Preatmospheric detection of a meter-sized Earth impactor

Abstract

On 2020 September 18 US Government sensors detected a bolide with peak bolometric magnitude of -19 over the western Pacific. The impact was also detected by the Geostationary Lightning Mapper (GLM) instrument on the GOES-17 satellite and infrasound sensors in Hawaii. The USG measurements reported a steep entry angle of $67^{\circ}$ from horizontal from a radiant $13^{\circ}$ E of N and an impact speed of 11.7 km s$^{-1}$. Interpretation of all energy yields produces a preferred energy estimate of 0.4 kt TNT, corresponding to a $23000$ kilogram $3$ meter diameter meteoroid. A post-impact search of telescopic images found that the ATLAS survey captured the object just 10 minutes prior to impact at an Earth-centred distance of nearly $11900$ kilometers with apparent magnitude $m\text{=}12.5$. The object appears as a $0.44^{\circ}$ streak originating on the eastern edge of the image extending one-third of the USG state vector-based prediction of $1.26^{\circ}$ over the 30 second exposure. The streak shows brightness variability consistent with small asteroid rotation. The position of Earth's shadow, the object's size, and its consistency with the reported USG state vector confirm the object is likely natural. This is the eighth preatmospheric detection of a Near-Earth Asteroid (NEA) impactor and the closest initial telescopic detection prior to impact. The high altitude of peak fireball brightness suggest it was a weak object comparable in many respects with 2008 TC3 (Almahata Sitta meteorite), with absolute magnitude $H=32.5$ and likely low albedo. Therefore we suggest the NEA was a C-complex asteroid.