[meteorite-list] Asteroid Impactor Reported over Indonesia

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 23 Oct 2009 14:07:57 -0700 (PDT)
Message-ID: <200910232107.n9NL7v1T023072_at_zagami.jpl.nasa.gov>

http://neo.jpl.nasa.gov/news/news165.html

Asteroid Impactor Reported over Indonesia
Don Yeomans, Paul Chodas, Steve Chesley
NASA/JPL Near-Earth Object Program Office
October 23, 2008

On October 8, 2009 about 03:00 Greenwich time, an atmospheric fireball
blast was observed and recorded over an island region of Indonesia. The
blast is thought to be due to the atmospheric entry of a small asteroid
about 10 meters in diameter that, due to atmospheric pressure, detonated
in the atmosphere with an energy of about 50 kilotons (the equivalent of
50,000 pounds of TNT explosives).

The blast was recorded visually and reported upon by local media
representatives. See the YouTube video at:

http://www.youtube.com/watch?v=yeQBzTkJNhs&videos=jkRJgbXY-90

A report from Elizabeth Silber and Peter Brown at the University of
Western Ontario indicates that several international very-long
wavelength infrasound detectors recorded the blast and fixed the
position near the coastal city of Bone in South Sulawesi, island of
Sulewesi. They note that the blast was in the 10 to 50 kT range with the
higher end of this range being more likely.

Assuming an estimated size of about 5-10 meters in diameter, we would
expect a fireball event of this magnitude about once every 2 to 12 years
on average. As a rule, the most common types of stony asteroids would
not be expected to cause ground damage unless their diameters were about
25 meters in diameter or larger.

A more extensive report by Elizabeth Silber and Peter Brown of the
University of Western Ontario is here.

------------------------------------------------------------------------


Summary of Preliminary Infrasonic Analysis of the Oct 8, 2009
Indonesian Superbolide

Elizabeth Silber and Peter Brown
Meteor Infrasound group
Dept. of Physics and Astronomy,
Univ. of Western Ontario
London, ON
N6A 3K7
CANADA

Released: October 19, 2009

On Oct 8, 2009, media reports appeared in the local press in Indonesia
concerning a loud air blast occurring near 11am local time (0300 UT).
Subsequent to these first media reports, additional English language
reports appeared suggesting the event was meteoritic.

http://thejakartaglobe.com/home/mysterious-explosion-panics-locals-in-south-sulawesi-police-still-investigating/334246

http://www.thejakartapost.com/news/2009/10/08/blast-may-be-result-falling-space-waste-or-meteorite-lapan.html

Indonesian language reports more clearly identify a bright fireball,
accompanied by an explosion and lingering dust cloud as the origin of
the air blast. Finally, a YouTube video posted on the same day appears
to show a large dust cloud consistent with a bright, daylight fireball.

http://www.surya.co.id/2009/10/09/ledakan-misterius-guncang-sulsel.html

http://www.youtube.com/watch?v=yeQBzTkJNhs&videos=jkRJgbXY-90

Based on these initial reports, a detailed examination was made of all
International Monitoring System (IMS) infrasound stations of the
Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). From this
initial examination, a total of 11 stations showed probable signals from
a large explosion centered near 4.5S, 120E, with an origin time near
0300 UT on Oct 8, 2009, consistent with the media reports. This signal
was notable for having been (a) detected at many IMS stations, including
five at ranges over 10,000 km (and one at a nearly 18,000 km range) and
(b) being confined to very low frequencies. Both of these observations
suggest the explosion source was of very high total energy. All signal
motions were between 0.27 - 0.32 km/s, consistent with stratospheric
signal returns.

We have used the Air Force Technical Application Centre (AFTAC)
period-yield relation as described by ReVelle (1997) as the most robust
basic indicator of source energy. To generate measured periods, the
average periods of all phase-aligned stacked waveforms at each station
were measured, according to the technique described in Edwards et al
(2006). These periods were then averaged to produce a single, global
average period of 13.4 sec and the AFTAC yield relation applied; this
produced an average source yield of 31 kT of TNT. Averaging the
individual yields from all stations produces a mean source energy near
50 kT of TNT while using only the eight stations having the highest
signal-to-noise-ratio (SNR) and using the local observed periods of the
waveform at maximum amplitude produces a yield estimate of 40 kT of TNT,
all of which are basically consistent. It is important to note, however,
that the standard deviation of this measurement is nearly 30 kT. That
is, the best source energy estimate would be 40 +/- 30 kT TNT. Note that
much of this variation may be due to the signal emanating from different
portions of the fireball trail as observed at different stations; each
period measurement is a "sample" of the size of the cylindrical blast
cavity at that particular segment of the trail detected by any one
station. As such, the out of atmosphere yield for this event is likely
higher than these measurements suggest - very probably in the ~50 kT range.

The yield estimates based on infrasonic amplitude are very uncertain in
this instance as the propagation distances are much larger than is
typical and outside the range limits where such relations have been
developed (e.g. Edwards et al, 2006) and hence the period relationship
(which was generated using a dataset of nuclear explosions having yields
in this range) is more applicable.

Some examples of the detected and processed waveforms are shown in the
appendix.

Based on these infrasound records, it appears that a large (40-50 kT
TNT) bolide detonation occurred near 0300 UT on Oct 8, 2009 near the
coastal city of Bone in South Sulawesi, Indonesia. The infrasonic
geolocation is not precise enough to determine if the bolide was over
water or land, but it was relatively near the coast.

Follow-on observations from other instruments or ground recovery efforts
would be very valuable in further refining this unique event.

Using an average impact velocity for NEAs of 20.3 km/s, the energy
limits (10 - 70 kT) suggested by this analysis correspond to an object
5-10 m in diameter. Based on the flux rate from Brown et al (2002), such
objects are expected to impact the Earth on average every 2 - 12 years

*References*

Brown P., Spalding R.E., ReVelle D.O., Tagliaferri E. and Worden S.P.
2002. The flux of small near-Earth objects colliding with the Earth,
Nature, 420, 314-316.

Edwards W.N., Brown P.G., ReVelle D.O., 2006. Estimates of Meteoroid
Kinetic Energies from Observations of Infrasonic Airwaves, Journal of
Atmospheric and Solar-Terrestrial Physics, 68: 1136-1160.

ReVelle D.O. 1997. Historical Detection of Atmospheric Impacts by Large
Bolides using Acoustic-Gravity Waves, Annals of the New York Academy of
Sciences, Near-Earth Objects - The United Nations International
Conference, editor J.L. Remo, New York Academy of Sciences, 822, 284-302.

*Appendix : Example waveform detections.*

In each of the following the infrasound signals across each station have
been array processed in windows (typically of 30-60 second length) to
search for coherent signals with consistent back-azimuth measurements.
The top panel in each display is the F-statistic, a measure of the
relative coherency of the signal across the array elements in any
particular window (essentially a SNR measure). The second window shows
the apparent trace velocity of the acoustic signal across the array in
the direction of the peak F-stat. Similarly, the third plot shows the
best estimate for the signal back-azimuth in the direction of maximum
F-stat for each window. The fourth plot shows the raw pressure signal
for one array element bandpassed according to the chosen Low - High
frequency combination, shown in the boxes of the lower plot.

[graphic]
Signal as detected at IS05AU (Australia) as a range of 5000 km - the
green area highlights the airwave signal

[graphic]
Signal as detected at IS07AU (Australia) from a range of 2300 km.

[graphic]
Signal as detected at IS13CL (Easter Island) at 13500 km range showing a
distinct signal with a dominant period near 16.5 seconds.
Received on Fri 23 Oct 2009 05:07:57 PM PDT