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, Jiří Borovička 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.

Needle in a haystack: tracking down the fragments of asteroid 2008 TC 3 On 6 October 2008, a small Earth-bound asteroid designated 2008 TC 3 was discovered by the Catalina Sky Survey. Some 19 hours — and many astronomical observations — later it entered the atmosphere and disintegrated at 37 km altitude. No macroscopic fragments were expected to have survived but a dedicated search along the approach trajectory in a desert in northern Sudan has recovered 47 meteorites, fragments of a single body named Almahata Sitta, with a total mass of 3.95 kg. The asteroid and meteorite reflectance spectra identify the asteroid as surface matter from a class 'F' asteroid, material so fragile that it was not previously represented in meteorite collections. To have recovered meteorites from a known class of asteroids is a coup on a par with a successful spacecraft sample-return mission — without the rocket science.

The fireball accompanying the Park Forest meteorite fall (L5) was recorded by ground-based videographers, satellite systems, infrasound, seismic, and acoustic instruments. This meteorite shower produced at least 18 kg of recovered fragments on the ground (Simon et al. 2004). By combining the satellite trajectory solution with precise ground-based video recording from a single site, we have measured the original entry velocity for the meteoroid to be 19.5 +/- 0.3 km/s. The earliest video recording of the fireball was made near the altitude of 82 km. The slope of the trajectory was 29degrees from the vertical, with a radiant azimuth (astronomical) of 21degrees and a terminal height measured by infrared satellite systems of 18 km. The meteoroid's orbit has a relatively large semi-major axis of 2.53 +/- 0.19 AU, large aphelion of 4.26 +/- 0.38 AU, and low inclination. The fireball reached a peak absolute visual magnitude of -22, with three major framentation episodes at the altitudes of 37, 29, and 22 km. Acoustic recordings of the fireball airwave suggest that fragmentation was a dominant process in production of sound and that some major fragments from the fireball remained supersonic to heights as low as similar to 10 km. Seismic and acoustic recordings show evidence of fragmentation at 42, 36, 29, and 17 km. Examination of implied energies/initial masses from all techniques (satellite optical, infrasound, seismic, modeling) leads us to conclude that the most probable initial mass was (11 +/- 3) x 10(3) kg, corresponding to an original energy of similar to 0.5 kt TNT (2.1 x 10(12) J) and a diameter of 1.8 m. These values correspond to an integral bolometric efficiency of 7 +/- 2%. Early fragmentation ram pressures of < 1 MPa and major fragmentations occurring with ram pressures of 2-5 MPa suggest that meter-class stony near-Earth asteroids (NEAs) have tensile strengths more than an order of magnitude lower than have been measured for ordinary chondrites. One implication of this observation is that the rotation period for small, fast-rotating NEAs is likely to be > 30 seconds.

Asteroids with diameters smaller than ∼50–100 m that collide with the Earth usually do not hit the ground as a single body; rather, they detonate in the atmosphere1. These small objects can still cause considerable damage, such as occurred near Tunguska2, Siberia, in 1908. The flux of small bodies is poorly constrained, however, in part because ground-based observational searches pursue strategies that lead them preferentially to find larger objects3. A Tunguska-class event—the energy of which we take to be equivalent to 10 megatons of TNT—was previously estimated to occur every 200–300 years, with the largest annual airburst calculated to be ∼20 kilotons (kton) TNT equivalent (ref. 4). Here we report satellite records of bolide detonations in the atmosphere over the past 8.5 years. We find that the flux of objects in the 1–10-m size range has the same power-law distribution as bodies with diameters >50 m. From this we estimate that the Earth is hit on average annually by an object with ∼5 kton equivalent energy, and that Tunguska-like events occur about once every 1,000 years.

Abstract— Data on the trajectory and orbit of an extremely bright bolide (superbolide) over Greenland on 1997 December 9 are given, and circumstances of the phenomenon and its observations are described. A surveillance video camera and satellite‐based records enabled computing the trajectory and orbit independently of visual sightings of casual observers. The superbolide body of about 36 000 kg penetrated the atmosphere with an initial velocity of 30.5 ± 1.7 km s−1. Its orbit was a long‐period orbit and seems to be at variance with the low value of ablation coefficient (0.017 kg MJ−1) derived from modeling the atmospheric trajectory. However, such an event has been documented previously. Also the intensity and brevity of the satellite‐detected light flares are highly unusual. The impact area of the main hypothetical remnant of the body is given. Search for meteorites was performed. No meteorites were recovered. Also analysis of snow samples gave no hint of meteoritic dust.

