The population of near-Earth asteroids

Abstract

We describe a methodology of estimating the size–frequency distribution (SFD) of near-Earth asteroids (NEAs). We estimate the completion versus size of present surveys based on the re-detection ratio, that is, the fraction of all detections over a recent period that are re-detections of already discovered objects rather than new discoveries. The re-detection ratio is a robust measure of completion, but must be corrected for the obvious bias caused by differences in ease of discovery due to specific orbital geometries. We do this with a computer survey simulation using a large set of synthetic orbital elements matching as best possible the distribution of the real NEA population. Once suitably “calibrated” to match re-detections of the real survey, the completion estimate versus size derived from the simulation can be extended both to large size where few if any new detections are recorded, and to small sizes beyond where re-detection numbers are statistically significant, thereby providing an estimate of the population and survey completion over the entire range from the largest NEAs down to the smallest sizes detected (∼3m diameter). Here we update our previous population estimates and survey progress, using discoveries by surveys from August, 2012 through July, 2014. We estimate that there are 990±20 NEAs larger than 1km in diameter (absolute magnitude H⩽17.75), of which about 90% have been discovered as of August, 2014. We confirm a “dip” in the SFD, in the range from a few tens to a few hundreds of meters diameter, which may be due to the transition from larger “rubble pile” bodies to smaller “monolithic” bodies. We compare our population estimate at the smallest sizes with recent ones based on bolide frequency and find excellent agreement, within estimated errors. The same survey simulation methodology can be used to investigate population and survey completion of various subset populations, for example Earth-Crossing Asteroids (ECAs, with orbits crossing 1AU heliocentric distance), Potentially Hazardous Asteroids (PHAs, with orbits passing within 0.05AU of the Earth’s orbit), or Interior to Earth Asteroids (IEOs, with orbits entirely interior to the Earth’s orbit). Lastly, we have investigated the population and completion of so-called “ARM-target” asteroids, of size ∼10m diameter in orbits passing within 0.03AU of the Earth’s orbit with very low Earth-encounter velocity, <2.5km/s. We find current ground-based surveys are remarkably efficient in detecting this subset of NEAs, and are currently about 1% complete, implying a total population of such bodies of only a few thousand.