Cosmic Visitor with a Red Glow: Looking Inside the Interstellar Comet 3I/ATLAS

Introduction
For the third time in recorded history, an interstellar object (ISO) has crossed our Solar System. Named 3I/ATLAS, this cosmic wanderer was discovered by the ATLAS survey in early July 2025. Unlike its predecessors—1I/‘Oumuamua, which appeared inert, and 2I/Borisov, which displayed typical cometary activity—3I/ATLAS has already revealed a rich, red-tinted dust coma. Early observations provide a critical window into the makeup of interstellar visitors, offering clues about planetary systems far beyond our own (1–3).

Artist's conception of Comet 3I/ATLAS in deep space © Jon Le-Bon -chronicles-stock.adobe.com

Eyes on the Comet: VLT/MUSE Observations
Just two days after discovery, an international team led by Cyrielle Opitom, Colin Snodgrass, and Emmanuel Jehin leveraged the European Southern Observatory’s Very Large Telescope (VLT) with the Multi-Unit Spectroscopic Explorer (MUSE) instrument to capture high-resolution optical spectra of 3I/ATLAS (1). Observing the comet at 4.47 astronomical units (au) from the Sun and 3.46 au from Earth, they employed MUSE’s integral-field spectrograph, which covers 4800–9200 Å (480–920 nm), allowing simultaneous acquisition of spectral and spatial information across the comet’s coma. This capability enables both continuum analysis and searches for faint emission lines from atomic or molecular species.

To avoid contamination from the dense star field near the galactic plane, the team acquired eight separate 300-second exposures, applying small dithers and rotations between frames. One contaminated frame was discarded, and the remaining seven were median-combined. Standard MUSE pipelines were used for sky subtraction, telluric correction, and flux calibration, with reference to a spectrophotometric standard observed the same night. The team also applied a dedicated search for the [O I] 6300 Å (630 nm) line using 2D mapping techniques to distinguish faint cometary emission from telluric lines (1).

Spectroscopic Measurements: Red but Featureless
The extracted spectrum revealed a continuum consistent with sunlight reflected by dust, but no significant emission lines from CN, C₂, NH₂, or [O I] were detected. Cyanogen radical (CN), dicarbon (C₂), amino radical (NH₂), and neutral atomic oxygen ([O I]) are common cometary gas species produced by photodissociation of parent molecules. Despite careful corrections for Doppler shift and atmospheric absorption, the faintness of the comet at this distance meant that atomic and molecular lines remained below the detection threshold. Using the Haser model and literature g-factors, the team calculated upper limits for CN and C₂ production rates, finding 3σ upper limits of ~molec/s for CN and ~molec/s for C₂ (1).

The reflectance spectrum, normalized at 6000 Å (600 nm) and corrected for solar contribution using a reference spectrum from the SOLar SPECtrometer (SOLSPEC), exhibited a red slope across the 5000–9000 Å (500–900 nm) range. Specifically, the normalized reflectivity gradient was measured as 18%/1000 Å (100 nm), with minor variations across 5000–7000 Å (500–700 nm) (15%) and 7000–9000 Å (700–900 nm) (21%), consistent with broadband TRAPPIST-North photometry. The lack of absorption or emission features indicates that the coma is dominated by dust rather than volatiles at this early stage.

Context in the Solar System and Beyond
Comparisons with previous interstellar objects show intriguing differences. While 1I/‘Oumuamua was relatively inert, and 2I/Borisov displayed typical cometary gas signatures, 3I/ATLAS appears intermediate, with a red, dust-rich coma but minimal detected gas. This could reflect the early onset of activity, revealing surface layers similar to outer Solar System bodies rather than freshly exposed interior material (1).

The findings support models suggesting a diverse population of interstellar comets, shaped by the metallicity and age of their home stars (2,3). With perihelion expected at 1.35 au in October 2025, ongoing observations will track the evolution of 3I’s activity and gas production, offering unprecedented insights into the chemistry of interstellar visitors (1).

Conclusion
3I/ATLAS provides a rare opportunity to study an alien comet in real time. Its red, featureless spectrum emphasizes a dust-dominated environment, while the absence of gas emissions at this distance hints at an evolving activity profile. Future VLT/MUSE monitoring will refine upper limits on gas production and track changes in spectral slope, helping astronomers place this interstellar visitor in context with Solar System comets and the broader galactic population (1).

References
(1) Opitom, C.; Snodgrass, C.; Jehin, E.; Bannister, M. T.; Bufanda, E.; Deam, S. E.; Dorsey, R. C.; Ferrais, M.; Hmiddouch, S.; Knight, M. M.; Kokotanekova, R.; Leicester, B.; Marsset, M.; Murphy, B.; Okoth, V.; Ridden-Harper, R.; Vander Donckt, M.; Ferellec, L.; Hutsemékers, D.; Lippi, M.; Manfroid, J.; Benkhaldoun, Z. Snapshot of a New Interstellar Comet: 3I/ATLAS Has a Red and Featureless Spectrum. Mon. Not. R. Astron. Soc., Lett. 2025, 544 (1), L31–L36. DOI: 10.1093/mnrasl/slaf095

(2) Hopkins, M. J.; Bannister, M. T.; Lintott, C. Predicting Interstellar Object Chemodynamics with Gaia. Astron. J. 2025, 169 (2), 78. DOI: 10.3847/1538-3881/ad9eb3.

(3) Hopkins, M. J.; Dorsey, R. C.; Forbes, J. C.; Bannister, M. T.; Lintott, C. J.; Leicester, B. From a Different Star: 3I/ATLAS in the Context of the Ōtautahi–Oxford Interstellar Object Population Model. Astrophys. J. Lett. 2025, 990 (2), L30. DOI: 10.3847/2041-8213/adfbf4.