Wayne N. Edwards

A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors

Most large (over a kilometre in diameter) near-Earth asteroids are now known, but recognition that airbursts (or fireballs resulting from nuclear-weapon-sized detonations of meteoroids in the atmosphere) have the potential to do greater damage than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects. Above the threshold size of impactor at which the atmosphere absorbs sufficient energy to prevent a ground impact, most of the damage is thought to be caused by the airburst shock wave, but owing to lack of observations this is uncertain. Here we report an analysis of the damage from the airburst of an asteroid about 19 metres (17 to 20 metres) in diameter southeast of Chelyabinsk, Russia, on 15 February 2013, estimated to have an energy equivalent of approximately 500 (±100) kilotons of trinitrotoluene (TNT, where 1 kiloton of TNT = 4.185×1012 joules). We show that a widely referenced technique of estimating airburst damage does not reproduce the observations, and that the mathematical relations based on the effects of nuclear weapons—almost always used with this technique—overestimate blast damage. This suggests that earlier damage estimates near the threshold impactor size are too high. We performed a global survey of airbursts of a kiloton or more (including Chelyabinsk), and find that the number of impactors with diameters of tens of metres may be an order of magnitude higher than estimates based on other techniques. This suggests a non-equilibrium (if the population were in a long-term collisional steady state the size-frequency distribution would either follow a single power law or there must be a size-dependent bias in other surveys) in the near-Earth asteroid population for objects 10 to 50 metres in diameter, and shifts more of the residual impact risk to these sizes.

Meteor Generated Infrasound: Theory and Observation

There are many impulsive sources that produce infrasound in the atmosphere that can be detected by ground-based microbarometers, but few match the range of potential source energies and altitudes of meteor-generated infrasound. Ranging from the scale of centimeters to 100s of meters in dimension, hypervelocity meteoroids impacting the earth’s atmosphere can range widely in energy from 10−5 to >104 kt (Kilotons) of TNT, producing infrasound that may be observed either locally or globally. In this review, the history and development of meteor-generated infrasound research is explored with focus on how observations of meteor infrasound are progressing our understanding of the interaction of meteoroids with the upper atmosphere and the physics of meteors in general. The theoretical development of cylindrical line source blast wave theory for meteors propagating in an inhomogeneous, stratified atmosphere is reviewed and shown how this approximation to a meteor’s hypersonic ballistic shock relates to both regional and global observations of meteor infrasound, with examples that both fit and challenge our current understanding. Today, modern sensor suites and technology, both ground- and space-based, are providing a plethora of new constraints and secondary data that are helping unravel the source regions, generating methods and physics of meteor infrasound, while detections of infrasound from meteors/fireballs/bolides are being observed at an unprecedented rate by both the CTBT/International Monitoring System (IMS) global network and regional research groups. Some of the newest findings are reviewed and discussed.

An estimate of the terrestrial influx of large meteoroids from infrasonic measurements

[1] The influx rate of meteoroids hitting the Earth is most uncertain at sizes of � 10 m. Here we make use of historical data of large bolides recorded infrasonically over a period of 13 years by the U.S. Air Force Technical Applications Center (AFTAC) to refine the terrestrial influx rate at these sizes. Several independent techniques were applied to these airwave data to calculate bolide kinetic energies. At low energies our flux results are within a factor of two in agreement with previous estimates. For 5–20-m diameter objects, however, our measurements of the cumulative number of Earth-impacting meteoroids are as much as an order of magnitude higher than estimates from telescopic surveys of near-Earth objects and satellite-detected bolides impacting the Earth. The precise cause of this disagreement is unclear, though we propose several possible explanations. From our infrasound study, our best estimate for the cumulative annual flux of impactors with energy equal to or greater than E (in kilotons of TNT equivalent) is N = 4.5 E � 0.6 .

Seismic observations of meteors: Coupling theory and observations

To date, coupling of acoustic waves directly with the Earth’s surface represents the bulk of the observed meteor-related seismic signals, while precursory and impact-related seismic waves remain an observational rarity. With proliferation of infrasound and seismic monitoring systems, new opportunities exist to explore the relationship between Earth’s atmosphere andsurface.Continued studyofmeteorseismologywillleadto new methods to constrain energies, sizes, and fluxes for moderately (cm to m) sized meteoroids on Earth and potentially on Mars.

