Where will be the next "Chelyabinsk" event?

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Are we in meteor fireball season?  This seems to be the case after the recent reports of large fireballs appearing in the sky of places as far as Detroit (USA) and Hong Kong (China).  

Images of the recent large Michigan Fireball (17 January 2018) taken from a dashboard camera in a highway in Michigan. Credit: AP

Yesterday (17 January 2018) what appeared to be a meteoroid a few meters across, entered into the atmosphere over Michigan and exploded above Detroit.  Tens to hundreds of thousands persons witnessed the blast produced by the object in the sky, a explosion that was even detected as a Magnitude 2 earthquake.  The object probably ended up as hundreds of little fragments that are now chased by meteorite collectors.  Few months ago (2 June 2017) a huge fireball having a visual magnitude of -19 (what makes it a so-called "superbolide") exploded in the skies of Arizona.  A recent report demonstrated that the fireball was produced by the entrance of an object with the size of a dinner table (~2 m in diameter), falling through the atmosphere at a speed of 60,000 km/h (38,000 mph). 

These two recent events are rather small as compared with the event that hit the atmosphere in the Chelyabinsk region (Russia) in February 2013.  In a cold winter morning, a explosion with the energy of a large nuclear bomb, that fortunately happened at an altitude of about 20 km, was witnessed by millions of (very fortunate) Russians.  The explosion was the most intense phase of a superbolide that for a few seconds was brighter than the Sun (magnitude -27).  The phenomenon, now called the "Chelyabinsk event", ended up in shattered glasses and other building damages in the affected cities, and more than a thousand people injured (it is fair to say that most of the injuries were not because of the large power of the explosion at the ground level, but because of the people curiosity and a natural ignorance about the risks involved at observing these phenomena).

But no. There is no fireball season at all. This is the natural way as the Solar System works.  We are just simply witnessing a regular planetary process, as we never did it before. 

Tens to hundreds of meter-sized Solar System debris, also called "meteoroids" by the experts, fall through the Earth's atmosphere every year.  Most of them produce bright fireball undetectable by humans or surveillance cameras. They happen more frequently in uninhabited continental regions or in the middle of the ocean.  Seismic and infrasonic sensors around the world and Earth observing satellites, are well aware of the fact that the Earth is being continuously bombarded.  This was clear after a series of public announcements of NASA in 2014, showing the location and time of large fireballs, detected by the "ears" and "eyes" of the American space administration and their allied institutions.  Only between 1994 and 2014 more than 500 nuclear-scale meteoroid-induced blasts happened up in the atmosphere.  This correspond to an (almost scaring) average rate of moderated meteoroid impacts per month.  The list of detected events is growing.

This diagram published by NASA media in november 2014 show the almost scaring fact that the Earth is being continuously bombarded by meter-sized objects able to produce nuclear-scale explosions. 
Image taken from: http://bit.ly/2DjXmBE. Credit: NASA/CNEOS/JPL.

One thing we know for sure: the Earth will have another "Chelyabinsk event" in the futureThe question is "when?" and "where?".  The "when" is hard to pin down. Researchers have estimated that impacts as large as Chelyabinsk happen with an average frequency of one each 30-50 years. Meteoroid impacts, however, are a random phenomena: it may equally happens tomorrow or in one century from now.

But... what about the "where"?.  This is precisely the question that two Colombian astronomersJorge I. Zuluaga and Mario Sucerquia from the Solar, Earth and Planetary Physics Group of the University of Antioquia, have attempted to pin down.  In a paper released today in the arxiv, they presented a novel method to calculate the relative probability of meteoroid impacts (RIP for short) of a given site on the Earth's surface.  In simple terms, the chances that a meteoroid impact occurs at some of place on Earth (a big city as New York or Paris) instead of any other place.

Zuluaga and Sucerquia start their paper by realizing that the two of the largest meteoroid impacts on Earth in recent history, the Tunguska and Chelyabinsk events, happened at about 2,400 km away.  This seems to be a large distance for humans standards, but it is a small figure, for a large planet as the Earth. 

