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Concurrent sound associated with very bright meteors manifests as popping, hissing, and faint rustling sounds occurring simultaneously with the arrival of light from meteors. Numerous instances have been documented with −11 to −13 brightness. These sounds cannot be attributed to direct acoustic propagation from the upper atmosphere for which travel time would be several minutes. Concurrent sounds must be associated with some form of electromagnetic energy generated by the meteor, propagated to the vicinity of the observer, and transduced into acoustic waves. Previously, energy propagated from meteors was assumed to be RF emissions. This has not been well validated experimentally. Herein we describe experimental results and numerical models in support of photoacoustic coupling as the mechanism. Recent photometric measurements of fireballs reveal strong millisecond flares and significant brightness oscillations at frequencies ≥40 Hz. Strongly modulated light at these frequencies with sufficient intensity can create concurrent sounds through radiative heating of common dielectric materials like hair, clothing, and leaves. This heating produces small pressure oscillations in the air contacting the absorbers. Calculations show that −12 brightness meteors can generate audible sound at ~25 dB SPL. The photoacoustic hypothesis provides an alternative explanation for this longstanding mystery about generation of concurrent sounds by fireballs.

The seismic network can be used for the localization of the fireball trajectory in the atmosphere. The types of sonic or blasting waves, created during the meteorid’s penetration of the atmosphere, are discussed. The practical use of the localization based on cylindrical or spherical waves is shown on the examples of fireballs Moravka (2000), Břeclav (2007), Jesenice (2009) and Kosice (2010).

The damage caused by the asteroid 17–20 metres in diameter that exploded over Chelyabinsk, Russia, on 15 February 2013 is estimated here to have an energy equivalent to about 500 kilotons of TNT. The fireball that streaked across the skies above Chelyabinsk in Russia on 15 February 2013 is providing astronomers with a wealth of information. Two papers in this issue present detailed reconstructions of the Chelyabinsk event. From an analysis of videos, Jiři Borovicka et al. determined the trajectory and velocity of the superbolide with high precision. Its orbit was similar to that of the 2-kilometre-diameter asteroid 86039 (1999 NC43), suggesting that the two bodies may be part of the same asteroid family. And they show that it broke into small pieces between the altitudes of 45 and 30 kilometres. In the companion paper, Peter Brown et al. analysed the damage caused by the airburst which they estimate was equivalent in energy to the detonation of 400 to 600 kilotons of TNT. They suggest that the number of impactors with diameters of tens of metres was an order of magnitude higher than current estimates, shifting much of the residual impact risk to these sizes. 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 damage1 than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects2. 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 wave3, but owing to lack of observations this is uncertain4,5. 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 (±100) kilotons of trinitrotoluene (TNT, where 1 kiloton of TNT = 4.185×1012 joules). We show that a widely referenced technique4,5,6 of estimating airburst damage does not reproduce the observations, and that the mathematical relations7 based on the effects of nuclear weapons—almost always used with this technique—overestimate blast damage. This suggests that earlier damage estimates5,6 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 techniques8,9. 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.

Analysis of video records of the Chelyabinsk superbolide of 15 February 2013 show that its orbit was sufficiently similar to the orbit of asteroid 86039 (1999 NC43) to suggest that the two were once part of the same object. The fireball that streaked across the skies above Chelyabinsk in Russia on 15 February 2013 is providing astronomers with a wealth of information. Two papers in this issue present detailed reconstructions of the Chelyabinsk event. From an analysis of videos, Jiři Borovicka et al. determined the trajectory and velocity of the superbolide with high precision. Its orbit was similar to that of the 2-kilometre-diameter asteroid 86039 (1999 NC43), suggesting that the two bodies may be part of the same asteroid family. And they show that it broke into small pieces between the altitudes of 45 and 30 kilometres. In the companion paper, Peter Brown et al. analysed the damage caused by the airburst which they estimate was equivalent in energy to the detonation of 400 to 600 kilotons of TNT. They suggest that the number of impactors with diameters of tens of metres was an order of magnitude higher than current estimates, shifting much of the residual impact risk to these sizes. Earth is continuously colliding with fragments of asteroids and comets of various sizes. The largest encounter in historical times occurred over the Tunguska river in Siberia in 1908, producing1,2 an airburst of energy equivalent to 5–15 megatons of trinitrotoluene (1 kiloton of trinitrotoluene represents an energy of 4.185 × 1012 joules). Until recently, the next most energetic airburst events occurred over Indonesia3 in 2009 and near the Marshall Islands4 in 1994, both with energies of several tens of kilotons. Here we report an analysis of selected video records of the Chelyabinsk superbolide5 of 15 February 2013, with energy equivalent to 500 kilotons of trinitrotoluene, and details of its atmospheric passage. We found that its orbit was similar to the orbit of the two-kilometre-diameter asteroid 86039 (1999 NC43), to a degree of statistical significance sufficient to suggest that the two were once part of the same object. The bulk strength—the ability to resist breakage—of the Chelyabinsk asteroid, of about one megapascal, was similar to that of smaller meteoroids6 and corresponds to a heavily fractured single stone. The asteroid broke into small pieces between the altitudes of 45 and 30 kilometres, preventing more-serious damage on the ground. The total mass of surviving fragments larger than 100 grams was lower than expected7.

