LETTER
doi:10.1038/nature12741
A 500-kiloton airburst over Chelyabinsk and an
enhanced hazard from small impactors
P. G. Brown
1,2
, J. D. Assink
3
, L. Astiz
4
, R. Blaauw
5
, M. B. Boslough
6
, J. Borovic
ˇ
ka
7
, N. Brachet
3
, D. Brown
8
, M. Campbell-Brown
1
,
L. Ceranna
9
, W. Cooke
10
, C. de Groot-Hedlin
4
,D.P.Drob
11
, W. Edwards
12
, L. G. Evers
13,14
, M. Garces
15
, J. Gill
1
, M. Hedlin
4
,
A. Kingery
16
, G. Laske
4
, A. Le Pichon
3
, P. Mialle
8
, D. E. Moser
5
, A. Saffer
10
, E. Silber
1
, P. Smets
13,14
, R. E. Spalding
6
, P. Spurny
´
7
,
E. Tagliaferri
17
,D.Uren
1
, R. J. Weryk
1
, R. Whitaker
18
& Z. Krzeminski
1
Most large (over a kilometre in diameter) near-Earth asteroids are
now known, but recognition that airbursts (or fireballs result-
ing from nuclear-weapon-sized detonations of meteoroids in the
atmosphere) have the potential to do greater damage
1
than prev-
iously thought has shifted an increasing portion of the residual
impact risk (the risk of impact from an unknown object) to smaller
objects
2
. Above the threshold size of impactor at which the atmo-
sphereabsorbs sufficientenergy to prevent a ground impact, most of
the damage is thought to be caused by the airburst shock wave
3
, but
owing to lack of observations this is uncertain
4,5
. 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 (6100) kilotons of trinitrotoluene (TNT,
where 1 kiloton of TNT 54.185310
12
joules).We show that a widely
referenced technique
4–6
of estimating airburst damage does not
reproduce the observations, and that the mathematical relations
7
based on the effects of nuclear weapons—almost always used with
this technique—overestimate blast damage. This suggests that earl-
ier damage estimates
5,6
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
8,9
. 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.
1
Department of Physics and Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada.
2
Centre for Planetary Science and Exploration, University of Western Ontario, London, Ontario
N6A 5B7, Canada.
3
Commissariat a
`
l’Energie Atomique, De
´
partement Analyse Surveillance Environnement (CEA/DAM/DIF), Bruye
`
res-le-Cha
ˆ
tel, 91297 Arpajon Cedex, France.
4
Laboratory for Atmospheric
Acoustics, Institute of Geophysics and Planetary Physics, University of California, San Diego, La Jolla, California 92093-0225, USA.
5
Marshall Information Technology Services (MITS)/Dynetics Technical
Services, NASA Marshall Space Flight Center, Huntsville, Alabama 35812, USA.
6
Sandia National Laboratories, PO Box 5800, Albuquerque, New Mexico 87185, USA.
7
Astronomical Institute, Academy of
Sciences of the Czech Republic, CZ 251 65 Ondrejov, Czech Republic.
8
International Data Center, Provisional Technical Secretariat, Comprehensive Test Ban Treaty Organization, PO Box 1200, A-1400
Vienna, Austria.
9
Bundesanstalt fu
¨
r Geowissenschaften und Rohstoffe, Stilleweg 2, 30655 Hannover, Germany.
10
Meteoroid Environments Office, EV44, Space Environment Team, Marshall Space Flight
Center, Huntsville, Alabama 35812, USA.
11
Space Science Division, Naval Research Laboratory, 4555 Overlook Avenue, Washington DC 20375, USA.
12
Natural Resources Canada, Canadian Hazard
Information Service, 7 Observatory Crescent, Ottawa, Ontario K1A 0Y3, Canada.
13
Seismology Division, Royal Netherlands Meteorological Institute, Wilhelminalaan 10, 3732 GK De Bilt, The Netherlands.
14
Department of Geoscience and Engineering, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands.
15
Infrasound Laboratory,
University of Hawaii, Manoa 73-4460 Queen Kaahumanu Highway, 119 Kailua-Kona, Hawaii 96740-2638, USA.
16
ERC Incorporated/Jacobs ESSSA Group, NASA Marshall Space Flight Center, Huntsville,
Alabama 35812, USA.
17
ET Space Systems, 5990 Worth Way, Camarillo, California 93012, USA.
18
Los Alamos National Laboratory, EES-17 MS F665, PO Box 1663 Los Alamos, New Mexico 87545, USA.
