Assessing the Vera Rubin Observatory’s Ability to Discover Asteroid Impactors before They Collide with Earth

Abstract Asteroid impactors larger than ∼10 m, from Chelyabinsk-scale to >300 m continental threats, remain the dominant planetary-defense risk. While the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will transform solar system science, its observing cadence and survey design were not optimized to discover imminent impactors. To assess its performance, we introduce a new method for efficiently generating synthetic impactor populations by minimally perturbing sampled NEOMOD3 orbits and evaluate their discovery efficiency with the Sorcha survey simulator. Our simulations show that LSST discovers 79.7% of large impactors (>140 m), decreasing to 50.3% for upper midsized (50–140 m), 26.8% for lower midsized (20–50 m), and 10.5% for small objects (10–20 m). Warning times scale similarly with size: small objects are discovered weeks before impact (median: 12.4 days), lower midsized within a month (median: 21.5 days), and upper midsized within a few months (median: 106.2 days). 39.0% of large impactors are discovered over a year before impact, lacking long-lead warning despite their brightness. A loss-mode analysis reveals that small impactors are limited mainly by photometric sensitivity, whereas midsized and large objects are missed primarily due to cadence and linking constraints from LSST and its Solar System Processing Pipelines. These results show that LSST excels at discovering faint, small impactors, but cannot by itself guarantee long-lead warning across the hazardous size spectrum. Coordinated multisurvey strategies will therefore be essential in the LSST era for robust planetary defense, and we study a complementary high-cadence, shallow-depth example with the Argus Array.