Disentangling geometric contributions to the observed opposition brightening of 3I/ATLAS

Determine, using dust-dynamical and radiative modeling tailored to the HST observing geometry, the respective contributions of orbital-plane crossing and dust-tail projection to the near-opposition brightening observed in the coma of interstellar object 3I/ATLAS, so that the geometric effects can be separated from the intrinsic dust-scattering phase behavior inferred from the postperihelion photometry.

Background

Analysis of postperihelion HST imaging of 3I/ATLAS revealed a statistically significant opposition surge (~0.2 mag with an e-folding width of ~3 degrees) in the dust-scattering phase function. The apparent brightening near opposition was modeled with a linear-exponential phase function.

The authors note that, in addition to any intrinsic backscattering properties of the dust, the observed enhancement may have been significantly influenced by two geometric effects: (i) crossing of the orbital plane and (ii) projection of the dust tail, both of which increase the effective dust cross-section in fixed-aperture photometry. These effects were not corrected for in the photometric model, and the authors explicitly defer separating these contributions to future dust-modeling work.

References

We leave the disentanglement of these competing factors to future dust-modelling work.

— Nucleus and Postperihelion Activity of Interstellar Object 3I/ATLAS Observed by Hubble Space Telescope  (2601.21569 - Hui et al., 29 Jan 2026) in Section 4.3 (Dust Activity and Scattering Phase Function)