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Academy of Sciences of the Czech Republic |
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Academy of Sciences of the Czech Republic |
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The April 6, 2002 fireball – the Pribram twin
| A very bright fireball illuminated large territory of Western Austria
and Southern Bavaria on Saturday evening, April 6 at 22:20:18 local time
(UT+2h). The fireball was observed by many casual witnesses over the territory
of almost whole Central Europe, but most observations were reported from
Bavaria and Western Austria. Except of numerous visual observations, the
fireball was recorded by several kinds of scientific instruments. The most
important records were obtained by the systematic long-term observational
photographic program – the European Fireball Network (EN). The records
were taken at 5 German, one Czech and one Austrian station of the EN. Each
of these stations is equipped with one all-sky camera, which is open whole
night and whole sky is photographed on one image. The German and Austrian
stations are equipped with mirror all-sky cameras and are operated by the
German Aerospace Center DLR, Berlin. The Czech stations of the EN are equipped
with very precise Zeiss Distagon fish-eye objectives and are operated by
the Astronomical Institute of the Academy of Sciences of the Czech Republic,
Ondrejov. Most Czech stations had cloudy skies on April 6, however. The
photographic records are most important for exact determination of the
fireball atmospheric trajectory, including prediction of meteorite impact
area and derivation of heliocentric orbit. In addition to these photographic
data, the fireball was recorded by three radiometric systems placed in
the Czech Republic at Ondrejov Observatory and Kunzak station, which gives
us basic information about light curve and maximum brightness of the fireball
and about exact time of the event. Furthermore the fireball was recorded
by at least at two infrasound stations, one located at Freyung, Germany
(see http://www.seismologie.bgr.de)
and second at Deelen, The Netherlands (see http://www.knmi.nl/~evers/infrasound/events/020406/bavaria-bolide.html)
and also at several seismic stations from Austria, Southern Germany and
Switzerland.
All data presented below are based only on above-mentioned photographic and radiometric data recorded within the EN observing program and are very close to final values. All records were measured, reduced and all computations were performed at the Ondrejov Observatory, the headquarters of the European Fireball Network. The fireball started its almost 92 km long luminous trajectory at an altitude of 85.6 km about 15 km NE from Innsbruck, Austria (longitude 11.564 deg E, latitude 47.304 deg N). Maximum brightness of about –18 absolute magnitude was reached in a bright flare at a height of 21 km near Garmisch-Partenkirchen, Germany (longitude 10.91 deg E, latitude 47.51 deg N). The fireball terminated at an altitude of only 15.8 km about 20 km W from Ga-Pa (longitude 10.85 deg E, latitude 47.53 deg N). Such deep penetration of a fireball is very scarce and this fireball belongs to the deepest ever-photographed fireballs in the history. It also implicates, that some part of the initial mass survived the ablation processes in the atmosphere and landed on the ground as meteorites. The slope of the atmospheric trajectory to the Earth's surface was 49.5 degrees. The fireball entered the atmosphere with the velocity of 20.9 km/s and during its flight substantially decelerated to the final value of only 4 km/s, when ablation process was stopped. According to the dynamic behavior in the atmosphere this fireball belongs to the fireball type I, which is usually identified with stony material, mostly ordinary chondrites. The initial dynamic mass of the entering meteoroid was about 500 kg and most of this mass was ablated and only about 30 kg of total mass could land on the ground in several fragments. The impact area is relatively large, it is at least several kilometers long and about 1km wide. The main fragments will lie eastwards from Schwangau, Germany. Smaller fragments could be found also around the Austria–Germany border westwards from Ga-Pa. The whole area is located in high mountains (the Alps), which is unfortunately very unfavorable for any systematic search. From the exact time of the fireball occurrence, its initial velocity, and the position of the radiant, we computed the heliocentric orbit. We found that the body, before its collision with Earth, orbited the Sun on an elliptic orbit defined by the following orbital elements: semimajor axis 2.4 AU, eccentricity 0.67, perihelion distance 0.79 AU, argument of perihelion 241.4 degrees, longitude of ascending node 16.8 degrees and inclination 11.4 degrees. Such kind of heliocentric orbit is quite usual for fireballs which penetrate very deep into the Earth's atmosphere and which can produce meteorites. The aphelion of these orbits lies in the main belt of Asteroids and therefore the asteroidal origin of these bodies is inferred. However, the heliocentric orbit of this fireball has one very significant exceptionality: we found that this orbit is the same as the orbit of the first photographed meteorite fall in the history – the Pribram meteorite fall on April 7, 1959. Both orbits are so close that there is no doubt that both bodies have the same origin. It is very important evidence for the existence of asteroidal streams and meteorite streams as suggested earlier by Halliday and others. From observations of both bolides we know that both bodies were far from each other in the orbit (probably about half of the period) when the Pribram collided with the Earth. It implies that many such bodies have to be on this orbit, because it is fantastic chance to photograph two meteorite falls from the same orbit on practically the same territory within only 43 years! It also substantiates why it is important to operate such long term observing program as the European Fireball Network is. Finally, from the perfect similarity of both heliocentric orbits we can predicate, that both bodies had also the same composition and therefore we can expect that meteorites produced by the April 6 fireball are H5 ordinary chondrites. Pavel Spurny
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