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.

Figures (9)

• Figure 1: Orbit of 3I in the heliocentric ecliptic reference frame. Perihelion is marked by a yellow star, and the endpoints of the observational arc used for the orbit determination in this study are indicated by open circles. Gray shading denotes the region where $Z_\epsilon < 0$. The Sun and the orbits of the major planets through Jupiter are shown for reference.
• Figure 2: Normalized post-fit residuals of gravity-only orbital fit (model gr0), in right ascension (top panels) and declination (bottom panels). Data points discarded as outliers by the fitting procedure are shown as gray crosses. The dashed vertical line marks the perihelion.
• Figure 3: Same as Figure \ref{['fig:residuals_gr0']}, but for dynamical model ng1, including NGA according to equation \ref{['g_func']} with peak outgassing coinciding with perihelion, applied to the entire dataset.
• Figure 4: Same as Figures \ref{['fig:residuals_gr0']} and \ref{['fig:residuals_ng1']}, but for dynamical model ng2, including NGA according to equation \ref{['g_func']} with peak outgassing offset with respect to perihelion by free parameter $DT$, applied to the entire dataset.
• Figure 5: Sampled posterior distribution for parameters of model ng1. Sampled parameters vector comprises the components of the state vector in the heliocentric equatorial frame and the NGA parameters. The nominal values of the parameters from the orbital fit along with their $1$- and $2$-$\sigma$ contour lines are also plotted in the off-diagonal panels (blue squares and blue lines, respectively).