This paper describes first results of common detections of fireballs by photographic cameras in Czech Fireball Network (CFN) and the new radiometric systems equipped with sensors with very high time and intensity resolutions placed at two stations of this network, Ondrejov Observatory and Kunzak. Since August 1999, when we started regular operation of two radiometric systems, we have detected 17 different fireballs. Eleven of them were recorded simultaneously by photographic cameras, another six were only single radiometric detections as radiometers can detect meteoric events also under cloudy conditions. From two most suitable common events we performed calibration of radiometers and we determined their sensitivity. We found significant differences between lightcurves of slow and fast meteors recorded by these techniques, and finally, we found substantial differences in shapes of lightcurves for fireballs belonging to the same meteor stream observed in only 2.5 hours time interval.

This paper describes the coordinated results of several sets of measurements of two Leonid meteor fireballs over northern New Mexico at 1:32 and 3:06 MST, respectively, on the night of 1998 November 17. The measurements included visible band photometry on both events, as well as filtered 5890 Angstrom all-sky images of the Na airglow. Also, for the 3:06 A.M. event, we obtained an infrasound measurement of the hydrodynamic yield. For the 1:32 A.M. event, we obtained a set of visible band charge-coupled device (CCD) camera images of the meteor train for times extending to 30 min after the initial impact. The measurement results have been combined to derive an optical efficiency for the intense early-time optical flash, and the total explosion yields and masses for both of the meteors.We have also done a set of numerical radiation, hydrodynamic, and chemistry computations to investigate the nature and distribution of the long-lasting airglow. We attribute the brightest visible airglow to atomic O 5577 Angstrom line emission, with additional contributions from atomic Na emission and NO2 chemiluminescence. The near-infrared atmospheric bands of molecular O-2 should be very strong as well. All of the band emissions are expected to show a hollow limb-brightened structure.

Abstract— On the early morning of 1994 January 18, a very bright luminous object crossed the sky of Santiago de Compostela, Spain. From visual sightings, it is concluded that the object wasn't a meteoric fireball (bolide). A surface “crater” in Cando (close to Santiago) with dimensions 29 × 13 m and 1.5 m deep was later discovered within 1 km of the projected “impact” point of the luminous object. At this site, in addition to the topsoil, full‐grown pine trees >20 m high were thrown downhill over a nearby road, leaving the down‐slope edge of the “crater” untouched and with a steep interior wall (this would not be the case if a regular landslide were responsible for the transport). Standing trees below the “crater” showed embedded soil and plant residues up to heights >3 m. No strange materials (meteorites or artifacts) were recovered in or close to the “crater”; all materials belonged to the site and were not shocked; thus, an impact is very improbable.

Radiation energies of bright flashes caused by disintegration of large meteoroids in the atmosphere have been measured using optical sensors on board geostationary satellites. Light curves versus time are available for some of the events. We have worked out several numerical techniques to derive the kinetic energy of the meteoroids that produced the hashes. Spectral opacities of vapor of various types of meteoroids were calculated for a wide range of possible temperatures and densities. Coefficients of conversion of kinetic energy to radiation energy were computed for chondritic and iron meteoroids 10 cm to 10 m in size using radiation-hydrodynamics numerical simulations. Luminous efficiency increases with body size and initial velocity, Some analytical approximations are presented for average conversion coefficients for irons and H-chondrites. A mean value of this coefficient for large meteoroids (1-10 m in size) is about 5-10%. The theory was tested by analyzing the light curves of several events in detail.Kinetic energies of impactors and energy-frequency distribution of 51 bolides, detected during 22 months of systematic observations in 1994-1996, are determined using theoretical values of luminous efficiencies and heat-transfer coefficients, The number of impacts in the energy range from 0.25 to 4 kt TNT is 25 per year and per total surface of the Earth.The energy-frequency distribution is in a rather good agreement with that derived from acoustic observations and the lunar crater record, Acoustic systems have registered one 1 Mt event in 12 years of observation. Optical systems have not detected such an event as yet due to a shorter time of observation. The probability of a 1 Mt impact was estimated by extrapolation of the observational data. (C) 1997 Academic Press.

