Modeling An Asteroid Strike Using A Nuclear Explosion Code

The above  (2D) computational simulation for the Chelyabinsk asteroid's atmospheric wake, after being adapted from Sandia's code for modeling nuclear explosions. Each panel is a cross-sectional slice at the same time location along the asteroid's path for different times after the energy was deposited. (From Physics Today, Sept.)

The Feb. 15, 2013 asteroid airburst over Chelyabinsk, Russia has now gone down in infamy after the 20 m diameter object, entering the Earth's atmosphere at 19 km/s ,  blew up and shattered windows miles away - shaking the residents from their day to day routines. Of course, getting an actual reliable estimate of the magnitude of the blast - which was compared to an airburst nuclear explosion - has proven difficult.

But, as reported in Physics Today (September, p. 32) this has now been accomplished thanks to a 3-dimensional simulation using a shock code developed at Sandia National Laboratories, originally intended to model nuclear explosions. The code, input into Sandia's 'Red Sky' supercomputer, showed that the Chelyabinsk blast was of at least a half megaton, or comparable to the yield of many U.S. warheads on ICBMs, like the 'Minuteman'.

Some of the information and insights that the Sandia team has assembled with other data:

- The entry at 19 km/s meant that it originated from the asteroid belt between Mars and Jupiter - not from a ballistically launched missile whose velocity would only be about 11.2 km/s or a short period comet with a mean speed of 35 km/s.

-  The altitude of the blast indicated the object was small and weak. The diameter of 20 m (66 feet) was estimated base on the observed velocity factored together with the assumed density of the material.

- The asteroid first felt the presence of Earth's atmosphere while it was thousands of miles above the Pacific Ocean and for a dozen minutes the 10,000 ton rock fell swiftly and unobserved passing at shallow angle through the atmosphere where the molecular mean free path was much greater than the 20 m diameter.

- When it crossed over the border into Russia at 3:20:20 UT and was 100 km in altitude 99.99997 % of the atmosphere still lay beneath it.

- For the better part of 10 seconds the asteroid hurtled through the air as a rigid body moving at a shallow angle, 17 degrees relative to the horizon and descending 1 km for every 3 km of flight.

- At about 45 km altitude the entry dynamics began to change. The dynamic pressure then built up from 0.7 Mpa (millions of Pascals, where 1 Pa = 1 atm equivalent), Within a couple more seconds, below 40 km, pressure on the now fracturing asteroid increased past 1 MPa, breaking it into a number of smaller fragments.

- As the pressure then grew exponentially the process cascaded and formed ever smaller fragments that rapidly increased the surface to volume ratio. As the fragments ablated the hot gas between them built up finally resulting in a chain reaction and a massive explosion converting the asteroid's kinetic energy into heat and pressure (yielding the shock wave that shattered windows).

- Only one significant piece-fragment remained post-explosion. This continued to fall like a ballistic missile in 'dark flight' at terminal velocity until it punched through the ice of frozen Lake Chebarkul.  This 1.5 m diameter boulder thereby became the largest Chelyabinsk object found.

While all these facts provoke interest, the authors are quick to point out that the asteroid airburst should not be simply compared to or called "an explosion".  They note that technically an explosion represents a "point source" with radial symmetry, i.e. energy is radiated in all directions from the source and the peak pressure on the ground below decays radially.

But the asteroid airburst does not act the same way and instead its profile - including shock waves  changes. For example, at the beginning of its flight it behaves more like a supersonic jet but at the end more like a textbook explosion. As noted by the authors (p. 35):

"In Chelyabinsk, the energy deposition that led to the explosion took place in stages and was spread out over a long distance because of the shallow entry angle. Energy was deposited at linear densities greater than 1 kiloton per kiliometer and rose to a peak of 80 kt/km; most of the energy deposition occurred at altitudes from about 38 km down to 23 km. It took four seconds for this to happen during which the asteroid left a 50 km wake of hot expanding gas and ablation products."

The preceding gives a good synopsis of the asteroid's behavior.  Sadly, as the authors also warn, it can't be generalized to extrapolate to all asteroids - even if they are roughly the same size.  The problem is the angle of entry which can vary markedly (obviously) and also the fact you're not going to get staged fragmentation over long distances. For example, nearly 24 years ago in Barbados I observed a large object blow up over Mt. Tenantry, Barbados that appeared to enter from a much steeper angle. Alas, when we went out to the site, nothing could be found, no residue or even a crater.

The authors themselves (ibid.) make reference to the Tunguska event in Siberia, in 1908 and note it was "a much more abrupt explosion".

Finally, in terms of energy released, we learn (p. 36):

"The Chelyabinsk airburst was roughly two orders of magnitude more energetic than the approximately 10 kiloton Sikhote-Alin asteroid event of 1947 and roughly an order of magnitude less energetic than the 3 - 15 megaton Tunguska blast of 1908."

This puts it at the half megaton release mark.  The authors admit there remains "incredible uncertainty in the energy estimates for smaller impact events making it difficult to quantify future hazards."  For that reason, it is "incredibly important" that "high precision" values have been forthcoming from analysis of the Chelyabinsk event.

Let us hope the Sandia nuclear shock code can be developed or refined for other objects and ultimately enable us to forecast in advance the devastation from future asteroids. That is, assuming we can find them before they strike!  There is also the potential - given we can find these objects - to maybe eliminate the threats entirely. See e.g.

http://brane-space.blogspot.com/2013/03/a-proposal-to-eliminate-smaller.html

TWO Cosmic Shots Over the Bow for Earth's Planetary Sleepers!