Calibrating infrasonic to seismic coupling using the Stardust sample return capsule shockwave: Implications for seismic observations of meteors

[1] Shock waves produced by meteoroids are detectable by seismograph networks, but a lack of calibration has limited quantitative analysis of signal amplitudes. We report colocated seismic and infrasound observations from reentry of NASA’s Stardust sample return capsule (SSRC) on 15 January 2006. The velocity of the SSRC (initially 12.5 km/s) was the highest ever for an artificial object, lying near the low end of the 11.2–72 km/s range typical of meteoroids. Our infrasonic/seismic recordings contain an initial N wave produced by the hypersonic shock front, followed � 10 s later by an enigmatic series of weak, secondary pulses. The seismic signals also include an intervening dispersed wave train with the characteristics of an air-coupled Rayleigh wave. We determine an acoustic-seismic coupling coefficient of 7.3 ± 0.2 m ms � 1 /Pa. This represents an energy admittance of 2.13 ± 0.15%, several orders of magnitude larger than previous estimates derived from earthquake or explosive analogs. Theoretical seismic response was computed using in situ VP and VS measurements, together with laboratory density measurements from samples of the clay-rich playa. Treatment of the air-ground interface as an idealized air-solid contact correctly predicts the initial pulse shape but underestimates its seismic amplitude by a factor of � 2. Full-wave synthetic seismograms simulate the air-coupled Rayleigh wave and suggest that the secondary arrivals are higher-order Airy phases. Part of the infrasound signal appears to arise from coupling of ground motion into the air, much like earthquake-induced sounds.

Frequency-Dependent Acoustic-Seismic Coupling of Meteor Shock Waves

Simultaneous infrasonic and seismic observations of meteor shock waves have been recorded at a collocated array in southwestern Ontario, Canada. Analysis of signals from seven events that exhibited acoustic-seismic coupling (representing 0.6% of the meteor flux detected using other systems) suggests signifi- cant filtering and amplitude enhancement indicative of a frequency-dependent acous- tic transfer function. The acoustic response at the site possesses the properties of an ∼10 Hz high-pass filter on incident shock waves, with ground motions up to an order of magnitude larger than theoretical predictions. In terms of energy, the air-to-ground energy coupling efficiency is measured at 1:3 � 0:8% above this cutoff. Overall cou- pling efficiencies, however, are much lower (between 0.0007% and 0.071%) as the dominant frequencies of the incident shock waves (typically 0.1-10 Hz) are weak or nonexistent in the seismic signals. Such strong frequency-dependent effects suggest that, without detailed knowledge of the site and its influences on the subsequent ground motion, transformation of seismic data to produce pseudopressure data for the purposes of quantitative source-energy estimation may be unreliable for seismom- eters located within thick overburden.

Listening to shower meteors with infrasound

Observations of the Leonid and Perseid meteor showers over the last few years in Europe using multiple camera systems, including the European Fireball Camera Network, have shown several bright meteors (abs. magnitudes −8 to −16) passing nearby to the IMS infrasound array in Freyung, Germany (I26DE). Subsequent checks of I26DE data show that these meteors were also detected infrasonically. This combination of optical location of the meteor in flight and microbarometer array beamforming has provided an excellent opportunity to delimit the altitudes at which these infrasound signals are being generated. UKMO temperature and wind data from the UARS satellite has been combined with MSIS and HWM models to reconstruct the acoustic velocity conditions present during observations. Using the reconstructed conditions, geometric ray tracing indicates that the source altitudes lie between approximately 80 to 105 kilometers; array back‐azimuths appear to confirm this conclusion. This is extraordinary since meteoroids o...

Estimation of bolide energy using observed infrasound signals

The acoustic amplitude‐yield relationships including formal errors for a population of large and well‐observed (greater than 0.05 kton) bolide events have been investigated. Using various signal measurements as a function of range, these data have been calibrated against optical yield estimates from satellite measurements. A correction for the presence of stratospheric winds has also been applied to the observations and is found to greatly improve the relationship correlation, remaining scatter may be due to variations amongst the fireball population such as differing burst altitudes, greater or lesser amounts of fragmentation or the variability in the magnitude of stratospheric winds, which can during certain times of the year be comparable to or exceed the strength of the winds themselves. Comparison to point source, ground‐level nuclear and high explosive airwave data with similar yields shows that observed bolide infrasound is consistently lower in amplitude. This downward shift relative to nuclear an...

An analysis of seismic and acoustic signals from the June 3, 2004 Washington state bolide

On June 3rd, 2004 a spectacular bolide was reported over British Columbia, Washington, Oregon and Idaho. In addition to eyewitness accounts and video recordings, the event was recorded on a number of seismometers in the Pacific Northwest Seismograph network and at infrasound arrays in Washington State and California. Using the NRL‐G2S atmospheric model for the time of the event, source locations have been determined with seismic and acoustic signals separately. Estimates of the yield and acoustic efficiency of the explosion have also been determined. By simulating the propagation of infrasound through the atmosphere, the arrival of discrete acoustic phases at the infrasound arrays has been modeled.