The Detectability of Lunar-Origin Asteroids in the LSST Era

While most near-Earth asteroids (NEAs) are thought to originate from the main belt, recent discoveries have suggested the existence of a lunar-derived NEA population, such as the asteroids Kamo’oalewa and 2024 PT5. These objects may hold key clues to the dynamical evolution of NEAs and the recent impact history of the Earth-Moon system. However, the population, distribution, and dynamical characteristics of these Lunar-Origin Asteroids (LOAs) remain poorly constrained. By combining the lunar ejecta production with N-body orbital simulations of the ejecta, we investigate their orbital evolution in the past millions of years and the current LOA population, revealing their significant potential for detection by future surveys. Specifically for the Vera C. Rubin Observatory’s upcoming Legacy Survey of Space and Time (LSST), we predict an average detection rate of about 6 LOAs (with D > 5 m) per year. Additionally, we find that the LOAs tend to approach from sunward and anti-sunward directions, with encounter velocities significantly lower than those of typical NEAs. These findings offer valuable insights in guiding targeted ground-based surveys and planetary defense efforts for LOAs in the future.

💡 Research Summary

The manuscript “The Detectability of Lunar‑Origin Asteroids in the LSST Era” presents a comprehensive quantitative assessment of a previously speculative population of near‑Earth objects that originate from lunar impact ejecta, termed Lunar‑Origin Asteroids (LOAs). The authors combine a lunar cratering‑frequency model with results from high‑resolution smoothed‑particle‑hydrodynamics (SPH) simulations to estimate the size‑frequency and velocity distributions of ejecta that exceed the lunar escape velocity (2.38 km s⁻¹). Two cratering scenarios are explored: a “background” model based on the long‑term average impact flux, and a “real” model that explicitly includes the rare but recent Giordano Bruno 22‑km crater, thereby capturing the upper tail of the impact‑size distribution.

Using these production functions, the authors generate a synthetic ensemble of 20 000 massless test particles (5 m – 100 m in diameter) launched from random lunar surface locations with a fixed 45° launch angle and velocities drawn from a power‑law distribution (α = −4, v_min = 2.38 km s⁻¹, v_max = 6 km s⁻¹). The dynamical evolution of the particles is simulated with the open‑source N‑body integrator REBOUND. Each of the 200 independent simulations follows the particles for 100 years under the combined gravity of the Sun, eight planets, and the Moon, after which the Earth–Moon system is replaced by its barycenter and the particles are integrated for up to 100 Myr. The Yarkovsky effect is incorporated via a size‑ and heliocentric‑distance‑dependent acceleration, yielding semi‑major‑axis drift rates up to ~10⁻² AU Myr⁻¹ for 5‑m objects, consistent with previous estimates for similar bodies (e.g., 2016 HO3). Solar radiation pressure is neglected because it is an order of magnitude weaker for the size range considered.

The dynamical results reveal a rapid attrition of the LOA population. The survival fraction drops from 74 % at 0.1 Myr to 61 % at 1 Myr, 42 % at 10 Myr, and only 1.6 % after 100 Myr. Approximately 25 % of the ejecta impact Earth within the first 0.1 Myr, confirming earlier work that lunar ejecta are most likely to strike Earth shortly after launch. Over longer timescales, the Yarkovsky effect drives many particles outward: by 10–100 Myr, 23 % have escaped beyond 6 AU, 14 % fall into the Sun, and 2.4 % collide with Mercury. The authors therefore argue that the long‑term fate of lunar ejecta is strongly influenced by non‑gravitational forces, a factor omitted in some earlier studies.

Integrating the production rates with the age‑dependent survival fractions yields an estimate of the present‑day LOA inventory. The “background” cratering model predicts roughly 2.6 × 10⁵ LOAs larger than 5 m, while the “real” model (including the Giordano Bruno event) suggests up to 5 × 10⁵ objects. This corresponds to less than 1 % of the total NEA population of comparable size (≈10⁸ objects). The authors emphasize that, despite their rarity, LOAs are dynamically distinct: they preferentially approach Earth from sunward and anti‑sunward directions and possess encounter velocities significantly lower than the typical NEA (~5 km s⁻¹ versus ~20 km s⁻¹). Such low velocities reduce impact energy and make LOAs attractive targets for planetary‑defense demonstrations (e.g., kinetic impactors).

The paper then evaluates the detectability of LOAs by the forthcoming Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST). Using the standard LSST single‑visit depth (V ≈ 24.5 mag) and a geometric albedo of 0.20 (consistent with lunar silicates), the authors compute apparent magnitudes for each simulated flyby. They define an Earth flyby as any passage within 0.05 AU (the typical MOID threshold for potentially hazardous asteroids). The simulated flyby frequency peaks for young ejecta (≈0.12 flybys per particle for 0.1 Myr‑old ejecta) and declines sharply for older populations. Summing over the entire synthetic population, the model predicts an average of about six detectable LOAs per year with LSST, most of which will be observed during brief windows (weeks to months) before either escaping, colliding with Earth, or becoming too faint.

The authors discuss the scientific and practical implications of these findings. LOAs provide a unique probe of recent lunar impact history, as their spectral signatures (lunar‑like silicate reflectance) can be directly compared to lunar samples. Their low encounter speeds also make them valuable test cases for mitigation techniques, where a lower kinetic energy impact can be studied with reduced risk. Moreover, the identification of temporary captures (mini‑moons) such as 2024 PT5 in the simulations demonstrates that LSST could discover additional short‑lived Earth‑bound objects, enriching the catalog of transient near‑Earth companions.

In conclusion, the study delivers the first robust, physics‑based estimate of the LOA population, quantifies their dynamical lifetimes, and demonstrates that LSST will be capable of detecting a modest but scientifically rich sample (≈ 6 yr⁻¹). These results lay the groundwork for targeted follow‑up observations, spectroscopic confirmation of lunar provenance, and the integration of LOAs into planetary‑defense planning. Future work is suggested to refine the lunar impact flux, incorporate rotational dynamics of ejecta, and validate the predictions against early LSST data releases.