Follow-Up Imaging Observations of the Interstellar Comet 3I/ATLAS

Left: Comet 3I/ATLAS, imaged (20x DIT=120s) on 22 November 2025, with the STK in the R-band. The direction to the Sun and the comet's heliocentric velocity are indicated by a yellow and a blue arrow, respectively. The plasma tail points towards the northwest and a prominent anti-tail extends in the southeast direction.

Right: The brightness of the coma of comet 3I/ATLAS in the R-band during our observation campaign. The comet's coma faded continuously by 3.1 ± 0.14 magnitudes between 22 October 2025 and 24 January 2026.

........................*** Orbital Elements of 3I/ATLAS ***

..............	Epoch	17.0 Dec 2025
	T	29.488 ± 0.003 Oct 2025
	q	1.3568 ± 0.0002 au
	ω	128.017 ± 0.006 °
	Ω	322.157 ± 0.002 °
	e	6.143 ± 0.002
	i	175.1139 ± 0.0003 °

based on 139 observations, taken between 21 Nov 2025 and 24 Jan 2026 RMS = 0.31 arcsec

Left: The derived absolute brightness of the coma of comet 3I/ATLAS, assuming pure reflection of sunlight. While the absolute brightness of the coma was 10.15 ± 0.09 mag during the first two observation epochs (21 and 22 October 2025, r~1.6 au), it was 11.15 ± 0.07 mag on average, during the rest of our observation campaign (13 December 2025 to 24 January 2026), showing no significant variation throughout this range of time, when the comet's distance from the Sun increased from 2.1 to 3.4 au.

Right: The orbital elements of comet 3I/ATLAS, derived from its STK astrometry using all planets, the Moon, as well as Pluto as gravitational perturbers.

M. Mugrauer 2026 - All imaging data are available upon request.