The Detectability of Lunar-Origin Asteroids in the LSST Era

TL;DR

This study addresses the detectability and dynamical evolution of Lunar-Origin Asteroids (LOAs) formed from lunar ejecta. By coupling a cratering-based ejecta production model with extensive N-body simulations that include the Yarkovsky effect, the authors predict a present-day LOA population of roughly $2.6\times10^5$ to $5\times10^5$ objects with $D>5$ m and estimate annual Earth flyby rates of about $40$–$76$ events, depending on model assumptions. Their LSST-focused analysis predicts roughly $3$–$6$ LOAs per year detectable during Earth flybys, with a strong bias toward older LOAs and sunward approach geometries; LOAs also exhibit lower encounter velocities ($v_{\infty}$ median $\approx$ 12 km s$^{-1}$) than typical NEAs, and a notable probability of origin from lunar ejecta when $v_{\infty}<2.4$ km s$^{-1}$ is observed. These results provide concrete guidance for survey strategies and planetary-defense planning in the LSST era, establishing LOAs as a quantitatively significant, albeit minority, component of near-Earth space that can be probed with ground-based optical surveys.

Abstract

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.

Figures (5)

• Figure 1: Cumulative size distribution of lunar craters.The black curves illustrate the cumulative size distribution of lunar craters over the past 10 Myr and 100 Myr, calculated from the background impactor flux. The x-axis scale of the red vertical lines indicates the minimum crater diameter necessary to launch lunar escape ejecta of certain sizes. For instance, producing a Kamo‘oalewa-sized (36 m) fragment requires a crater of at least $\sim$14.4 km in diameter.
• Figure 2: Dynamical fates and flyby frequency of globally sourced lunar ejecta. (a) illustrates the temporal evolution of 20,000 test particles, which simulate lunar ejecta launched from the entire lunar surface. Presented on a log-log scale, the colored curves show the fraction of these particles in various dynamical states: survival, impact (with specific planets or the Sun), reaching an aphelion of $Q > 6$ AU, and others. The final fraction for each state at 100 Myr is annotated on the right side of the plot. For comparison, the survival fractions reported by jiao2024asteroid and gladman1995dynamical are also plotted. (b) shows the present-day LOA flyby frequency. The bars (left y-axis) present $\lambda(t)$, the average flyby frequency per initial ejecta for LOAs of that age; the curve (right y-axis) is the production‑rate‑weighted cumulative number of annual LOA flybys up to that age, equal to $N_\text{flyby}$ at 100 Myr.
• Figure 3: Illustrative distinct modes of LOA Earth flybys. The trajectories of LOAs passing within 0.05 AU of Earth are projected onto the ecliptic (x-y) plane in the geocentric Sun– Earth co-rotating frame. An arrow at the end of each trajectory indicates the particle’s direction of motion; the positive y-axis points toward Earth's leading side, and the red arrow points toward the Sun. (a) shows two low-energy capture modes: the orange trajectory represents a 'minimoon' state (e.g., 2024 PT5), characterized by a close passage ($<$ 0.01 AU) and negative geocentric Keplerian energy. The green trajectory depicts a close flyby, where the object slows to a near-zero velocity with a significant change in direction. (b) illustrates a quasi-satellite state (e.g., Kamo‘oalewa), where the object closely co-orbits Earth for decades.
• Figure 4: Detection efficiency distribution for LOA flybys. (a--c) show heatmaps of the annual number of LOA detections by LSST (a), Pan-STARRS (b) and ATLAS (c),binned by asteroid age and the observable duration. We apply survey-specific detection limits: LSST ($V_\text{trail}<24$, $\omega<10~\text{deg day}^{-1}$), Pan-STARRS ($V_\text{trail}<22$, $\omega<5~\text{deg day}^{-1}$), and ATLAS ($V_\text{trail}<20$, $\omega<10~\text{deg day}^{-1}$). Above each heatmap is the cumulative age-distribution curve, equal to $N_\text{det}$ at 100 Myr. (d) shows the relative frequency distribution of geocentric ecliptic longitude for LOA flybys at 0.05 AU from Earth, where 0$^{\circ}$ marks the anti-solar direction and 90$^{\circ}$ the direction of Earth's motion. The red and orange arcs indicate the detectable range ($V<24$) for asteroids with diameters of $D=10~\text{m}$ and $D=5~\text{m}$, respectively.
• Figure 5: Distributions of encounter velocity. Here we present the relative frequency distributions of LOAs' and NEA's velocity at infinity ($v_{\infty}$, with a dedicated bin for LOAs with a negative geocentric Keplerian energy.