Ce n'est jamais que le énième bolide depuis deux mois... mais de loin le plus gros.
Voilà l'annonce publiée par la Nasa vendredi 23 octobre, deux semaines après le passage de ce gros bolide:
http://neo.jpl.nasa.gov/news/news165.html Asteroid Impactor Reported over IndonesiaDon Yeomans, Paul Chodas, Steve ChesleyNASA/JPL Near-Earth Object Program Office
October 23, 2009 On October 8, 2009 about 03:00 Greenwich time, an atmospheric fireballblast was observed and recorded over an island region of Indonesia.
Theblast is thought to be due to the atmospheric entry of a small asteroidabout 10 meters in diameter that, due to atmospheric pressure, detonatedin the atmosphere with an energy of about 50 kilotons (the equivalent of50,000 pounds of TNT explosives).
The blast was recorded visually and reported upon by local mediarepresentatives. See the YouTube video at:
https://www.youtube.com/watch?v=yeQBzTkJNhs&videos=jkRJgbXY-90 A report from Elizabeth Silber and Peter Brown at the University ofWestern Ontario indicates that several international very-longwavelength infrasound detectors recorded the blast and fixed the position near the coastal city of Bone in South Sulawesi, island ofSulewesi. They note that the blast was in the 10 to 50 kT range with thehigher end of this range being more likely.
Assuming an estimated size of about 5-10 meters in diameter, we wouldexpect a fireball event of this magnitude about once every 2 to 12 yearson average. As a rule, the most common types of stony asteroids wouldnot be expected to cause ground damage unless their diameters were about25 meters in diameter or larger.
A more extensive report by Elizabeth Silber and Peter Brown of theUniversity of Western Ontario is here.
------------------------------------------------------------------------ Summary of Preliminary Infrasonic Analysis of the Oct 8, 2009 Indonesian Superbolide
Elizabeth Silber and Peter Brown
Meteor Infrasound groupDept. of Physics and Astronomy,
Univ. of Western OntarioLondon, ONN6A 3K7CANADA
Released: October 19, 2009
On Oct 8, 2009, media reports appeared in the local press in Indonesiaconcerning a loud air blast occurring near 11am local time (0300 UT).Subsequent to these first media reports, additional English languagereports 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 ofthe air blast. Finally, a YouTube video posted on the same day appearsto show a large dust cloud consistent with a bright, daylight fireball.
http://www.surya.co.id/2009/10/09/ledakan-misterius-guncang-sulsel.html https://www.youtube.com/watch?v=yeQBzTkJNhs&videos=jkRJgbXY-90 Based on these initial reports, a detailed examination was made of allInternational Monitoring System (IMS) infrasound stations of theComprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). From thisinitial examination, a total of 11 stations showed probable signals froma large explosion centered near 4.5S, 120E, with an origin time near0300 UT on Oct 8, 2009, consistent with the media reports. This signalwas notable for having been (a) detected at many IMS stations, includingfive 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 observationssuggest the explosion source was of very high total energy. All signalmotions were between 0.27 - 0.32 km/s, consistent with stratosphericsignal returns.
We have used the Air Force Technical Application Centre (AFTAC)period-yield relation as described by ReVelle (1997) as the most robustbasic indicator of source energy. To generate measured periods, theaverage periods of all phase-aligned stacked waveforms at each stationwere measured, according to the technique described in Edwards et al(2006). These periods were then averaged to produce a single, globalaverage period of 13.4 sec and the AFTAC yield relation applied; thisproduced an average source yield of 31 kT of TNT. Averaging theindividual yields from all stations produces a mean source energy near50 kT of TNT while using only the eight stations having the highestsignal-to-noise-ratio (SNR) and using the local observed periods of thewaveform 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. Thatis, the best source energy estimate would be 40 +/- 30 kT TNT. Note thatmuch of this variation may be due to the signal emanating from differentportions of the fireball trail as observed at different stations; eachperiod measurement is a "sample" of the size of the cylindrical blastcavity at that particular segment of the trail detected by any onestation. As such, the out of atmosphere yield for this event is likelyhigher than these measurements suggest - very probably in the ~50 kT range.
The yield estimates based on infrasonic amplitude are very uncertain inthis instance as the propagation distances are much larger than istypical and outside the range limits where such relations have beendeveloped (e.g. Edwards et al, 2006) and hence the period relationship(which was generated using a dataset of nuclear explosions having yieldsin this range) is more applicable.
Some examples of the detected and processed waveforms are shown in theappendix.
Based on these infrasound records, it appears that a large (40-50 kTTNT) bolide detonation occurred near 0300 UT on Oct 8, 2009 near thecoastal city of Bone in South Sulawesi, Indonesia. The infrasonicgeolocation is not precise enough to determine if the bolide was overwater or land, but it was relatively near the coast.
Follow-on observations from other instruments or ground recovery effortswould be very valuable in further refining this unique event.
Using an average impact velocity for NEAs of 20.3 km/s, the energylimits (10 - 70 kT) suggested by this analysis correspond to an object5-10 m in diameter.
Based on the flux rate from Brown et al (2002), suchobjects 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 MeteoroidKinetic Energies from Observations of Infrasonic Airwaves, Journal ofAtmospheric and Solar-Terrestrial Physics, 68: 1136-1160.
ReVelle D.O. 1997. Historical Detection of Atmospheric Impacts by LargeBolides using Acoustic-Gravity Waves, Annals of the New York Academy ofSciences, Near-Earth Objects - The United Nations InternationalConference, 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 havebeen array processed in windows (typically of 30-60 second length) tosearch for coherent signals with consistent back-azimuth measurements.The top panel in each display is the F-statistic, a measure of therelative coherency of the signal across the array elements in anyparticular window (essentially a SNR measure). The second window showsthe apparent trace velocity of the acoustic signal across the array inthe direction of the peak F-stat. Similarly, the third plot shows thebest estimate for the signal back-azimuth in the direction of maximumF-stat for each window. The fourth plot shows the raw pressure signalfor one array element bandpassed according to the chosen Low - Highfrequency combination, shown in the boxes of the lower plot.
[graphic]
Signal as detected at IS05AU (Australia) as a range of 5000 km - thegreen 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 adistinct signal with a dominant period near 16.5 seconds.
20-100
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Sujet: Rentrée d'astéroïde 5-10 m le 8 octobre 2009 en Indonésie 
Dim 25 Oct - 15:36
