Evidence for meter-sized NEO rubble piles from fireball data

The structure of small NEOs is of both practical concern for planetary defence and provides insights into their origin and evolution. From their rotation rates, it is known that most large NEAs are likely rubble-pile agglomerations, while some smaller (<100m) NEAs have sufficiently fast spin rates to require significant internal strength. However, at meter-sizes limited information on strength is available. Only a handful of proxy strength measures are available for decameter and smaller NEAs. One data source which can address the question of meter-sized NEO structural strength is the recent release of over 800 light curves for bolides detected by US Government sensors since 1988. A subset of these events (about 270) also have trajectory, speed and height at peak brightness information. These data, together with the recently available lightcurves, afford the possibility of applying ablation modelling to infer strength based on fragmentation heights. Among these 270 bolides with photometric and metric information, we found a dozen events which stood out from the rest of the population in that they reached peak brightness at very high altitudes (>50 km). Among these dozen, two were clearly on cometary-type orbits, while the remainder were on asteroidal orbits. Of the asteroidal bodies, all showed global strengths of order ~100 kPa or lower. Here we report on ablation modelling to assess the potential structural makeup of these bodies. Our modelling suggests these meteoroids are the closest yet measured to having properties of a rubble pile structure at meter-sizes. They comprise ~5% of all meter-sized Earth impactors detected by the USG sensors. We suggest this is a lower limit to the fraction of meter-sized NEOs impacting Earth which are true rubble piles. We compare our analysis with similar data from the impact of 2008 TC3, the most confidently identified meteorite producing fireball which was undoubtedly a rubble-pile.