Periodic Wobble of the Post-Perihelion Jet Structure Around 3I/ATLAS

Abstract: We analyse data on the post-perihelion morphology, including jet position angles (PAs) and coma dominated photometry of the interstellar object 3I/ATLAS. From Hubble Space Telescope (HST) images processed with a Larson Sekanina rotational gradient filter, we measure the PAs of three main persistent jet like features between November 30 and December 27, 2025 and fit a weighted Fourier model in a period scan. The dominant jet PA wobble yields Pjet = 7.20 +/- 0.05 h. An independent Gr (R band) time-series photometry data set, using two different apertures from MPC station L92, analyzed with nightly offsets and 30 minute binning, gives Pphot = 7.136 +/- 0.001 h (formal 1sigma), with a semiamplitude A about 0.311 mag and scatter sigmajit about 0.089 mag. The close agreement beetwen the periods supports a characteristic post-perihelion period of about 7.1 h. We interpret this period as an attitude precession/nutation (non principal axis rotation) traced by jet orientation and coma flux redistribution. The jet structure precesses about the rotation axis with a characteristic angular excursion of order about 20 degs, and the rotation axis is aligned with the sunward direction to within about 20 degs.