The fall of the St‐Robert meteorite

Abstract

Abstract The St‐Robert (Québec, Canada) meteorite shower occurred on 1994 June 15 at 0h02m UT accompanied by detonations audible for >200 km from the fireball endpoint. The fireball was recorded by visual observers in Vermont, New York State, New Hampshire, Québec and Ontario as well as by optical and infrared sensors in Earth‐orbit. Penetration to an altitude of 36 km occurred ∼60 km to the northeast of Montreal, where the bolide experienced several episodes of fragmentation. A total of 20 fragments of this H5 chondrite, comprising a total mass of 25.4 kg, were recovered in an ellipse measuring 8 × 3.5 km. One fragment of the shower partially penetrated the aluminum roof of a shed.Interpretation of the visual and satellite data suggests that the fireball traveled from south‐southwest to north‐northeast, with a slope from the horizontal of 55°–61°. A statistical evaluation of the likely heliocentric orbits for the body prior to collision with the Earth, coupled with theoretical modeling of the entry, suggests an entry velocity in the range of 12.7–13.3 km/s; the meteoroid had moved in a low‐inclination orbit, with orbital perihelion located extremely close to the Earth's orbit.From satellite optical data, it is found that the photometric mass consumed during the largest detonation is ∼1200 kg. Estimation of the amplitude of the acoustic signal detected by the most distant observer yields a source energy near 0.5 kt TNT equivalent energy, which corresponds to a mass of order 10 metric tonnes. This measure is uncertain to approximately one order of magnitude. Theoretical modeling of the entry of the object suggests a mass near 1600 kg. Cosmogenic radionuclide activities constrain the lower initial mass to be ∼700 kg with an upper limit near 4000 kg. Seismic data possibly associated with the fireball suggest extremely poor coupling between the airwave and the ground.The total mass estimated to have reached the ground is ∼100 kg (in material comprising >55 g fragments), while the preatmospheric mass is found to be most probably in the range of 1200–2000 kg.