We report exceptional fireball activity of the Orionid meteor shower in 2006. During four nights in October 2006 the autonomous fireball observatories of the Czech part of the European Fireball Network (EN) recorded 48 fireballs belonging to the Orionids. This is significantly more than the total number of Orionids recorded during about five decades long continuous operation of the EN. Based on precise multi-station photographic and radiometric data we present accurate atmospheric trajectories, heliocentric orbits, light curves and basic physical properties of 10 Orionid fireballs with atmospheric trajectories that were long enough and, with one exception, were observed from at least three stations. Seven were recorded in within a 2-h interval in the night of 20/21 October. Their basic parameters such as radiant positions and heliocentric orbits are very similar. This high fireball activity originated from a very compact geocentric radiant defined by α = 95.10° ± 0.10° and δ = 15.50° ± 0.06°. These fireballs most likely belonged to a distinct filament of larger meteoroids trapped in 1:5 resonance with Jupiter. From detailed light curves and basic fireball classification we found that these meteoroids appertain to the weakest component of interplanetary matter.

We present an analysis of sporadic meteor number 07406018, observed by image intensified video cameras at two stations, which showed a pronounced deceleration along its trajectory. We have applied the erosion model to analyze simultaneously the deceleration and light curve. We have found that the meteoroid had a low density of about 500 kg m−3, consistent with its cometary orbit. The meteoroid structure was, nevertheless, markedly different from the Draconid meteoroids, studied recently with the same model. The size of the constituent grains was larger and the erosion energy was higher than in Draconids. The meteor spectrum was also different from Draconid spectra and showed very bright Na lines. The meteoroid composition was probably different from normal cometary composition.

We present detailed data on 8 bright meteors recorded simultaneously by different observational techniques. All meteors were recorded by all-sky cameras at the Czech stations of the European Fireball Network and by image intensified TV cameras placed at Ondrejov and Kunzak observatories. As well as direct photographic and LLLTV recordings, most of meteors were recorded also by the spectral TV camera and some also by photographic spectral cameras. For 6 cases, lightcurves from radiometers with very high time resolution (1200 s−1) are also available. From all these detections we found a significant difference between TV and photographic beginning heights. TV beginnings are in average about 40 km higher than the photographic ones. We found that meteor brightness is up to 2 magnitudes higher in the photographic system than in the TV system. This difference for high velocity meteors is mainly caused by the presence of strong Ca+ lines in the blue part of the spectrum, where the image intensifier is only marginally sensitive. At heights above 110 km, the Na line is usually brighter than the Mg line, while at lower heights both lines have comparable brightness. In one of two captured spectra of short duration luminous trains, a small initial brightening of the Mg and Na lines caused by recombination processes was observed.

We present results on the MFM analysis (Meteor fragmentation model recently proposed by Ceplecha and ReVelle, 2005) of EN270200, a type-I fireball of maximum absolute magnitude ???10 from February 27, 2000, 19 h 22 m 57 s UT, based on observational data from four Czech stations of the European Fireball Network equipped with fish-eye cameras. Radiometer record of the light curve with time resolution of 1200 records per second is also available. Two grating spectra point to composition of an ordinary chondrite. Data on height and stellar magnitude were available at 73 individual trajectory points. The standard deviation of our fit was ??28 m for height as function of time and ??0.11 magnitude for the light curve. Ablation coefficient was found to be constant during the entire trajectory (0.004 s2 km???2). The shape-density coefficient varied in a wide range of values (1.65 and 0.12 c.g.s.). We found 17 fragmentation events during the entire photographic light curve, 7 of them larger than 1% of the main body mass. All fragmentation occurred in a form of clusters of tiny fragments. The initial mass resulted as 3.6 ?? 0.2??kg at a height of 80.144??km with velocity of 18.790??km s???1, and the terminal mass was 0.27 ?? 0.02??kg at a height of 31.62??km with velocity of 6.64 ?? 0.12??km s???1. We were able to explain the very short millisecond flare by the MFM model.

We present preliminary results obtained with the European Observation Network. This network consists of 9 observatories in the Czech Republic, Germany and Bulgaria and has been involved in the BACODINE activities since April 1, 1994. We also discuss related problems such as the background of unknown variable stars and suggest a strategy for work in this area.

Many Perseid meteors were photographed in the Czech part of the European Fireball Network during the activity of the new strong and sharp maximum on August 12, 1993. Basic data on many of them were evaluated and radiants, atmospheric trajectories and some orbital elements are presented here and compared with atmospheric and orbital data of regular Perseids, which were photographed outside the new activity in 1993 and preceding years. No substantial difference between these two groups of Perseid meteors was found.