Time (s)
–3.0 –2.0 –1.0 0.0 1.0 2.0 3.0
Absolute brightness
(magnitude)
–28
–26
–24
–22
–20
–18
a
Hei
g
ht (km)
20253035404550
Energy deposition per unit height
(kt km
–1
)
0
20
40
60
80
100
b
Figure 1
|
Light curve of the Chelyabinsk airburst. a, The brightness profile
for the Chelyabinsk airburst, based on indirect illumination measured from
video records. The brightness is an average derived from indirect scattered sky
brightness from six videos proximal to the airburst, corrected for the sensor
gamma setting, autogain, range and airmass extinction, following the
procedure used for other airburst light curves generated from video
24,25
.The
light curve has been normalized using the US government sensor data peak
brightness value of 2.7 3 10
13
Wsr
21
, corresponding to an absolute
astronomical magnitude of 228 in the silicon bandpass. The individual video
light curves deviate by less than one magnitude between times 22 and 11.5
with larger deviations outside this interval. Time zero corresponds to
03:20:32.2
UTC on 15 February 2013. b, The energy deposition per unit height
for the Chelyabinsk airburst, based on video data. The conversion to absolute
energy deposition per unit path length assumes a blackbody emission of
6,000 K and bolometric efficiency of 17%, the same as the assumptions used
to convert earlier US government sensor information to energy
26
. The heights
are computed using the calibrated trajectory
10
and features of the light
curves common to different video sites, resulting in a height accuracy of
about 1 km. The total energy of the airburst found by integrating under the
curve exceeds 470 kt. The half-energy-deposition height range is 33–27 km;
these are the heights at which energy deposition falls below half the
peak value of approximately 80 kt per kilometre of height, which is reached
at an altitude near 29.5 km.
238 | NATURE | VOL 503 | 14 NOVEMBER 2013
Macmillan Publishers Limited. All rights reserved
©2013
The Chelyabinsk airburst
10
was observed globally by multiple
instruments—including infrasound, seismic, US government sensors
and more than 400 video cameras—at ranges up to 700 km away. The
resulting airblast (shock wave travelling through the air from an explo-
sion) shattered thousands of windows in urban Chelyabinsk, with
flying glass injuring many residents.
Data from US government sensors timed the peak brightness to
03:20:32.2
UTC (coordinated universal time) on 15 February 2013 with
an integrated radiated energy of 3.75 3 10
14
J and a peak brightness of
2.7 3 10
13
Wsr
21
. These values correspond to an estimated energy
equivalent of about 530 kt of TNT. The peak brightness was equivalent
to an absolute astronomical magnitude of 228 (referenced to a range
of 100 km) in the silicon bandpass, making the airburst appear 30 times
brighter than the Sun to an observer directly under this point. The
airburst’s light curve has been reconstructed by considering the mea-
sured light production from several video records (see Supplementary
Information for details) as shown in Fig. 1. We note that point-like
models
4–6
of airburst energy deposition, which treat the impactor as a
strengthless, liquid-like material, predict that the height range in which
the energy deposition per unit path length falls to half its maximum
value is less than 2 km for impacts as shallow (17u from the horizontal)
10
as that of Chelyabinsk, which is less than one-third of the observed
value (more than 6 km). (We note that any object striking the Earth or
its atmosphere is an impactor; a ground impactor creates a crater, but
most burn up before that, releasing a large amount of energy into the
atmosphere as an airburst.) Airburst energy estimates from four dif-
ferent techniques are summarized in Table 1. Our preferred mean
energy estimate is in the range of 400–600 kt. Details of the analysis
Table 1
|
Energy estimates for the Chelyabinsk airburst
Technique Best estimate (kt) Range (kt)
Seismic 430 220–630
Infrasound (mean period) 600 350–990
US government sensor 530 450–640
Video-derived lightcurve .470
Here ‘kiloton’ refers to the energy equivalent to a kiloton of TNT. To estimate the energy from infrasonic
airwaves, all 42 infrasound stations of the International Monitoring System
23
were examined. Of these,
20 stations showed clear signals from the airburst. Our infrasound energy estimates are based on the
average observed dominant infrasound period from 12 stations that have stratospheric returns
showing the highest signal-to-noise ratio. Seismic Rayleigh waves generated by the airburst
shock wave impinging on the Earth’s surface just south of Chelyabinsk were detected by about 70
seismic stations at ranges in excess of 4,000 km. The amplitude of these waves in specific
passbands as calibrated to nuclear airbursts
19
were used as an independent estimate of source
energy.
