Semisecular apsidal resonances within binary asteroid dynamical evolution

Abstract Small binary asteroid systems with primaries under 10 km undergo complex secular evolution driven by multiple perturbations. The YORP effect likely forms satellites around small rubble piles, while the binary YORP (BYORP) effect can expand or contract a near-synchronous binary’s orbit and induce satellite tumbling. Mutual body tides dissipate energy, damping satellite libration and gradually expanding the orbit. Together, BYORP and tides shape binary asteroid dynamical end-states. A less-explored factor is solar tides, which alter mutual orbit precession rates and can induce semisecular resonances between the binary’s orbit and the heliocentric mean motion. We present a novel method to identify apsidal semisecular resonances in non-Keplerian systems, applied to near-Earth binary asteroid (66391) 1999 KW4. We find that binary asteroids are not limited to the 1:1 evection resonance. Higher-order resonances can occur when the mutual orbit precesses N times over one or two heliocentric orbits. We map the locations of these apsidal resonances as a function of heliocentric semimajor axis and satellite elongation. We observe that strong resonances can trigger tumbling during orbital expansion, while contraction may trap systems in resonance near tidal-BYORP equilibrium. Additionally, resonance excitations can be avoided by satellite libration. Both prograde and retrograde systems on eccentric heliocentric orbits can experience apsidal resonance effects. Lastly, our analysis suggests 1999 KW4 could enter a tumbling state within the next several hundred years if tidal dissipation is weak. This work highlights the critical role of solar-induced semisecular resonances in shaping binary asteroid dynamics, complementing BYORP and tidal evolution models.