Dynamics of Lunar Impact Ejecta: Escape, Co-Orbitals, and Impacts

Abstract

Impact events on the Moon can generate high-velocity ejecta capable of escaping the lunar gravitational field and reaching Earth and interplanetary space. This process plays a significant role in material exchange within and beyond the Earth--Moon system, with implications for planetary evolution, meteoroid fluxes, and the provenance of some near-Earth objects (NEOs). This dissertation investigates the comprehensive dynamics and fate of lunar impact ejecta through complementary analytical and numerical approaches to assess the contribution of lunar ejecta to Earth impacts and the NEO population. The fate of impact-generated lunar ejecta was examined in previous studies but has garnered renewed attention following the discovery of NEOs exhibiting lunar-like spectral characteristics. Notable examples include Kamo'oalewa (2016 HO3), whose lunar origin is supported by its Earth-like orbit and reflectance spectrum consistent with lunar silicates, and the recently discovered 2024 PT5, which also shows spectral properties resembling lunar samples. These discoveries provide compelling evidence that lunar ejecta may contribute to the NEO population and persist in Earth's co-orbital space for extended periods. We present simplified analytical estimates based on patched-conic approximations to assess whether impact ejecta launched from satellite surfaces can escape the gravitational influence of the planet--satellite system and enter heliocentric orbit. Despite its simplicity, this approach provides clear physical insights into the fundamental dynamics governing satellite ejecta escape. We derive straightforward thresholds for escape in terms of just two dimensionless parameters: the satellite-to-planet mass ratio and the ratio of the satellite's orbital speed to its escape speed. This framework reveals that the Earth--Moon system occupies a unique dynamical situation among Solar System planet--satellite pairs, where ejecta launched at barely above lunar escape speed can reach heliocentric space within a narrow range of launch speeds---a property not found in other planetary satellite systems. The simplified analysis demonstrates that the Moon's long--term migration has not significantly altered its propensity to produce escaping ejecta, reinforcing the plausibility of a lunar origin for some near-Earth asteroids during historical heavy bombardment epochs, and also highlighting the distinctive characteristics of lunar ejecta dynamics. Using modern numerical methods, we employ the high-accuracy IAS15 integrator within the REBOUND N-body package to track thousands of test particles launched from various lunar latitudes and longitudes over extended timescales. The simulations incorporate a realistic power-law velocity distribution derived from impact simulations, representing a major advancement over earlier studies that assumed uniform velocity distributions. The numerical integrations include gravitational influences from the Sun, all planets, and the Moon, providing high-fidelity calculations of ejecta trajectories and outcomes. The results reveal that lunar ejecta constitute a previously unrecognized source of Earth impactors, with most impacts occurring within the first few thousand years following impact ejection. We show that these escaping lunar fragments present patterns that differ from the general NEO population and that a small fraction of particles escape the Earth--Moon system entirely, contributing to the interplanetary NEO population.We demonstrate that lunar impact ejecta can, in rare cases, enter long-lived co-orbital configurations with Earth, including transitions between quasi-satellite and horseshoe orbital states consistent with the observed evolution of objects like Kamo'oalewa. These outcomes appear across a wide range of initial orbital configurations, identifying viable pathways by which lunar ejecta can join Earth's co-orbital NEO population---outcomes that were likely missed in earlier computational studies due to technical limitations. The findings have significant implications for interpreting the origin of certain small NEOs, understanding the dynamics of objects in the cis-lunar space -- the region of space on the Earth side of the Moon’s orbit -- and the exchange of material across the Solar System.