Pavel Spurný

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.

The trajectory, structure and origin of the Chelyabinsk asteroidal impactor

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, producing 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 Indonesia in 2009 and near the Marshall Islands in 1994, both with energies of several tens of kilotons. Here we report an analysis of selected video records of the Chelyabinsk superbolide 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 meteoroids 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 expected.

Photographic observations of neuschwanstein, a second meteorite from the orbit of the Přibram chondrite

Photographic observations of meteoroids passing through the atmosphere provide information about the population of interplanetary bodies in the Earths vicinity in the size range from 0.1 m to several metres. It is extremely rare that any of these meteoroids survives atmospheric entry to be recovered as a meteorite on the ground. Příbram was the first meteorite (an ordinary chondrite) with a photographically determined orbit; it fell on 7 April 1959 (ref. 1). Here we report the fourth meteorite fall to be captured by camera networks. We determined the atmospheric trajectory and pre-atmospheric orbit of the object from the photographic records. One 1.75-kg meteorite—named Neuschwanstein and classified as an enstatite chondrite—was recovered within the predicted impact area. The bolides heliocentric orbit is exceptional as it is almost identical to the orbit of Příbram, suggesting that we have discovered a ‘stream’ of meteoritic objects in an Earth-crossing orbit. The chemical classifications and cosmic-ray exposure ages of the two meteorites are quite different, however, which implies a heterogeneous stream.

An anomalous basaltic meteorite from the innermost main belt

The Meteorite Who Fell to Earth Orbital data is available for only a handful of meteorites. Some are found long after they fell to Earth. Others are recovered after they have been observed falling through the atmosphere, but their trajectories are rarely recorded. Bland et al. (p. 1525) used a photographic camera network located in the Australian desert to track a fireball in the sky, find the meteorite, and establish its orbit. The meteorite is a basaltic achondrite; most such rocks have been traced to the major asteroid Vesta. In this case, the meteorites isotopic composition and orbital properties suggest a distinct parent asteroid—a different source of basaltic material residing in the innermost main belt. This meteorite’s composition and orbital properties are such that it cannot be traced to the parent asteroid. Triangulated observations of fireballs allow us to determine orbits and fall positions for meteorites. The great majority of basaltic meteorites are derived from the asteroid 4 Vesta. We report on a recent fall that has orbital properties and an oxygen isotope composition that suggest a distinct parent body. Although its orbit was almost entirely contained within Earth’s orbit, modeling indicates that it originated from the innermost main belt. Because the meteorite parent body would likely be classified as a V-type asteroid, V-type precursors for basaltic meteorites unrelated to Vesta may reside in the inner main belt. This starting location is in agreement with predictions of a planetesimal evolution model that postulates the formation of differentiated asteroids in the terrestrial planet region, with surviving fragments concentrated in the innermost main belt.

Atmospheric deceleration and light curves of Draconid meteors and implications for the structure of cometary dust

Aims. The observation of Draconid meteors was used to infer information on the structure, porosity, strength, and composition of the dust of comet 21P/Giacobini-Zinner. Methods. Stereoscopic video and photographic observations of six faint and one bright Draconid meteors provided meteor morphologies, heights, light curves, and atmospheric decelerations. The spectrum of the bright meteor was also obtained. We developed a simple model of meteoroid ablation and fragmentation. The model assumes that cometary meteoroids are composed of constituent grains. Results. By fitting the observed decelerations and light curves, we have found that the grain mass range was relatively narrow in all meteoroids but differed from case to case. Some meteoroids were coarse grained with grain masses 10 −9 to 10 −10 kg, others were fine grained with grain masses one order of magnitude lower. Individual mm-sized meteoroids contained tens of thousands to almost a million grains (assuming grain density close to 3000 kg m −3 ). The meteoroids were porous aggregates of grains, having porosities of about 90% and bulk densities of 300 kg m −3 . Grain separation started after the surface of the meteoroid received energy of 10 6 Jm −2 . The separation continued during the first half of meteor trajectories. We call this phase erosion. The energy needed for grain erosion was 15−30× lower than the energy of vaporization. However, 30% of the largest meteoroid was resistant to thermal erosion; this part disrupted later mechanically under a very low dynamic pressure of 5 kPa. The relative abundances of Na, Mg, and Fe were nearly chondritic, but differential ablation caused preferential loss of sodium at the beginning of the trajectory.

