2,3,2,4,3,4,2,1,4,5,6,7,2,- 1INAF - OAR (Osservatorio Astronomico di Roma), Via Frascati 33, 00040 Monte Porzio Catone (Roma), Italy (elena.mazzottaepifani@inaf.it)
- 2INAF – IAPS (Istituto di Astrofisica e Planetologia Spaziali), Via Fosso del Cavaliere 100, 00133 Roma, Italy
- 3IAA-CSIC (Instituto de Astrofísica de Andalucía - CSIC), Glorieta de la Astronomía 3, 18008 Granada, Spain
- 4INAF - OAPd (Osservatorio Astronomico di Padova), Vicolo Osservatorio 5, 35122 Padova, Italy
- 5Escuela de Ingeniería de Bilbao, Univ. del País Vasco, UPV/EHU, Plaza Ingeniero Torres Quevedo 1, 48013 Bilbao, Spain
- 6School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
- 7German Aerospace Center (DLR), Institute of Planetary Research, Rutherfordstr. 2, 12489 Berlin, Germany.
- *A full list of authors appears at the end of the abstract
In July 2025 the third known member of the Interstellar Objects (ISOs) class, 3I/ATLAS, was discovered on its inbound orbital branch, at a heliocentric distance rh = 4.5 au. An intense world-wide campaign started immediately after the discovery and continued up to a few weeks before its perihelion passage. It was possible to depict a scenario of a cometary body redder than most of the Solar System comets, with a deep and narrow negative polarization branch, and a CO2-dominated coma. While approaching its perihelion, a rapid rise in comet’s brightness and a steep heliocentric-distance scaling for the production rates of Ni and CN were observed [1] [2] [3] [4] [5] [6] [7].
Due to reciprocal positions of comet and Earth, ground-based observations through the perihelion passage (2025 October 29) were unfortunately impossible, but instead the JUICE spacecraft [8], on its cruise phase towards the Jovian system, was very well placed to observe 3I/ATLAS during those days. Therefore, an observational campaign has been designed and planned, involving several instruments onboard the spacecraft, including the JANUS visible camera [9].
123 JANUS images were obtained in 5 observing slots on 2025 November 5, 6, 12, 19, and 25, using 7 filters to cover the spectral range from 340 to 1080 nm. Figure 1 shows an example of image sequence in the F01 (panchromatic) filter obtained from November 5 to November 25, while the comet’s distance from the Sun varied from 1.39 to 1.67 au and the JUICE’s distance from the comet varied from 64 to 188 million km.
In this work, the JANUS images of 3I/ATLAS and the first results of the JANUS observing campaign will be presented and discussed.
References: [1] Seligman D.Z., Micheli M., Farnocchia D. et al., ApJL 2025. 989, L36-L46. [2] Opitom C., Snodgrass C., Jehin E. et al., MNRAS 2025, 544, 1, L31-L36. [3] Gray Z., Bagnulo S., Borisov G. et al., ApJL 2025, 992, 2, L29-L37. [4] Cordiner M.A., Roth N.X., Kelley M.S.P. et al., ApJL 2025, 991, 2, L43-L52. [5] Rahatgaonkar R., Carvajal J.P., Puzia T.H. et al., ApJL 2025, 995, 1, L34-L46. [6] Hoogendam W.B., Shappee B.J., Wray J.J. et al., 10.48550/arXiv.2510.11779. [7] Lazzarin M., Mura A.C., La Forgia F. et al., ApJL 2026, 998, 1, L30-L35. [8] Grousset O., Dougherty M.K., Coustenis A. et al., PSS 2013, 78, 1-2. [9] Palumbo P., Roatsch T., Lara L.M. et al., SSR 221, 3, id.32
I. Bertini, S. Fornasier, A. Aboudan, M. Aye, E. Kersten, K.-D. Matz, R. Politi, F. Trauthan, A. Zinzi, R. Evill, A. Dietz, I. Belgacem, J. M. Castro Marin, V. Della Corte, S. Hviid, T. Roatsch, N. Schmitz, full JANUS team
How to cite: Mazzotta Epifani, E., Tubiana, C., Lara, L. M., Agostini, L., Penasa, L., Rizos, J. L., Lucchetti, A., Tosi, F., Dotto, E., Pajola, M., Hueso, R., Patel, M. R., Portyankina, G., and Palumbo, P. and the JANUS team: The interstellar comet 3I/ATLAS as seen by JANUS onboard the JUICE spacecraft, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-605, https://doi.org/10.5194/epsc2026-605, 2026.
