Snapshot of a new interstellar comet: 3I/ATLAS has a red and featureless spectrum
ABSTRACT The interstellar comet 3I/ATLAS is only the third interstellar object to be discovered. Pre-perihelion measurements provide a unique opportunity to study its activity and composition, which may alter as it is heated in the coming months. We provide an initial baseline from optical spectroscopic observations obtained only two days after discovery, using the MUSE instrument on the VLT on 2025 July 3, while 3I was at 4.47 au from the Sun and 3.46 au from the Earth. These observations confirm the cometary nature of 3I, and reveal a red coma with a spectral slope of $(18\pm 4)\, \%/1000$ Å in the 5000-9000 Å range, redder than most Solar System comets but similar to the surface colour of some Solar System Trans-Neptunian Objects or Centaurs. We searched for but did not detect gas emission from C$_2$, NH$_2$, CN, and [O i], which is consistent with volatile non-detections for Solar System comets at this heliocentric distance. At present, the coma appears entirely dusty. Future observations of 3I as it comes closer to the Sun will provide an invaluable opportunity to witness the evolution of its activity, study its composition, test predictions of interstellar object population models, and compare 3I to Solar System comets.
Authors
- Said Hmiddouch (ORCID: https://orcid.org/0000-0001-7624-0738)
- Marin Ferrais (ORCID: https://orcid.org/0000-0002-0535-652X)
- Mathieu Vander Donckt (ORCID: https://orcid.org/0000-0002-2380-6889)
- Michele T. Bannister (ORCID: https://orcid.org/0000-0003-3257-4490)
- Emmanuël Jehin (ORCID: https://orcid.org/0000-0001-8923-488X)
- Damien Hutsemékers
- Brian Murphy (ORCID: https://orcid.org/0000-0002-8137-5132)
- Sophie Deam (ORCID: https://orcid.org/0000-0003-1955-628X)
- J. Manfroid
- C. Snodgrass (ORCID: https://orcid.org/0000-0001-9328-2905)
- Z. Benkhaldoun (ORCID: https://orcid.org/0000-0001-6285-9847)
- Rosita Kokotanekova (ORCID: https://orcid.org/0000-0003-4617-8878)
- Erica Bufanda (ORCID: https://orcid.org/0000-0002-0406-8518)
- Ryan Ridden-Harper (ORCID: https://orcid.org/0000-0003-1724-2885)
- Rosemary Dorsey
- Matthew M. Knight (ORCID: https://orcid.org/0000-0003-2781-6897)
- Léa Ferellec (ORCID: https://orcid.org/0000-0002-2088-3889)
- Cyrielle Opitom (ORCID: https://orcid.org/0000-0002-9298-7484)
- Michaël Marsset (ORCID: https://orcid.org/0000-0001-8617-2425)
- Manuela Lippi
- Vincent Okoth
- Brayden Leicester
Institutions
- University of Central Florida (US)
- Cadi Ayyad University (MA)
- Planetary Science Institute (US)
- University of Helsinki (FI)
- University of Liège (BE)
- University of Canterbury (NZ)
- United States Naval Academy (US)
- Curtin University (AU)
- Arcetri Astrophysical Observatory (IT)
- Helsinki Institute of Physics (FI)
- Royal Observatory (GB)
- Institute of Astronomy and National Astronomical Observatory (BG)
- European Southern Observatory (CL)
- International Centre for Radio Astronomy Research (AU)
- International Space Science Institute (CH)
- University of Edinburgh (GB)
Publication Details
- Journal
- Monthly Notices of the Royal Astronomical Society Letters
- Published
- 2025-09-04
- DOI
- https://doi.org/10.1093/mnrasl/slaf095
- Citations
- 22
- Primary Topic
- Astro and Planetary Science
- Type
- article
- Field-Weighted Citation Impact
- 25.41
Funders
- European Southern Observatory