Overpressure (kPa)
10
Source height (km)
20
1
25
30
35
40
Originating height (km)
15 20 25 30 35 40
Time residual (observed – expected)
–6
–5
–4
–3
–2
–1
0
Cloud width from video (km)
0.0
0.5
1.0
1.5
2.0
Time residual
Cloud width
b
a
d
c
T (K)
550
50.02 seconds
500
450
400
350
20 km
Figure 2
|
Observed and predicted shock characteristics for the Chelyabinsk
airburst. a, Theoretical airblast overpressures using standard nuclear weapons
relations
7
and the cylindrical-line source blast theory
13
(which assumes the
explosion occurs so swiftly that it is equivalent to a single instantaneous
detonation of a long cylindrical line of explosive charge) that is appropriate to
central Chelyabinsk. The nuclear relation curves (in black) assume a spherical
point source at a specific height and show assumed yields of 500 kt (dashed line)
and 1 Mt (dotted line). The cylindrical-line source airblast model (red line) uses
the energy deposition per unit length from Fig. 1b to define an equivalent blast
radius as the source and assumes that the shock is linear at the ground (linear
means its amplitude is low enough to be well approximated as moving at
the local ambient speed of sound and non-linear effects are negligible).
b, Travel-time residuals between the time of airburst passage at each height and
the main airblast arrival for 38 videos (see Supplementary Table 5). The
residuals (black circles) show the observed arrival time (corrected for fireball
motion) minus the expected time, calculated assuming propagation at the local
adiabatic sound speed and incorporating winds
11
. For comparison, the width of
the visible cloud trail is shown (red line). This is consistent with the shock wave
travelling faster than the ambient sound speed near the airburst. The minimum
timing residuals suggest that shock source heights vary between 30 km and
23 km across Chelyabinsk, as opposed to originating from a point source.
c, Modelling the temperature of the trail (colour scale). The apparent cross-
section of CTH simulation 50 s after 0.5 Mt was released into segmented
cylinders of air. The outer blue-grey shaded area reveals an envelope of shocked
air—note that the dominant shape of the shock is cylindrical. d, For comparison
with c, a video frame of the dust cloud taken 250 km to the southwest of the
airburst path looking North is shown (http://www.youtube.com/
watch?v5lCv9S0Z0e0E, taken by E. Volkov). Approximately 140 km of the end
portion of the airburst trail is shown, some 40–60 s after the passage of the
fireball.
LETTER RESEARCH
14 NOVEMBER 2013 | VOL 503 | NATURE | 239
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©2013
procedures and measurements are given in the Supplementary
Information.
To establish the nature and source height of the airblast that caused
damage in Chelyabinsk, we used the known trajectory
10
and a suite of
videos (see Supplementary Table 5) that recorded both the airburst
and the main airblast arrival. We computed acoustic travel times from
each point on the airburst trajectory to each video location using a
propagation model including winds that was developed for earlier
airburst infrasound analyses
11
. The results show that the first airblast
wave (which also produced the damage) arrived from different alti-
tudes at different sites, consistent with a cylindrical shock from the
extended airburst, as opposed to a more point-like explosion. The
timing residuals between the observed and expected arrivals follow
the bolide trail size, as shown in Fig. 2b, consistent with the shock
being strong early in its propagation. The airblast reaching the city of
Chelyabinsk was generated at altitudes of 24–30 km, roughly from the
peak to the end of the main airburst.
In Fig. 2a we show overpressure predictions from standard airblast
relations based on nuclear explosions
7
, as used by most impact-effect
models
4,6,12
. For comparison, the predictions of cylindrical-line source
airblast theory applied to meteor entry
13
are also shown. The airblast
overpressure in Chelyabinsk from window breakage measurements is
3.2 6 0.6 kPa (see Supplementary Information for details). We note
the overestimation of overpressure using the nuclear blast relations
7
,
an effect others have suggested in connection with airbursts
4
. Given
that nuclear explosions release half their energy as radiation
7
, thus
reducing the effective yield of airblast energy, the nuclear curve in
Fig. 2a that is most appropriate to Chelyabinsk is about 1 Mt.
To examine whether a fragmentation model
14
is consistent with the
observed data and estimated object size, we have applied an entry
code based on a progressive fragmentation model of the initial object.
Assuming an initial meteoroid of diameter 19 m and a tensile strength
at first fragmentation of 0.7 MPa (ref. 10), with ablation ending at about
27 km once most of the energy has been lost, we find a reasonable
match to both the light curve and early dynamics. The final main
fragmentations in this model occur near 4 MPa, very similar to those
observed (1–5 MPa) in the most severe fragmentation portion of the
airburst
10
. The dynamics and light production from the model are not
realistic near the terminal phase of the airburst because the model
assumes that all fragments split identically at each fragmentation
epoch. This is in contrast to observations at the end of the airburst
where one leading fragment was observed
10
(as opposed to dozens of
identically sized individuals).