Atmospheric trajectories and light curves of shower meteors

Double station data on 496 meteors belonging to several meteor showers were obtained within the program of the video meteor observations during years 1998-2001. Analyzed meteors cover a range of photometric masses from 10 -7 to 10 -4 kg with a corresponding range of maximum brightness from +4.7 to -2.1 absolute magnitude. Atmospheric trajectories of Perseid, Orionid and Leonid meteors are analysed. These typical cometary high velocity meteors are compared to Geminid meteors with probable asteroidal origin and Taurid meteors - another cometary shower with significantly lower entry velocity. The light curses of the studied meteors vary widely, but generally are nearly symmetrical with the point of maximum brightness located close the to middle of the luminous trajectory. Small differences between showers are reported. We found that the height data are in good agreement with the dust-ball model predictions. The only difference is the beginning height behaviour. The beginning heights of cometary meteors increase with increasing photometric mass. These meteoroids probably contain a volatile part which starts to ablate before we are able to detect the meteors. The Geminid meteors are a different case. They start to ablate suddenly and their beginning height is almost constant in the whole range of studied meteoroid masses. In this case we observe real beginnings of meteor ablation.

Recent fireballs photographed in central Europe

Abstract The atmospheric trajectories, velocities, brightnesses and orbits of 17 exceptional fireballs photographed in central Europe during 1990–1993 are presented. This survey contains, among others, one Earth-grazing fireball, two fireballs with predicted multiple meteorite fall and four well defined fireballs which are members of major meteor streams (Geminid, Quadrantid, Southern Taurid and α Capricornid). The operation of the Czech part of the European Fireball Network is briefly described.

Automation of the Czech part of the European fireball network: equipment, methods and first results

In the last several years the manually operated fish-eye cameras in the Czech part of the European fireball Network (EN) have been gradually replaced with new generation cameras, the modern and sophisticated completely autonomous fireball observatories (AFO), which were recently developed in the Czech Republic. The main motivation for construction of this new observing system was to continue in regular fireball observations and to make these observations more complex and efficient. In this paper we briefly describe basic design and work of this new instrument and its deployment at the Czech stations of the EN. The current dislocation of the individual stations and their equipment is also discussed. Along with this new modern instrument we developed also new software for measurement of photographic negatives which makes this time consuming work more efficient and easier. The AFOs provide us with data on fireballs far richer and more interesting than those we were able to get in the past. This is illustrated by the cases of two recently observed fireballs which were recorded by the AFOs. We describe the high precision of all the measureded values as well as the very detailed information about light curves in both cases.

The Australian Desert Fireball Network: a new era for planetary science

Through an international collaboration between Imperial College London, the Ondřejov Observatory in the Czech Republic and the Western Australian Museum, the installation of the Australian Desert Fireball Network in the Nullarbor Region of Western Australia was completed in 2007. Currently, the Network, which is the first to be established in the southern hemisphere, comprises four all-sky autonomous observatories providing precise triangulation of fireball records to constrain pre-atmospheric orbits and fall positions of meteorites over an area of approximately 200 000 km2. To date, the Network has led to the successful recovery of two observed meteorite falls. The first recovery was three fragments (174, 150 and 14.9 g) of the same meteorite fall recorded on 20 July 2007 at 19 h 13 m 53.2 s±0.1 s UT that were found within 100 m of the predicted fall line. Named Bunburra Rockhole, the meteorite is a basaltic achondrite with an oxygen isotopic composition (Δ17O = −0.112 ‰) distinguishing it from basaltic meteorites belonging to the Howardite–Eucrite–Diogenite clan thought to be derived from asteroid 4Vesta, and therefore must have come from another differentiated asteroid in the terrestrial planet region. Bunburra Rockhole was delivered to Earth from an Aten-like orbit that was almost entirely contained within the Earths orbit. The second recovered fall was detected by the Network on 13 April 2010 and led to the recovery of a 24.54 g meteorite fragment that is yet to be fully described. To date, the Network has recorded ∼550 fireballs. Records from which precise orbits and trajectories can be determined number ∼150. In addition to the two recovered falls twelve fireballs are considered to have resulted in meteorite falls. Of these, four are probable falls (10s–100 g), and five are certain falls (>100 g). Having proved the potential of the Network, ultimately a large dataset of meteorites with orbits will provide the spatial context for the interpretation of meteorite composition that is currently lacking in planetary science.

Optical observations of enhanced activity of the 2005 Draconid meteor shower

Context. The enhanced activity of the Draconid meteor shower was observed on October 8, 2005 using video and photographic cameras. Aims. The aim of this paper is to use a higher than usual number of recorded meteors to look at some physical properties of the Draconid meteoroids, to describe the activity profile, and to infer meteor orbits. Methods. Video data on meteors are used for the determination of the meteor shower activity. Double station data provide precise beginning heights of the meteors as well as their radiants and orbits. Beginning heights and light curves of all meteors are used for investigation of meteoroid properties. Results. Only the descending branch of the enhanced activity was observed between 17:30 and 19 UT. The mass distribution index is similar to the 1998 return. Beginning heights of the Draconid meteors are several kilometres higher in comparison with other meteors of similar velocity. Light curves are nearly symmetrical, with a slight preference of early maxima. Both results are consistent with the very fragile nature of Draconid meteoroids.