Abstract
The Oort cloud is thought to be a reservoir of icy planetesimals and a source of long-period comets (LPCs) implanted from the outer Solar System during the time of giant planet formation. The presence of rocky ice-free bodies is much harder to explain. The rocky fraction in the Oort cloud is a key diagnostic of Solar System formation models as this ratio can distinguish between "massive" and "depleted" proto-asteroid belt scenarios and thus disentangle competing planet formation models. Objects of asteroidal appearance have been telescopically observed on LPC orbits, but from reflectance spectra alone it is uncertain whether they are asteroids or extinct comets. Here we report a first direct observation of a decimeter-sized rocky meteoroid on a retrograde LPC orbit (e ≈ 1.0, i = 121°). The ~2 kg object entered the atmosphere at 62 km/s. The associated fireball terminated at 46.5 km, 40 km deeper than cometary objects of similar mass and speed. During its flight, it experienced dynamic pressures of several MPa, comparable to meteorite-dropping fireballs. In contrast, cometary material measured by Rosetta have compressive strengths of ~1 kPa. The earliest fragmentation of this fireball occurred at >100 kPa, indicating it had a minimum global strength well in excess of cometary. A numerical ablation model produces bulk density and ablation properties consistent with asteroidal meteoroids. We estimate the flux of rocky objects impacting Earth from the Oort cloud to be ~0.7 × 106 km2 per year to a mass limit of 10 g. This is ~6% of the total flux of fireballs on LPC-orbits to these masses. Our results suggests there is a high fraction of asteroidal material in the Oort cloud at small sizes and gives support to migration-based dynamical models of the formation of the Solar System which predict that significant rocky material is implanted in the Oort cloud, a result not explained by traditional Solar System formation models.