To further define the nature of the shock, we have used the well
known CTH simulation framework used for the Tunguska
15
airburst
and impactors
1
comparable to the asteroid causing the Chelyabinsk
airburst. The simulation used all the observed trajectory parameters
10
and the observed energy as a function of height (Fig. 1b) to mimic the
entry process by creating an instantaneous energy release in a sequence
of momentum-preserving air cylinders along the airburst path, scaled
such that the total integrated energy is 500 kt. Figure 2c and d shows
the result of this simulation and comparison to a video record of the
dust cloud generated by the airburst at a similar time. The notable
characteristics are that the primary shock is cylindrical, in contrast to
point-source energy release airburst models
4–6
, which have a strong
spherical shock component. Instabilities that result from fast-rising
buoyant air in the simulation produce similar structures to those seen
in videos of the dust cloud.
Model overpressures for central Chelyabinsk are found to be 3 kPa,
consistent with observations. Our estimates of overpressure are based
on window breakage (see Supplementary Fig. 5) confined to a small
region in Chelyabinsk. The CTH simulations were run for more than
three minutes after the airburst, producing model variations of over-
pressure across the entire city of Chelyabinsk which were smaller than
the differences produced by local effects, such as shock reflections from
buildings, numerical uncertainty in the simulation and our generally
small number statistics. This limits our ability to validate the simulated
CTH overpressure spatially.
Using our best estimate for the Chelyabinsk airburst energy, of
about 500 kt, we have estimated the bolide flux at the Earth over the
period from 1994 to mid-2013. This estimate is based on 20 years of
total global coverage by the US government or infrasound sensors,
more than doubling the earlier time coverage
16,17
. All events with esti-
mated yields in excess of 1 kt are included. Figure 3 shows that this
bolide flux at small sizes (less than 5 m in diameter) is in agreement
within uncertainties with telescopic
8
data and earlier infrasonic
18
influx estimates. However, at larger diameters (15–30 m), both the
bolide and infrasound
18
flux curves show an apparent impact rate at
the Earth an order of magnitude larger than either that estimated by
Bolide energy (kt)
Cumulative number impacting the Earth per year
10
2
10
1
10
0
10
–1
10
–2
10
–3
10
–4
10
–5
10
2
10
1
10
0
10
–1
10
–2
10
3
10
4
10
5
Equivalent diameter (m)
1 10 100
Lunar craters (ref. 9)
NEAT (ref. 27)
Spacewatch (ref. 27)
Infrasound bolide ux (ref. 18)
Power-law t to ref. 16
Ref. 8
Bolide ux 1994–2013
Bolide ux 1994–2013 t
Tunguska
Figure 3
|
The estimated cumulative flux of impactors at the Earth. The
bolide impactor flux at the Earth (bolide flux 1994–2013; black circles) is based
on about 20 years of global observations from US government sensors and
infrasound airwave data. Global coverage averages 80% among a total of 58
observed bolides with E . 1 kt and includes the Chelyabinsk bolide (rightmost
black circle). This coverage correction is approximate and the bolide flux curve
is probably a lower limit. The brown line represents an earlier power-law fit
from a smaller data set for bolides 1–8 m in diameter
16
. Error bars represent
counting statistics only. For comparison, we plot de-biased estimates of the
near-Earth-asteroid impact frequency based on all asteroid survey telescopic
search data until mid-2012 (green squares)
8
and other earlier independently
analysed telescopic data sets
27
including the NEAT discoveries (pink squares)
and the Spacewatch survey (blue squares), where diameters are determined
assuming an albedo of 0.1. From the telescopically determined number of near-
Earth asteroids and their typical orbits we can compute the average interval
between Earth impactors of a given energy. Energy for telescopic data was
computed assuming a mean bulk density of 3,000 kg m
23
and average impact
velocity of 20.3 km s
21
. The intrinsic impact frequency for these telescopic data
was found using an average probability of impact for near-Earth asteroids of
2 3 10
29
per year for the entire population of asteroids. Lunar crater counts
converted to equivalent impactor flux and assuming a geometric albedo of
0.25 (grey solid line) are shown for comparison
9
, although we note that
contamination by secondary craters and modern estimates of the near-Earth-
asteroid population that suggest lower albedos will tend to shift this curve to the
right and downwards. Finally, we show the estimated influx from global
airwave measurements conducted from 1960 to 1974, which detected larger
(5–20 m) bolide impactors (red triangles)
18
using an improved method for
energy estimation compared to earlier interpretations of the same data.
