The size of 3I/ATLAS from non-gravitational acceleration

Abstract: The third macroscopic interstellar object detected in the solar system recently passed through perihelion, with the best-fitting models of its trajectory now featuring non-gravitational accelerations. We assess how much mass loss is required to produce plausible non-gravitational acceleration solutions and compare with estimates of the mass loss. We find that they are consistent when the nucleus of 3I/ATLAS is around 1 km in diameter. For a recent solution with a time lag in the acceleration from Eubanks et al, we find diameters between 820 meters and 1050 meters, assuming an outgassing asymmetry factor $ζ=0.5$ and a density of the comet nucleus $ρ=0.5$ g cm$^{-3}$. The limits on the diameter scale as $(ζ/ρ)^{1/3}$. Substantial extrapolation is required in general to compare non-gravitational accelerations to mass loss rates, so reliable estimates of the mass loss rate at other stages of the comet's trajectory will substantially reduce the systematic uncertainty in this estimate.

• The paper constrains the nucleus diameter of interstellar object 3I/ATLAS (820–1050 m) by linking non-gravitational acceleration with observed mass loss.
• It applies a lagged NGA model to reconcile multi-instrument CO₂ production rates, highlighting a delayed outgassing response.
• Results emphasize the need for composite models integrating time-lag dynamics and multi-species volatile activity in comet characterization.

Size Constraints on Interstellar Object 3I/ATLAS via Non-gravitational Acceleration

Introduction

The recent perihelion passage of 3I/ATLAS, the third confirmed macroscopic interstellar object traversing the Solar System, presents a unique case for the study of cometary nuclei and non-gravitational dynamics. This investigation applies astrometric measurements of non-gravitational accelerations (NGAs) to constrain the nucleus size, bridging NGA-derived mass loss requirements with observational estimates of volatile production. By leveraging updated CO$_2$-driven NGA solutions and a comprehensive set of contemporaneous gas production rate data, the study reconciles dynamical and physical observations, yielding robust limits on the nucleus diameter.

Methodology and Data Sources

The astrometric trajectory of 3I/ATLAS deviates measurably from gravitational expectation, implying significant momentum exchange via asymmetric outgassing. NGAs, parameterized primarily by their heliocentric distance dependence and functional form (power-law and/or time-lag as in Eubanks et al., 2025), serve as proxies for the sublimation-driven mass loss process. Translation from NGA to nucleus mass (and therefore size) proceeds under the impulse approximation, formalized as:

$$M |\vec{a}_\mathrm{ng}| = \zeta \dot{M} v_\mathrm{th}$$

where $\zeta$ is the asymmetry factor and $v_\mathrm{th}$ a physically-motivated outflow velocity. Outgassing rate measurements from JWST, SPHEREx, ALMA, TRAPPIST-North, and Swift provide multi-species $\dot{M}$ constraints, with particular emphasis on contemporaneous CO$_2$ data, reflecting its dominance in perihelion activity. Nucleus density ($\rho$) and asymmetry factor are standardized to 0.5 g/cm$^3$ and 0.5, respectively, for baseline calculations.

Figure 1: NGA solutions (top), corresponding mass loss rates (middle), and CO$_2$ production constraints (bottom) for 3I/ATLAS, enabling cross-validation of dynamic and physical models.

Core Findings

Combining the lagged NGA solution of Eubanks et al. (2025) with the multi-instrument volatile production rates yields a self-consistent nucleus diameter between 820 m and 1050 m (scaling as $(\zeta/\rho)^{1/3}$). This result is anchored by:

• Upper bounds from the JWST CO$_2$ production rates, serving as robust limits due to the spectroscopic sensitivity and unambiguous tracing of the active volatile phase.
• Lower bounds from ALMA and TRAPPIST-North, which, despite limitations in CO$_2$ access, provide critical constraints on the minimum required $\dot{M}$.

These cross-linked constraints are only simultaneously satisfied by the Eubanks solution incorporating a time-delay term, implying that outgassing response to insolation and/or nucleus geometry must be non-instantaneous and that perihelion-observed activity features this temporal offset. The canonical NGA parameterization employed by the JPL Small-Body Database does not reproduce the observed $\dot{M}$ upper/lower limits concurrently, suggesting that time-lag and/or species composition variations are essential for proper dynamical modeling.

Discussion and Implications

The analysis exposes several critical aspects of interstellar comet characterization:

• Systematic dependence on physical parameters: The strongest uncertainty arises from the choice of $\zeta$ and $\rho$. Deviations from assumed values—such as through enhanced jet collimation or anomalous bulk density—scale the inferred diameter accordingly. Observational signatures of jet structure (e.g., via KCWI and direct imaging) and symmetry (e.g., JWST) motivate further refinement of $\zeta$.
• Dynamical-physical model tension: The inability of simple NGA models to satisfy all observational constraints underscores the necessity for temporally and compositionally resolved outgassing physics. Multi-species and evolving-volatility models, as supported by ALMA and JWST's distinct molecular detections, must be incorporated.
• Astrometric time-lag as a diagnostic: The apparent requirement for a lag in acceleration response highlights either delayed mass loss (potentially due to thermal inertia or changes in jet/active area) or post-perihelion activation asymmetry. This signature may serve as a diagnostic for nucleus structure and rotational state.
• Context with previous interstellar interlopers: Compared to 1I/‘Oumuamua and 2I/Borisov, 3I/ATLAS demonstrates an intermediate-sized nucleus with a CO$_2$-rich activity regime. The reconciliation of dynamical and physical mass loss metrics in this case strengthens the case for interstellar comets displaying diversity between Oort Cloud analogs and Solar System standards.

Future Prospects

Refinement of mass loss rates at epochs distinct from perihelion is essential to constrain pre- and post-perihelion evolution. Future prospective surveys (e.g., LSST/Rubin, further JWST epochs) with dedicated molecular line sensitivity will be key. Theoretical work must generalize NGA models to incorporate both temporal response and multi-species composition under varying thermophysical regimes. As the population of observed interstellar comets grows, comparative NGA/mass loss analysis will clarify the degree of heterogeneity in nucleus structure and volatile inventory among extrasolar planetesimals.

Conclusion

The synthesis of non-gravitational acceleration analysis with gas production observational data robustly constrains the nucleus diameter of interstellar comet 3I/ATLAS, yielding 820–1050 m under standard physical parameter assumptions. The requirement for time-lagged NGA reflects the complexity of real comet activity and the importance of comprehensive, multi-wavelength monitoring. These results further bridge dynamical and physical perspectives on cometary nuclei and establish methodologies essential for future interstellar object characterization.

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Reference: "The size of 3I/ATLAS from non-gravitational acceleration" (2512.18341)