess what action to take in case of an incoming asteroid. For small objects, there is not much risk and we all can head out to do some fun field work with our favorite instruments (green zone, Figure 5). Once incoming space rocks get sizeable, our decisions are determined by how much lead time we have. With enough time we can nudge asteroids to a new orbit. If the object is large and we have over a year to prepare, we could exercise the nuclear option and blast it. But if the object is large and we don’t have enough time, our remaining choice is to brace for impact, shown in red as the “pray for a miracle” area (Figure 5).



Figure 5. Decisions support volume with 200-m asteroid size plane, and 10-year time plane. From Boslough, 2013b.

I’m glad somebody has thought this through, and that there is an international response plan involving rockets, space travel, and explosives. I humbly recommend the addition of high-power lasers, and assume robots are already included. Since I am personally averse to the red “zone of despair” in Figure 5, I fully support the UN Asteroid Warning initiative and the Planetary Defense community’s aims of figuring out how to detect and deflect large asteroids from hitting Earth. In the meantime, us ground-pounding geoscientists will keep collecting asteroid impact data with increasing temporal and spatial resolution to help us better prepare for (and respond to) unwelcome aggressive visitors from space. 

References
1. Threat of space objects demands international coordination, UN team says (20 February 2013).
2. Antarctic Sound Check (25 February 2013). http://www.youtube.com/watch?v=t270pSxorho.
3. Brown P. et al. (2013). A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors, Nature, doi:10.1038/nature12741.
4. Borovicka J. et al. (2013). The trajectory, structure, and origin of the Chelyabinsk asteroidal impactor, Nature, doi:10.1038/nature12671.
5. Fall AGU Meeting, Tuesday 10 December 2013. Session NH23. The Chelyabinsk Meteor Event.
6. Boslough, M. (2013a) Airburst warning and response, Acta Astronautica, http://dx.doi.org/ 10.1016/j.actaastro.2013.09.007.
7. 2011 Planetary Defense Conference, Bucharest, Romania, http://www.pdc2011.org/
8. Covey, C., S. L. Thompson, P. R. Weissman, M. C. MacCracken (1994).
Global climatic effects of atmospheric dust from an asteroid or comet impact on Earth
Global and Planetary Change, Volume 9, Issues 3–4, December 1994, Pages 263–273
9. Boslough, M. (2013b), Impact decision support diagrams, Acta Astronautica, http://dx.doi.org/ 10.1016/j.actaastro.2013.08.013.

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