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

TL;DR

This paper estimates the size of interstellar object 3I/ATLAS based on non-gravitational acceleration data, suggesting a nucleus around 1 km in diameter consistent with mass loss estimates.

Contribution

It provides a method to infer the size of interstellar objects from non-gravitational acceleration measurements, incorporating mass loss considerations.

Findings

Estimated diameter of 3I/ATLAS is approximately 820-1050 meters.

Non-gravitational acceleration is consistent with mass loss for a ~1 km nucleus.

Size estimates depend on outgassing asymmetry and density assumptions.

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 $\zeta=0.5$ and a density of the comet nucleus $\rho=0.5$ g cm$^{-3}$. The limits on the diameter scale as $(\zeta/\rho)^{1/3}$. 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.