Upper Limit on the Non-Gravitational Acceleration and Lower Limits on the Nucleus Mass and Diameter of 3I/ATLAS

Abstract: We use astrometric data on 3I/ATLAS compiled by the Minor Planet Center from May 15 to September 23, 2025, and derive an upper limit on any statistically significant deviation from the best-fit gravity-based trajectory. The residuals imply that the non-gravitational acceleration is smaller than $\sim 3 \times 10^{-10}\,{\rm au\,d^{-2}}$. Based on the total mass-loss rate and outflow speed inferred from JWST data on August 6, 2025, we derive lower limits on the mass and diameter of 3I/ATLAS of $3.3 \times 10^{16}\,{\rm g}$ and $5\,{\rm km}$, respectively.