Today, not one but TWO cosmic shots over our planetary bow have been received - well, ok, the 2nd is the asteroid DA 14 2012, scheduled to make a close pass at 2.25 p.m. ET today. (See graphic attached). Will we take note or continue living in a fool's paradise oblivious to the threats of large rocks from space landing on us at any time? One wonders! We are supposedly the species with the large brains and high intelligence, unlike the dinosaurs who were basically taken out 65 million years ago by the KT boundary event.  The dinos at least had an excuse, since their average brain size was about the size of a walnut so they'd never be able to configure rockets or telescopes to provide advance warning systems for asteroids or large meteoroids. (Apart from the fact they lacked the digits to render such machines, even if they had the brain power to conceive them.)

Anyway, the first 'shot over the bow' occurred with the explosion of an 11 ton (~2.2. x 10 ⁴ kg) meteoroid over the Russian Urals barely 12 hours ago by Russian local time. The blast broke windows in 6 cities according to an MSNBC report this morning, and injured 1,000 people, while 3,000 buildings sustained damage mostly from the blast shock wave. See, e.g.

http://www.cnn.com/2013/02/15/world/europe/russia-meteor-shower/index.html

  The incident recalled for many the Tunguska Siberian event in 1908 which knocked down nearly 80 million trees and decimated nearly 820 square miles, about two-thirds the size of Rhode Island.  And this was from an object perhaps ten times the size of the 11 ton meteoroid that just hit the Urals this morning.

Meanwhile, a much larger asteroid, the one identified as DA 14 2012 is making its closest pass barely three hours from now. It packs a mass of 130,000 metric tons (~ 2.9 x 10⁸  kg) and with the explosive release equivalent of a 20 megaton nuclear bomb.  Incredibly, the object was first discovered back in February last year by a DENTIST using a high powered telescope! Why the hell had no national scientific agencies made the discovery? What would have been the case if,  instead of a minor midget asteroid making this close pass, it was a planet killer 6 km across or larger? (As opposed to only 150'  or 45 m  DA 14 2012) Would we be ready to do anything other than piss, moan and pray?

It seems doubtful! This despite evidence that we truly are living in a fool's paradise of false security! As far back as 1979, in an interview given to the BBC, Dr. Basil Boothe warned it was most unwise of humans to bet on their future security from approaching cosmic objects. This would be by assuming that their lone home planet would remain untouched, unaffected and they could sleep in peace. Not so! He estimated at least a 50-50 chance of a strike by a planet killer within 250,000 years. Sadly, this long timeline - misunderstood by the budget cutters- has been taken to mean we can do nada for a quarter million years, as opposed to a strike happening at ANY time between NOW and a quarter million years hence.

Indeed, as recently as 1994 Earthlings also observed transfixed as nearly 21 immense fragments from Comet Shoemaker Levy 9 crashed into Jupiter. Had they instead smashed into our own planet, we might not even be having this discussion.

Meanwhile, Projects Space watch and Spaceguard remain on relative life support, funding wise. The first, based at the University of Arizona, featured generous grant allocations in its early years when it discovered over 1,300 Earth -approaching asteroids by 2002. As the austerity mindedness infected all science research areas, that capability has since wound down a lot, leaving Spacewatch mostly in the hands of amateurs. Not that the latter aren't observationally capable, but we also need professionals to spot these things and then (using the well known equations of celestial mechanics) to obtain the orbital parameters as fast as possible.

Spaceguard extends the detection to focus more on NEOs (near Earth objects) as opposed to singling out near Earth asteroids (NEAs).  Again budget limitations have loomed and imposed  truncated observation time frames and selectivity.

Although there have been many discoveries of near-Earth asteroids  the Spaceguard Project has been criticized for not having an in-depth master plan if our humble planet is directly in the crosshairs of a giant asteroid. It has also been criticized of having false alarms that could lead to dangerous implications.

NASA published an article in April of 2001 to clarify many questions that have come up with this project. The report distinguished the difference between NEOs and NEAs, why the organization selected NEAs with a diameter of at least one kilometer in diameter, why the number of 90 percent was chosen and many other questions that have perplexed the general public.

Neither Space watch or Spaceguard is adequately funded, which means that either an NEO or NEA can escape through the observational 'net' and could wreak havoc, especially if the size and mass approaches that of a planet killer such as exterminated the dinosaurs.

In addition, there is no plan of which anyone is aware  for taking out a planet killer - say 6-10 km across, if one does target Earth. There have been past ruminations about using nuclear weapons to try to split the object into smaller pieces, but that was put aside when the ''architects" decided a number of smaller (1 km ) strikes might not be so good either. But still, a planet killer of 6 km arriving intact is not something to dismiss lightly and our space gurus may well have to opt on the side of losing half of the earthly populace to losing ALL of it, say from a nuclear winter onset as a result of some 10 ²⁸ kg of dust and debris in our atmosphere, cutting off sunlight.

We have the brains, supposedly, and given we aren't planning to abandon 'Mother' Earth any time soon, we had better damned well come up with a credible plan to deal with a monster planet killing asteroid if and when one appears and approaches us - whether that be next year, or in ten thousand. It is either that, or we go the way of the dinosaurs, in which case, our vaunted intellect wouldn't have made a dime's worth of difference!