The Colombian researchers estimates that the geographical area enclosing both impact sites, represents only the 3.4% of the total area of the Earth.  They calculated that the chances that two almost consecutive large impacts happen in such a small area is about 10%, meaning that we need to wait up to 4,000 years, for such a tantalizing coincidence to happen again.

Are the spatial "coincidence" of Chelyabinsk and Tunguska events a demonstration that west Siberia is an unlucky place to be in the Solar System?. Not at all.  As the researches found in the paper, although this region of the planet seems to be different than other regions in the world, at least in what respect to impact probability, other areas such as northern Canada, Norway and Alaska, seem to be as prone to impacts as west Siberia.

Maps showing the relative impact probability for all Earth's surface at the date and time of the Tunguska (left) and Chelyabinsk events (right).  The black stars mark the position of the impacts.  Red colors indicate places of larger RIP while blue ones correspond to the sites less prone to impacts at that precise time.  Credit: Zuluaga and Sucerquia (2018)

What is most interesting in the work by Zuluaga and Sucerquia is the fact that the method they devised, and that was conveniently called "Gravitational Ray Tracing" or GRT for short, is inspired in a computational technique used by the animation industries to produce photo realistic images for games and movies.  The technique is called "Ray Tracing" and has been used for decades by companies like Pixar to produce their more iconic movies such as Toy Story.  Moreover, Ray Tracing is a classical and well-known computer graphics algorithm, used today for a huge diversity of applications.  Now, the Colombian astronomers have found a clever application of the algorithm for assessing asteroid impact risk.

GRT works by "reversing" asteroid impacts in time.  Instead of considering the Earth as the target of space rocks, our planet is assumed to be the source of those objects. But, how is that?

Given a particular geographical location, let's say the Champ de Mars in Paris, GRT launches hypothetical rocks (test particles) in tens to hundreds of random directions in the sky.  Each particle has also a random velocity.  The trajectories of these hypothetical rocks are propagated towards the interplanetary space, until they reach a stable orbit around the Sun.  Once there, the algorithm compares the orbit of the test particle with the orbits of actual Near Earth Asteroids (NEAs).  If the orbit of the test particle has properties similar to that of actual NEAs, the launch is considered a "hit" (a hypothetical impact could actually come from that direction and with that velocity).  If on the other hand, the hypothetical rock ends up in an unreal orbit or it impacts again the Earth, the Moon or the Sun, the test particle is discarded.  The relative impact probability of the site is estimated by comparing the number of hits that the site has with the same number computed for a reference place. Everything, without interrupting with a nuclear-scale blast a beautiful afternoon in Paris.

Shematic representation of the way the Gravitational Ray Tracing method works (right) as compared with the Ray Tracing algorithm used in computer graphics.  Credit: Zuluaga & Sucerquia (2018)

It seems a rather simple idea, but, as Jorge I. Zuluaga points out "the actual implementation of the method is harder than thought; you need to be sure that the results are not just 'numerical artifacts' arising from a limited statistics or an incomplete knowledge of the NEA distribution", and adds that "you also need powerful computer resources to thrown the hundred of thousands of test particles required to evaluate the RIP of the whole Earth at just one single moment in time". 

As every concerned astronomer, Zuluaga and Sucerquia are pretty aware of the urgency of assessing the risk posed by asteroid impacts to our ever growing (and sensitive) civilization.  As Mario Sucerquia points out, "[our method] is a 'safety tool' that could be used for decision making in the face large and dangerous impact events".  He adds that "assessing the hazard of meteoroid impacts deserves to be a practical goal supported by policy makers in all nations".  Sucerquia finally concludes "despite the difficulties we have faced designing and publishing the method, GRT is our contribution to this important goal".

Additional information:

  • https://youtu.be/yaVHvvDXCgg: Video showing the evolution of relative impact probability (RIP) as a function of time during a specific day and day during a typical year.

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Original sources:

  • Jorge I. Zuluaga & Mario Sucerquia, Towards a theoretical determination of the geographical probability distribution of meteoroid impacts on Earth, Monthly Notices of the Royal Astronomical Society, Volume 477, Issue 2, 21 June 2018, Pages 1970–1983, https://doi.org/10.1093/mnras/sty702 [arXiv:1801.05720]