Non-Gravitational Acceleration in 3I ATLAS: Constraints on Exotic Volatile Outgassing in Interstellar Comets

TL;DR

This study investigates whether the non-gravitational acceleration observed for the interstellar comet 3I/ATLAS can be explained by ordinary volatile-driven outgassing. Using a self-consistent thermophysical model with diurnal-averaged energy balance and empirical vapor-pressure relations, coupled to Monte Carlo jet-configuration sampling, the authors test CO, CO$_2$, NH$_3$, and CH$_4$ as drivers. They find CO-dominated sublimation can reproduce the measured acceleration for nucleus radii between 0.5 and 3 km with plausible active-area fractions; CO$_2$ remains radiatively dominated and ineffective, NH$_3$ and CH$_4$ underproduce unless near the Sun, and mixed CO–CO$_2$ solutions provide a formal lower bound but require larger, directionally consistent active areas. Overall, the results show that 3I/ATLAS’s acceleration is consistent with standard volatile-driven outgassing and does not require exotic physics, while outlining the thermophysical conditions under which interstellar comets can exhibit measurable deviations from purely gravitational motion. The work informs expectations for future interstellar visitors and provides a framework to interpret potential activity in similar objects.

Abstract

The interstellar comet 3I/ATLAS displayed a small but statistically significant non-gravitational acceleration during its passage through the inner Solar System. Using a thermophysical model coupled with stochastic sampling of jet configurations, we investigate whether standard volatile-driven activity can account for the observed acceleration. The model includes diurnal and obliquity-averaged energy balance, empirical vapour-pressure relations, and collimated outflow from localized active areas. We find that CO-dominated activity can reproduce the magnitude of the acceleration inferred from the Marsden non-gravitational parameters for nucleus radii between 0.5 and 3 km with active-area fractions that are substantial but thermodynamically plausible. Less volatile species, including NH_3 and CH_4, contribute less efficiently and cannot provide the required recoil when acting alone, while CO_2 remains radiatively dominated and dynamically ineffective over the heliocentric-distance range relevant to the observations. These results show that the measured acceleration of 3I/ATLAS is consistent with ordinary CO-driven outgassing and does not require unusual physical properties. The analysis delineates the thermophysical conditions under which interstellar comets can exhibit measurable deviations from purely gravitational motion.

Figures (3)

• Figure 1: Diurnal-averaged non-gravitational acceleration as a function of heliocentric distance for CO, CO$_2$, NH$_3$, and CH$_4$. Accelerations are computed from the full thermophysical energy-balance model using laboratory vapor-pressure relations, a diurnal-average illumination geometry, and a jet-collimation factor $\eta = 2.5$. The example shown adopts a nucleus radius $R_{\mathrm{nuc}} = 5.6$ km, bulk density $\rho = 500$ kg m$^{-3}$, and total active fraction $f_{\mathrm{act}} = 0.02$. The curves illustrate the relative efficiency of different volatile species in producing recoil acceleration at heliocentric distances relevant to the 3I/ATLAS observations.
• Figure 2: Required total active fraction as a function of nucleus radius for single-volatile and mixed-volatile sublimation models. For each radius, the scalar non-gravitational acceleration produced by the model is matched to the observed magnitude at $r_H = 1.36$ AU. Single-species curves (CO, CO$_2$, NH$_3$) illustrate the increase in required active area with nucleus size due to the mass-to-area scaling of recoil acceleration. The CO--CO$_2$ (50/50) curve represents a strict lower bound obtained by scalar combination of species contributions and does not constitute a physically unique solution; any directionally consistent model would require equal or larger active fractions.
• Figure 3: Latent-to-radiative power ratio $\Lambda = (Z L) / (\varepsilon \sigma T^4)$ as a function of heliocentric distance for CO, CO$_2$, NH$_3$, and CH$_4$, computed using diurnal-averaged illumination and the full thermophysical energy-balance model. Values $\Lambda \gg 1$ correspond to sublimation-dominated surface energy balance, while $\Lambda \ll 1$ indicates radiative dominance. CO and CH$_4$ remain sublimation-dominated across $0.4$--$3$ AU, whereas CO$_2$ is radiatively dominated throughout, limiting its ability to generate significant recoil acceleration.