Near-Earth asteroids in Main Belt-crossing orbits

Abstract

We study the dynamical and collisional evolution of Near-Earth asteroids (NEAs) in Main Belt-crossing orbits (NEACs). We select NEACs with H < 18 and integrate their orbits for 1e7 yr with N-body simulations. Objects are grouped by initial semi-major axis (G1: a < 2.06 au; G2: 2.06 < a < 2.5 au; G3: a > 2.5 au). We compute the fraction of each orbit spent within the main belt (MB), dynamical occupancy maps in the (a,e) plane, and median lifetimes. Using collisional evolution, we obtain size-dependent timescales, the change in the NEA size-frequency distribution (SFD) over 1 Myr, and impactor and crater SFDs on 150 m to 1 km targets, representative of NEAs visited by space missions. Median dynamical lifetimes decrease with increasing a: ~1.3e7 yr (G1), ~2.1e6 yr (G2), and ~0.9e6 yr (G3). NEACs in G2-G3 maintain nearly constant MB residence fractions with short intervals of full containment, while G1 exhibits stronger 0-0.8 oscillations (median ~0.55 for ~1e6 yr). DART-analog impacts occur on ~1e5 yr timescales for targets smaller than about 300 m (rising to ~1e6 yr for larger bodies), whereas catastrophic collisions are negligible within NEAC lifetimes. Over 1 Myr, collisional erosion reduces the meter-size NEA population by only 0.1-1.4% depending on Q_D*. Comparison with the observed crater SFDs on Bennu, Didymos, and Ryugu indicates target strengths of Y ~ 100 Pa for Bennu, young effective surface ages for Didymos, and short crater-retention times of order 1e4-1e5 yr for craters with diameters smaller than 100 m on Ryugu, consistent with rapid resurfacing. NEACs spend a substantial fraction of their lifetimes inside the MB and undergo frequent small-scale impacts, yet collisions weakly modify the global NEA SFD on Myr timescales. Our combined dynamical-collisional framework constrains NEAC lifetimes, orbital pathways, collisional timescales, and surface processing.

Figures (9)

• Figure 1: Distribution of semi-major axes $a$ and eccentricities $e$ of NEAs. The color bar indicates the fraction of time that NEAs spend within the MB relative to their orbital period. The curve $q=1.3$ au marks the boundary between NEAs and the MB (gray region), while NEACs correspond to the region to the right of the curve $Q=2.06$ au. The black dot indicates the $a$ and $e$ values of Didymos.
• Figure 2: Cumulative percentage of escaped NEACs as a function of time for Groups 1–3. Escapes are defined as reaching a heliocentric distance $>100$ au or colliding with the Sun or a planet. Solid lines show the evolution for G1 (blue), G2 (orange), and G3 (green), while vertical dashed lines and markers indicate the median lifetime.
• Figure 3: Dynamical occupancy maps in the $a$–$e$ plane for G1 (left panel), G2 (middle panel) and G3 (right panel). Colors indicate the fraction of total integration time spent in each of the 1000 grid cells. Solid black curves mark perihelion $q=1.3$ au and aphelion $Q=2.06$ au; dashed vertical lines denote the group boundaries at $a=2.06$ au and $a=2.5$ au.
• Figure 4: Evolution of the fraction of time spent in the MB relative to the orbital period, $\Delta P_\mathrm{MB}/P$, for each NEAC (red) and the median at each time step (black) for G1 (left panel), G2 (middle panel), and G3 (right panel).
• Figure 5: $Q_D^*$ as a function of target diameter $d_\mathrm{tgt}$ for rubble‐pile asteroids with different large‐boulder volume fractions raducan2024lessons. $Q_\mathrm{DART}$ corresponds to the 30% boulder‐volume case scaled to the DART impact energy.