Institution(s): 1. University of Helsinki
Recent years have witnessed a renewed interest in the true population-level characteristics of near-Earth objects (NEOs). This interest has, at least partly, been driven by ongoing and planned NEO surveys as well as the desire to better characterize the impact threat from small NEOs. I will review the latest advances in NEO population models with a particular emphasis on the latest model (Granvik et al., in preparation; hereafter the NEO model) which describes the debiased orbital and absolute-magnitude distributions.
The parameters of the NEO model are calibrated by using about 4500 distinct NEOs detected by the Catalina Sky Survey (CSS) during 2005-2012. It accounts for the statistically-distinct orbital evolution of NEOs from six different source regions in the main asteroid belt in addition to Jupiter-family comets. An individual absolute-magnitude distribution is estimated for each source region and its functional form allows for a wavy shape but does not require it. The predicted number of large NEOs is in agreement with the results of other contemporary estimates and the overall shape of the absolute-magnitude distribution is very similar to predictions by other authors. For the first time ever, the NEO model predicts a rather complex variation of the orbital distribution with absolute magnitude.
A particularly intriguing finding during the development of the NEO model was that there should be more objects on orbits with small perihelion distances than what is observed. This suggests that a significant fraction of all NEOs disrupt at small perihelion distances and can thus no longer be detected. The assumption that, on average, NEOs disrupt at perihelion distances less than about 20 solar radii leads to a virtually perfect agreement between observations and theory that increasingly complicated NEO population models otherwise fail to achieve. The physical mechanisms responsible for the disruptions are still unknown but I will discuss some alternatives.
I will end my talk by speculating on near-future advances in NEO population models such as the inclusion of proxies for surface properties.