Dynamical evolution of near-Earth objects

An apparent discrepancy between the number of observed near-Earth objects (NEOs) with small perihelion distances (q) and the number of objects that models
predict, has led to the conclusion that asteroids get destroyed at non-trivial distances from the Sun. Consequently, there must be a, possibly thermal,
mechanism at play, responsible for breaking up asteroids asteroids in such orbits.

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
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
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
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.

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
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
accuracy of our model is ~96%.

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
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
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
this measure with the recorded resonances, we can get a sense of the timescale of each q-lowering mechanism.

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
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.
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
heliocentric distance.

By following this approach, we derived categories of typical evolutions of NEOs that reach the average disruption distance. In addition, since we have the
information concerning the escape route from the main asteroid belt followed by each NEO, we linked the q-lowering mechanism and the associated orbital
evolutions in the range below the orbit of Mercury, to their source regions and thus were able to draw conclusions abour their physical properties.