• An entry model for the Tagish Lake fireball using seismic, satellite and infrasound records

      Brown, P. G.; ReVelle, D. O.; Tagliaferri, E.; Hildebrand, A. R. (The Meteoritical Society, 2002-01-01)
      We present instrumental observations of the Tagish Lake fireball and interpret the observed characteristics in the context of two different models of ablation. From these models we estimate the pre‐atmospheric mass of the Tagish Lake meteoroid to be ~56 tonnes and its porosity to be between 37 and 58%, with the lowest part of this range most probable. These models further suggest that some 1300 kg of gram‐sized or larger Tagish Lake material survived ablation to reach the Earth's surface, representing an ablation loss of 97% for the fireball. Satellite recordings of the Tagish Lake fireball indicate that 1.1 x 10^12 J of optical energy were emitted by the fireball during the last 4 s of its flight. The fraction of the total kinetic energy converted to light in the satellite pass band is found to be 16%. Infrasonic observations of the airwave associated with the fireball establish a total energy for the event of 1.66 +/- 0.70 kT TNT equivalent energy. The fraction of this total energy converted to acoustic signal energy is found to be between 0.10 and 0.23%. Examination of the seismic recordings of the airwave from Tagish Lake have established that the acoustic energy near the sub‐terminal point is converted to seismic body waves in the upper‐most portion of the Earth's crust. The acoustic energy to seismic energy coupling efficiency is found to be near 10^(-6) for the Tagish Lake fireball. The resulting energy estimate is near 1.7 kT, corresponding to a meteoroid 4 m in diameter. The seismic record indicates extensive, nearly continuous fragmentation of the body over the height intervals from 50 to 32 km. Seismic and infrasound energy estimates are in close agreement with the pre‐atmospheric mass of 56 tonnes established from the modeling. The observed flight characteristics of the Tagish Lake fireball indicate that the bulk compressive strength of the pre‐atmospheric Tagish Lake meteoroid was near 0.25 MPa, while the material compressive strength (most appropriate to the recovered meteorites) was closer to 0.7 MPa. These are much lower than values found for fireballs of ordinary chondritic composition. The behavior of the Tagish Lake fireball suggests that it represents the lowest end of the strength spectrum of carbonaceous chondrites or the high end of cometary meteoroids. The bulk density and porosity results for the Tagish Lake meteoroid suggest that the low bulk densities measured for some small primitive bodies in the solar system may reflect physical structure dominated by microporosity rather than macroporosity and rubble‐pile assemblages.
    • The Morávka meteorite fall: 1. Description of the events and determination of the fireball trajectory and orbit from video records

      Borovička, J.; Spurný, P.; Kalenda, P.; Tagliaferri, E. (The Meteoritical Society, 2003-01-01)
      The Morávka (Czech Republic) meteorite fall occurred on May 6, 2000, 11:52 UT, during the daytime. Six H56 ordinary chondrites with a total mass of 1.4 kg were recovered. The corresponding fireball was witnessed by thousands of people and also videotaped by 3 casual witnesses. Sonic booms were recorded by 16 seismic stations in the Czech Republic and Poland and by one infrasonic station in Germany. A total of 2.5% of the fireball eyewitnesses reported electrophonic sounds. Satellites in Earth orbit detected part of the fireball light curve. In this first paper from a series of 4 papers devoted to the Morávka meteorite fall, we describe the circumstances of the fall and determine the fireball trajectory and orbit from calibrated video records. Morávka becomes one of only 6 meteorites with a known orbit. The slope of the trajectory was 20.4 degrees to the horizontal, the initial velocity was 22.5 km/s, and the terminal height of the fireball was 21 km. The semimajor axis of the orbit was 1.85 AU, the perihelion distance was 0.982 AU, and the inclination was 32.2 degrees. The fireball reached an absolute visual magnitude of -20 at a height of 33 km.
    • The orbit, atmospheric dynamics, and initial mass of the Park Forest meteorite

      Brown, P.; Pack, D.; Edwards, W. N.; ReVelle, D. O.; Yoo, B. B.; Spalding, R. E.; Tagliaferri, E. (The Meteoritical Society, 2004-01-01)
      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 29 from the vertical, with a radiant azimuth (astronomical) of 21 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 ~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) 10^3 kg, corresponding to an original energy of ~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.