Post-perihelion Sunward Tail Detected on Interstellar Comet 3I/ATLAS: A Geometric Projection of the Dust Tail

ATel #17551; M. Serra-Ricart (Light Bridges, IAC, ULL), J. Licandro (IAC, ULL), M. R. Alarcon (Light Bridges, IAC, ULL)
on 19 Dec 2025; 11:54 UT
Distributed as an Instant Email Notice Comets
Credential Certification: Miquel Serra-Ricart (mserra@iac.es)

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Interstellar comet 3I/ATLAS (C/2025 N1; hereafter 3I) has been extensively monitored since 2025 November 29 using the imaging capabilities of the Two-Meter Twin Telescope (TTT) and the Transient Survey Telescope (TST) both located at the Observatorio del Teide (IAC, Tenerife, Spain). We confirm (ATel #17490) the detection of a sunward-oriented tail-like feature in deep TST imaging of 3I. The structure appears as a narrow elongation extending sunward from the central condensation of the coma, with a projected length that increases as the Sun-observer-target angle increases (see linked figure). On 2025 December 19 (perigee), the projected length of the feature is approximately 450000 km. A detailed geometric analysis based on images obtained at different epochs (2025 November 29 and December 19), together with the absence of a fan-shaped structure in the Laplacian-filtered images, indicates that this feature is best explained as a geometric projection of the curved dust tail, rather than as a physically sunward-directed outflow. The measured position angles (PA) of the dust tail are 287.4 +/- 0.5 deg on November 29 and 285.4 +/- 0.5 deg on December 19 (red lines in linked figure). The observed change in the dust-tail PA, 2 +/- 1 deg, is consistent with the corresponding change in the projected velocity-vector PA (from 289.94 deg to 288.63 deg, a difference of 1.3 deg), supporting a geometric interpretation of the observed sunward feature. This feature is compatible with older, large grains confined near the orbital plane in a simple synchrone/syndyne analysis. A faint ion tail is also detected on the December 19 image (yellow line in linked figure). The measured PA angle of ion tail 293.8 +/- 0.5 deg is coincident with the anti-solar PA direction 292.38 deg (yellow arrows in linked figure). Projection-induced sunward tails are well known in solar system comets, but this detection demonstrates that interstellar comets can exhibit comparable dust-tail geometries when observed under favorable viewing conditions. Continued monitoring of 3I/ATLAS as the phase angle evolves will help to further constrain the dust dynamics and grain properties of this object. We encourage additional imaging and modeling efforts to better constrain the dust size distribution and emission history responsible for the observed morphology. The linked figure presents the unprocessed images (right panels) and the corresponding Laplacian-filtered images (left panels) of 3I. The observation date (YYYY-MM-DD), start and end times (UTC), number of sidereal-tracking exposures, and total integration time are indicated above each panel. The projected velocity vector (red arrow) and the antisolar direction (yellow arrow) are shown, together with the image scale and orientation. Red crosshairs mark the comet optocenter. The position angles of the dust and ion tails are indicated by thin red and yellow lines, respectively. Isophotal contours of the original unfiltered image are overplotted on the filtered frame using ten logarithmically spaced levels between the 80th and 95th percentiles of the pixel-intensity distribution. The TST, a 1 m telescope, is equipped with FERVOR-L, a 14304 x 10748 Sony IMX411 BSI sCMOS detector providing a pixel scale of 0.60 arcsec per pixel and a field of view of 2.4 x 1.8 degrees. For the study of fine coma and tail structures, observations were also obtained with TTT3, a 2 m f/6 Ritchey-Chretien telescope currently in its commissioning phase. TTT and TST are operated by Light Bridges in Tenerife, Canary Islands (Spain). The observation time rights (DTO) used for this research were consumed in the PEI -PLANETIX25-. This research used storage and computing capacity in ASTRO POCs EDGE computing center at Tenerife under the form of Indefeasible Computer Rights (ICR).

Laplacian-filtered and original images of 3I obtained on November 29 and December 19