* Astronomy

Title: The Fall of the St. Robert Meteorite: Interpretation of Eyewitness Accounts, Satellite Data, Short-Lived Isotope Activity, and Infrasound
Authors: Brown, P.; Hildebrand, A.; Green, D.; Page, D.; Jacobs, C.; Revelle, D.; Tagliaferri, E.; Wacker, J.

The St. Robert meteoroid (a monomict H5 breccia) entered the Earth's atmosphere at 00:02 UT on June 15, 1994 approximately one hour before local sunset. The resulting daylight fireball was widely observed from the provinces of Ontario and Quebec and the states of New York, Vermont and New Hampshire. The fireball was first observed over New York state at an altitude of ~60 km traveling in a N-NE direction to its point of terminal burst ~60 km northeast of Montreal. At least one observer noted electrophonic sounds heard simultaneously with the passage of the fireball. Several episodes of fragmentation occurred at the end point near an altitude of ~33 km with observers reporting several clumps of dust along the trajectory. Eyewitnesses to the explosion described multi-directional debris dispersal. A prominent dust trail persisted for ~10 minutes after the passage of the fireball. The terminal burst produced loud detonations audible for more than 200 km and of sufficient strength to shake buildings throughout metropolitan Montreal. Twenty fragments of this meteorite have been recovered in a fall ellipse of 7.5 x 4 km located near the farming community of St. Robert. Total recovered mass to date is ~25.4 kg, but the shower of meteorites was sufficiently dense, in at least the uprange part of the ellipse, so that one fragment partially penetrated the roof of a farmer's shed, and two fragments were found on roads. The most productive UTM grid square of 1 km sides yielded 6 meteorites. From the searched fraction of this square km, and a search efficiency of ~0.5 due to ground conditions and subsequent ground disturbance by farming, we estimate that ~25 meteorites fell in this grid square. This concentration implies that as many as 100 fragments greater than 55 g (the smallest recovered) may have fallen. Eighteen of the recovered fragments were completely covered by dark fusion crusts with surfaces showing varying degrees of ablation in accord with the multiple fragmentation episodes observed. Most fragments were found in shallow pits up to ~50 cm deep in the soft clay and sand soils of the region. Dedicated searches by interested local residents and members of the Meteorites and Impacts Advisory Committee to the Canadian Space Agency (and friends) recovered half of the known fragments. Interpretation of the eyewitness data suggest that the fireball traveled from SSW to NNE with a moderate slope from the horizontal of 15-35 degrees. An evaluation of the probable orbits for the meteoroid suggests an entry velocity in the range 12 -15 km/s. The object moved in a low- inclination orbit with perihelion very near the Earth's orbit. The total mass estimated to have reached the ground is 50-100 kg while the pre-atmospheric mass derived from visual observations is found to be of order 1,000 kg. The fireball of the St. Robert meteorite shower was also observed from above by sensors located on satellites of the Department of Defense. In the visual the fireball reached a peak magnitude of -18 during its terminal flare and the observations suggest a lengthy period of fragmentation lasting perhaps as long as one second near the endpoint. Data reduction is proceeding on infrared observations of the fireball, and initial mass estimates will be derived for the pre-atmospheric meteoroid from infrasound considerations, short lived isotope measurements for 8 of the 20 fragments, and dynamical information from eyewitness data in addition to satellite measurements. The St. Robert meteorite shower affords the first opportunity to combine satellite and eyewitness observations of the hypervelocity entry of a natural object into the Earth's atmosphere together with "ground truth" from the surviving remnants of the object's atmospheric passage.

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