Dynamical evolution of near-Earth objects

Abstract

<p><span>An apparent discrepancy between the number of observed near-Earth objects (NEOs) with small perihelion distances (q) and the number of objects that models <br />predict, has led to the conclusion that asteroids get destroyed at non-trivial distances from the Sun. Consequently, there must be a, possibly thermal, <br />mechanism at play, responsible for breaking up asteroids asteroids in such orbits.<br /><br />We studied the dynamical evolution of ficticious NEOs whose perihelion distance reaches below the average disruption distance q_dis=0.076 au, as suggested by <br />Granvik et al. (2016). To that end, we used the orbital integrations of objects that escaped from the main asteroid belt (Granvik et al. 2017), and entered the <br />near-Earth region (Granvik et al. 2018). First, we investigated a variety of mechanisms that can lower the perihelion distance of an object to a small-enough <br />value. In particular, we considered mean-motion resonances with Jupiter, secular resonances with Jupiter and Saturn (v_5 and v_6) and also the Kozai resonance.<br /><br />We developed a code that calculates the evolution of the critical argument of all the relevant resonances and identifies librations during the last stages of <br />an object's orbital evolution, namely, just before q=q_dis. Any subsequent evolution of the object was disregarded, since we considered it disrupted. The <br />accuracy of our model is ~96%.<br /><br />In addition, we measured the dynamical 'lifetimes' of NEOs when they orbit the innermost parts of the inner Solar System. More precisely, we calculated the <br />total time it takes for the q of each object to go from 0.4 au to q_dis (τ_lq). The outer limit of this range was chosen such because it is a) the approximate <br />semimajor axis of Mercury, and b) an absence of sub-meter-sized boulders with q smaller than this distance has been proposed by Wiegert et al (2020). Combining <br />this measure with the recorded resonances, we can get a sense of the timescale of each q-lowering mechanism.<br /><br />Next, for a more rigorous study of the evolution of the NEOs with q<0.4 au, we divided this region in bins and measured the relevant time they spend at <br />different distances from the Sun. Together with the total time spent in each bin, we kept track of the number of times that q entered one of the bins. <br />Finally, we computed the actual time each object spends in each bin during its evolution, i.e., the total time it spends in a specific range in radial <br />heliocentric distance.<br /><br />By following this approach, we derived categories of typical evolutions of NEOs that reach the average disruption distance. In addition, since we have the <br />information concerning the escape route from the main asteroid belt followed by each NEO, we linked the q-lowering mechanism and the associated orbital <br />evolutions in the range below the orbit of Mercury, to their source regions and thus were able to draw conclusions abour their physical properties.</span></p>