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

Abstract

We present a detailed analysis of the trajectory of interstellar comet 3I/ATLAS, with emphasis on the determination of its non-gravitational acceleration (NGA) parameters and their associated uncertainties. Using astrometric observations spanning both pre- and post-perihelion phases, we compute orbital solutions assuming gravity-only, symmetric, delayed-peak, and asymmetric outgassing NGA models. To mitigate the effects of heterogeneous astrometric quality and potential tailward biases, we adopt a tiered uncertainty model that inflates the uncertainties of lower-fidelity observations, and assess solution robustness through validation against non-fitted data. We find clear evidence that NGA is required to reproduce the observations. Solutions obtained from pre- and post-perihelion arcs separately, however, yield mutually incompatible NGA parameters, suggesting either sensitivity to residual astrometric systematics or intrinsic limitations of constant parameter outgassing models. For the full data set, the inferred acceleration is in very good agreement with the latest JPL solution, supporting its use for physical interpretation and nucleus size estimates. Models incorporating delayed or asymmetric activity with respect to perihelion improve the fit relative to a symmetric prescription, suggesting that part of the observed asymmetry may be physical. We conclude that, while the NGA of 3I is robustly determined, systematic uncertainties arising from both residual astrometric biases and simplifications in the outgassing modeling prescriptions exceed the formal fitting errors, and should be taken into account when deriving physical properties such as the nucleus size.