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Nucleus shape of 3I/ATLAS is unconstrained by existing photometry

Determine whether the nucleus of 3I/ATLAS is highly aspherical, akin to 1I/’Oumuamua, or more nearly round like typical small solar system bodies, given that current coma-dominated photometry does not constrain the nucleus shape or rotation.

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Background

Because the optical signal is dominated by coma, rotational lightcurve signatures from an aspherical nucleus are suppressed on timescales shorter than the coma residence time. Reports of modest periodic variation contrast with flat lightcurves, reinforcing that the available data are insufficient to infer nucleus properties.

Consequently, the authors state that the nucleus’ shape and rotation cannot be assessed at present, leaving open whether 3I/ATLAS resembles the extreme elongation inferred for 1I/’Oumuamua or the more typical roundish shapes of small solar system bodies.

References

For the moment, we must conclude that existing photometry of 3I provides no sufficient basis on which to assess either the shape or the rotation of the nucleus of 3I. Significantly, we cannot know from the existing data if its shape is highly aspherical, like that of 1I/'Oumuamua, or more nearly round, like typical small solar system bodies.

— Hubble Space Telescope Observations of the Interstellar Interloper 3I/ATLAS (2508.02934 - Jewitt et al., 4 Aug 2025) in Discussion