Dynamics of small earth-approachers on low-eccentricity orbits and implications for their origins

The population of Near-Earth Asteroids (NEAs) appears to be overabundant at sizes smaller than 50m, compared to a power-law extrapolation from kilomet er-sized objects. Several of these small NEAs are also concentrated on low- eccentricity orbits, where a few larger Earth-crossers are observed, and ar e called Small Earth-Approachers (SEAs). Their source region as well as the dynamical mechanisms involved in their transport close to the Earth on low -eccentricity orbits have not yet been determined. In this paper, we presen t our numerical and statistical study of the production and dynamical evolu tion of these SEAs. We first show that three main sources of Earth-crossers which are, according to recent simulations, the 3/1 and v(6) resonances in the main belt, and the Mars-crosser population, are not able to produce as many bodies on SEAs-like orbits compared to other Earth-crossing orbits as has been inferred from observations. From these sources, SEAs-like orbits are reached through the interplay of two required mechanisms: secular reson ances and planetary close approaches. However, the time spent on these orbi ts remains smaller than I Myr as confirmed by the study of the evolutions o f 11 observed SEAs which also reveal the action of various mechanisms such as close approaches to planets and/or secular resonances. Therefore, our re sults present some mechanisms which can be responsible for their production but none that would preserve the lifetime of the SEAs sufficiently to enha nce their abundance relative to other Earth-crossing orbits at the level ob served. The overabundance of the SEA population, if real, remains a problem and could be related to the influence of collisional disruption and tidal splitting of Earth-crossers, as well as to observational biases that might account for a discrepancy between theory and observation.