Jets in the inner coma of comet 3I/ATLAS
ATel #17555; Federico Manzini (Stazione Astronomica di Sozzago), Virginio Oldani (Stazione Astronomica di Sozzago), Paolo Ochner (UniPd; INAF-OAPd), Luigi R. Bedin (INAF-OAPd), Andrea Reguitti (INAF-OAPd), Alessandra Mura (UniPd; INAF-OAPd), Andrea Farina (UniPd)
on 23 Dec 2025; 10:37 UT
Credential Certification: Andrea Reguitti (andreareguitti@gmail.com)
Interstellar comet C/2025 N1-3I, after passing perihelion on October 29.5, reached its minimum distance
from Earth on December 19, 2025. Near perigee, the comet was observed by HST on November 30 (P.I.
Jewitt, prop. ID 17830), with a resolution of 56 km/pixel. We analyzed the images taken by the WFC3
camera on HST, with the F350LP broadband filter and peak transmittance at 585.1 nm.
Throughout 2025, the angle between the comet's orbital plane and that of the Earth has remained
between -1 deg and +2 deg, which, together with a peculiar geometric configuration between the Sun,
Earth and comet, could lead to the observation of an anti-tail. The median stack of the HST images has
indeed confirmed the presence of an anti-tail (PA 113 deg) and a tail (PA 295 deg) respectively in solar and
antisolar directions, almost exactly aligned with the direction vector of the Sun, as already reported by
Serra-Ricart et al. (ATel #17551).
The analysis of the inner coma best highlighted its morphology with the application of a 1/r and a
Larson-Sekanina filter (alpha = 15 deg), revealing the presence of six faint, straight jets propagating outward
from the nucleus, losing collimation after a few thousand kilometers due to the diffusion of dust in the inner
coma. The jets apparently form three pairs symmetrical with respect to a direction assumed to be that of
the nucleus' spin axis, with each pair representing the sides of the emission cone of its active area located
on the rotating nucleus.
Measurements at 2000 km and 3000 km from the optocenter, assumed to be the nucleus, allowed the
identification of the symmetric jet pairs at PA 62/150 deg, PA 36/182 deg, PA 25/205 deg. Three bisectors
were calculated at approximately PA 106 deg, 109 deg, and 115 deg, respectively. The associated
uncertainty for each value (±2.5 deg) includes the propagation of experimental errors (orientation of the
original images, position measurement and jet identification). The weighted average of these directions
more accurately places the projected spin axis direction along the line: PA 110-290 deg, with an estimated
uncertainty of ±2.5 deg on each direction. This result confirms the initial estimates by Oldani et al. (ATel
#17350) and implies that the recent cometary activity originates from a hemisphere of the nucleus
opposite to the one active during the July HST observations by Jewitt et al. (2025, ApJL, 990 L2).
This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from
the Space Telescope Science Institute, which is operated by the Association of Universities for Research in
Astronomy, Inc.
Figures and captions are available at:
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