Influx of meteoroids onto Earth is reasonably well known for sizes of up to about 1 meter applying results from three independent Photographic Bolide (Fireball) Networks. The change of cumulative numbers with increasing mass was recently improved by new calibration at 100 kg masses from a very self-consistent solution for motion and ablation of the Lost City bolide (bulk density and terminal mass known from the recovered meteorite). The influx onto Earth of small asteroids observed in reflected sunlight is also reasonably well established from observations of the Space-Watch Telescope down to sizes of somewhat less than 10 meters. Cumulative fluxes as function of total kinetic energy at entry of these bodies into the Earth's atmosphere are used as standards for comparison with relative fluxes derived from cumulative numbers of 21 meteoric events observed by satellites (SA bolides). Absolute calibration is realized by comparing cumulative numbers of 43 Prairie Network (PN) bolides brighter than absolute magnitude –10 with the same flux curve and using the same method. Dynamically determined masses of the 43 PN bolides are used to define the mass scale of the satellite-observed (SA) bolides. Definitions of differential and total luminous efficiency are presented, and data on total luminous efficiencies as well as on velocities, masses, ablation coefficients, and on total radiated energies of 33 PN bolides are given. For the average luminous efficiency, the masses of the SA bolides resulted between 6 x 10 6 kg and 3500 kg with the median value of 80000 kilograms. Corresponding sizes are from 15 m to 1.2 m with the median at 3.4 m.

Observational data on luminosity of bolides observed from ground-based stations as well as from satellite systems can be converted into masses or sizes of bodies assuming we know the luminous efficiency of the atmospheric interaction process. Problems of luminous efficiency were very much simplified until recently. Traditional approach assigned just a single value for each velocity and the calibration was derived only from early experiments with artificial meteors produced by masses of the order of grams. A recent analysis of the Lost City fireball enabled a precise and reliable determination of masses from the motion of the body and revealed that the differential luminous efficiencies for bodies in a mass ranges of hundred kilograms are out 10 times larger than the traditionally used values. This paper presents results on luminous efficiencies from detailed analysis of 29 Prairie Network (PN) bolides brighter than magnitude minus 10. This analysis is based on independent solutions for motion of the body, combines them with the experimental results for gram masses and applies them to satellite observed bolides. Masses resulting from such dynamic solutions are compared to radiated energies and the luminous efficiencies are computed by two different approaches: (1) total radiated energy is compared to the initial kinetic energy of the body resulting in total luminous efficiency (The only concept used on space observed bolides so far); (2) time change of kinetic energy of ablated mass is compared to power of radiation for each time-mark on the photographic record resulting in differential luminous efficiency (the traditional quantity used in meteor physics). Within a factor of two, the total luminous efficiency is identical to the differential one for majority of the 29 PN bolides. Some values of the total luminous efficiency are smaller than the differential ones due to velocity dependence and due to a significant part of kinetic energy going into changing the momentum of the body. Some values of the total luminous efficiency are greater than the differential ones due to sudden release of radiation energy at discrete points (bolides with large flares of many stellar magnitudes over the smooth light curve). Values of the differential luminous efficiencies for 29 PN bolides were derived altogether at 1324 different points of their trajectories. Correlations of these values with height, velocity, deceleration, and brightness were used to get correction factors for the actual bulk density and shape of the body (comparing these correlations with data on the Lost City bolide). These corrections were applied to the values of the total luminous efficiencies. The resulting total luminous efficiencies mostly depend on total radiated energy and, to a lesser extent, also on velocity. No reliable correlations with other parameters were found. Each bolide from these 29 behaves as an independent individual from the point of view of radiation efficiency in analogical way to ablation efficiency (Figure 8). There is no statistical correlation between ablation and radiation efficiency, but we can find groups of similar behavior: the most frequent are poor 'ablators' an good radiators (13 cases); the second in importance are good 'ablators' and poor radiators (9 cases). It is highly probable that this individualistic behavior is also valid for the much brighter bolides observed by DOD satellites. The average total luminous efficiency for the 29 PN bolides resulted as (tau) t equals 1.36% of total kinetic energy amounting to an average of 5 multiplied by 107 J of total radiated energy, which corresponds to the average initial mass of 15 kg for this sample of 29 PN bolides. These values were compared with experimental data on gram size meteoroids. Total luminous efficiencies and their expected errors for radiated energies in the range 109 to 1013 were predicted (Table 5). Resulting total luminous efficiencies were applied to data published on 21 very bright bolides observed by sensors on DOD satellites and their masses were determined (Table 6).