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Infrasound informs fresh look at Russian meteor fall

Russian meteor fall
Screen capture from dash-cam video of Feb 15, 2013 meteor fall (courtesy Astrobob).

The Russian meteor that rocked the world on 15 February 2013 appeared 30 times brighter than the sun, and we may be in for more airbursts of this size than we had previously anticipated.  These are some of the new findings about the Chelyabinsk meteor fall included in the Nov. 6 electronic edition of the scientific journal Nature.

Much of the damage from a Chelyabinsk-sized meteor impact is caused by its airburst shock wave—the energy released in its fiery detonation in the atmosphere.  A shock wave can break windows, blow down trees or unsecured structures, or even destroy free-standing buildings if it is big enough.  The biggest low-altitude airbursts can also cause heavy thermal damage, incinerating everything within their fireball zone.

Milton Garces
Milton Garces, associate researcher, Hawaii Institute of Geophysics and Planetology

Chelyabinsk observers, including UH Mānoa associate researcher Milton A. Garces, a co-author on one of two new Nature papers, think that such airbursts may occur more frequently than previously predicted.

However, the damage from an asteroid airburst is often overestimated.  The mistake comes from using mathematical expressions derived from nuclear explosions:  asteroidal impactors distribute their total kinetic energy over their trajectory, instead of instantaneously, as in a nuclear explosion.

Writing about Chelyabinsk in his personal blog, “Infrasound Hunter,” Garces presents a quick review of the physics of an asteroid airburst:

 …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 (airburst) impact on the ground will depend on the asteroid kinetic energy and the height of energy release…”

Chelyabinsk, with a diameter of about 19 meters, had the energy equivalent of about 500 kilotons of TNT, the researchers found.

Garces helped determine the infrasonic energy of the Chelyabinsk meteor fall, based on data from the nearest recording station in Kazakstan. This infrasound observation, along with other actual seismic, satellite, and video observations, was later used to inform the energy equivalence measurement.

Chelyabinsk Infrasound
The infrasonic signature of the Chelyabinsk meteor, radiating from 8 Hz down to 0.004 Hz (via @isoundhunter)

Infrasound uses very sensitive microphones to listen to low-frequency sounds in the atmosphere.  These sounds, known as infrasound because they are too low in frequency to be audible to the human ear, can carry through the atmosphere for thousands of miles.  A specially tuned acoustic array network allows infrasound scientists to recognize the propagation speed and angle of arrival of acoustic signals within a very specific frequency band, such that they can isolate the specific sounds that they wish to study.

The infrasound laboratory at the University of Hawai’i, ISLA, is primarily focused on operating listening stations that are part of the International Monitoring System of the United Nations’ Comprehensive Nuclear-Test-Ban Treaty.  But the same equipment and methods used to detect a bomb test across the oceans can also tell you a lot about the blast power of a meteor fall.

About Chelyabinsk, Garces said:  “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.”

The new Nature papers used video, audio, satellite, infrasound, and seismic data to infer the energetics, trajectory, and fragmentation chronology of the Chelyabinsk impactor in unprecedented detail.  These studies concentrated on the observations closest to the source, which are a fraction of the observations available worldwide.

Deep infrasound from the meteor blast propagated around the world and was recorded at least twice in Antarctica.  Modeling how this ultradeep sound propagates through the dynamic spherical shell that is Earth’s atmosphere is an active topic of research.

“The Chelyabinsk event is so important, it will take years for some of us to complete our studies,” Garces said.

VIDEO: Meteor Infrasound: Antarctic sound check (by Milton Garces)

Nature article abstract “A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors” (November 6, 2013) :  http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12741.html

Milton A. Garces is an associate researcher at the Hawai‘i Institute of Geophysics & Planetology and the Director of the Infrasound Laboratory (http://www.isla.hawaii.edu/).  Garces will co-chair the special natural hazards session on the Chelyabinsk meteor at the American Geophysical Union Fall 2013 Meeting in San Francisco in December 2013. His blog site is Infrasound Hunter; his Twitter account is @isoundhunter.