Systematic and Statistical Uncertainties in the Non-Gravitational Acceleration of 3I/ATLAS

Abstract

We present a detailed analysis of the trajectory of the interstellar comet 3I/ATLAS, focusing on its non-gravitational acceleration (NGA) parameters and their uncertainties. Orbital solutions are computed with models that implement symmetric, time-offset, and asymmetric radial dependence of the outgassing law relative to perihelion. We assess solution robustness through multiple data-selection strategies and comparison with independent determinations. The radial and normal NGA components (A_1 and A_3) are broadly consistent across all configurations, whereas the transverse component (A_2) is more sensitive to data selection, parameter correlations, and orbital phase coverage. Models with asymmetric radial decay slopes marginally improve the fit, but they also introduce additional degeneracies, contributing to systematic uncertainties. The magnitude of the NGA scaled to 1 au constrains the nucleus size of 3I. While our total acceleration estimates agree well with that of JPL's Small-Body Database solution, inclusion of systematic modeling effects implies a significantly larger uncertainty in the inferred radius of 3I.