Predictions of Imminent Earth Impactors Discovered by LSST

Abstract

Imminent impactors are natural bodies discovered in space before impacting the Earth. They provide a rare opportunity to characterize individual near-Earth objects (NEOs) in great detail as asteroids in space, meteors in Earth's atmosphere and meteorites on the ground. The Vera C. Rubin Observatory's upcoming Legacy Survey of Space and Time (LSST) is expected to transform our understanding of the NEO population. In this work, we evaluate LSST's expected discovery performance for imminent impactors using $343$ meter-size objects previously recorded in NASA's CNEOS database as fireballs impacting Earth's atmosphere. We simulate pre-impact observations of these CNEOS impactors with the Sorcha survey simulator under LSST's default three-night discovery strategy and a one-night strategy for fast-moving objects that relies on matching aligned streaks in two exposures on the same night. We estimate that LSST will discover $\sim1-2$ meter-size and larger imminent impactors per year, representing $\sim4\%$ of all Earth impactors $\gtrsim1$ m in diameter and almost doubling the current discovery rate of imminent impactors. The median time of discovery and median time of first observation for impactors discovered in our simulations are $\sim1.57$ and $\sim3.06$ days before impact, respectively. The spatial distribution of the 11 previously discovered imminent impactors is biased towards the Northern Hemisphere, where the observatories that discovered them are located. We find a similar trend towards Southern Hemisphere impacts in our simulated LSST detections of the CNEOS impactors, suggesting Rubin will provide a powerful counterpart to existing asteroid surveys primarily located in the Northern Hemisphere.

Figures (8)

• Figure 1: An equal-area Mollweide projection map showing the impact locations (red dots) of all $343$ Earth impactors with known state vectors and impact energies recorded in NASA's Center for Near-Earth Object Studies (CNEOS) Fireball and Bolide Database from 1994 February 1 to 2026 January 1. The estimated diameter of each object (ranging from $\sim0.75-24$ m) is indicated by the relative size of its dot. The spatial distribution of impacts over Earth's surface appears generally uniform.
• Figure 2: The apparent LSST $r$-band magnitude of $343$ CNEOS-recorded Earth impactors (black solid lines) at one-hour intervals during the $14$ days prior to impact. LSST's approximate single-image $r$-band depth of $m_r \sim 24.0$ is indicated by the red solid line. Most impactors would remain too faint to be detectable by LSST on their final approach until a few days before impact. However, nearly all of them ultimately cross the fiducial magnitude threshold, bringing them into the range of LSST visibility (see Figure \ref{['fig:cumulative_visible']}).
• Figure 3: The cumulative fraction of CNEOS-recorded impactors that would become detectable by LSST ($r$-band apparent magnitude $m_r < 24.0$) as a function of time before impact, for nominal orbits (orange line) and when generating $1000$ clones per object (blue line). The black lines indicate the time before impact at which $10\%$ and $50\%$ of nominal CNEOS impactors become visible to LSST.
• Figure 4: All $18$ CNEOS impactors observed by LSST in our Sorcha simulations, including $14$ objects successfully linked and discovered before impact and $4$ that only have precovery observations (were not linked and discovered before impact).
• Figure 5: The distribution of on-sky angular motions for all observations of the CNEOS impactors in our Sorcha simulations. The angular motions of observed objects are quite small; most move less than $\sim2^\circ$/day, with the largest on-sky motion still less than $10^\circ$. This suggests that LSST observations of imminent impactors are unlikely to be affected by the maximum $10^\circ$/day motion threshold for reporting.