RESEARCH LETTER
240 | NATURE | VOL 503 | 14 NOVEMBER 2013
Macmillan Publishers Limited. All rights reserved
©2013
telescopic surveys or the longer-term average impact rate provided by
lunar cratering. In both cases these deviations well above the constant
power-law slope of ref. 16 are due to single large events, so caution
must be exercised owing to the small number statistics. A best-fit
regression line to the bolide flux is given by N 5 aE
2b
, where N is
the cumulative number of objects with energy E (in kilotons) or more
that impact the Earth per year, a 5 3.31 6 0.11 and b 520.68 6 0.06.
We note that excluding the rightmost two points in Fig. 3 (representing
Chelyabinsk and two other events larger than 30 kt) produces a nearly
identical slope.
Using the telescopic impact frequency
8
(green squares in Fig. 3) as a
baseline for the 20-year period of the bolide survey, there is only a 13%
chance that any random 20-year period would have an airburst as large
or larger than Chelyabinsk. The independent 14-year survey by infra-
sound
18
(1960–74) detected a probable ,1.5-Mt airburst on 3 August
1963. Such a large event would be expected at the ,3% level during
such a survey period. Although these deviations may be attributable to
small number statistics, we note that Tunguska, with a source energy
(energy released at the location of the explosion) of the order of
3–15 Mt (refs 15,19; shown as a horizontal line in Fig. 3) is also an
extreme outlier (expected at the 2–10% level to have occurred during
the past century). These events, taken together with Chelyabinsk, are
increasingly suggestive of non-equilibrium in the impactor flux for
near-Earth asteroids that are 10–50 m in diameter. This is manifested
as a change in the power-law energy–frequency distribution at these
sizes, similar to changes in the power law at other sizes
20
. This is also
consistent with the recent origin of Chelyabinsk as a single near-Earth
asteroid and a possible link to asteroid 86039 (ref. 10). Our findings
support earlier interpretations of an influx maximum at this size
range
21,22
. We note that telescopic surveys have only discovered about
500 near-Earth asteroids that are 10–20 m in diameter
8
(comparable to
the Chelyabinsk asteroid) of an estimated near-Earth asteroid popu-
lation (http://ssd.jpl.nasa.gov) of around 2 3 10
7
, implying that a non-
equilibrium impactor population at these sizes could be present but
not yet apparent in the discovered near-Earth asteroid population.
Received 27 June; accepted 3 October 2013.
Published online 6 November 2013.
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Acknowledgements Funding was provided by the NASA co-operative agreement
NNX11AB76A and the Czech institutional project RVO:67985815. D.P.D.
acknowledges support from the Office of Naval Research. We appreciate discussions
with F. Gilbert (of UCSD), J. Stevens (of SAIC), P. Earle and J. Bellini (of USGS).
D. Dearborn provided assistance with video reductions.
Author Contributions P.G.B., N.B., D.B., L.C., W.E., L.G.E., M.G., A.L.P., J.D.A., P.M.,
P. Smets and R.W. performed various aspects of the identification, measurement and
interpretation of infrasound records. L.A., C.d.G.-H., M.H. and G.L. collected and
identified the airburst signals in seismic recordings as well as analysing and
interpreting the seismic data. P.G.B., R.B., J.B., W.C., J.G., A.K., D.E.M., R.W., A.S. and
P. Spurny helped in identifying important videos and their geolocation and various
aspects of their measurements. M.B.B. and M.C.-B. performed bolide entry modelling.
D.P.D. provided atmospheric model data and interpretation. Z.K., J.G. and R.J.W.
performed video lightcurve analysis and calibrations and helped with their
interpretation as well as performing measurements of video dust cloud features. R.E.S.
and E.T. facilitated and interpreted US Government Sensor data. D.U. performed
window breakage analysis. P.G.B. and E.S. performed analysis of acoustic propagation
and associated computer code development. P.G.B. wrote the manuscript. All authors
commented on the manuscript.
Author Information Reprints and permissions information is available at
www.nature.com/reprints. The authors declare no competing financial interests.
Readers are welcome to comment on the online version of the paper. Correspondence
and requests for materials should be addressed to P.G.B. (pbrown@uwo.ca).
LETTER RESEARCH
14 NOVEMBER 2013 | VOL 503 | NATURE | 241
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