Interstellar comet 3I/ATLAS grew brighter as it moved inward before perihelion, while its optical colors stayed broadly red and nearly unchanged, according to a new observational study. The same campaign found a persistent sunward jet, an antisolar dust tail and a coma whose broadband light was dominated by dust, with only a small estimated gas contribution.
The results offer a detailed snapshot of the comet’s inbound activity, but the inferred dust composition is a model-based estimate rather than a direct measurement of the bulk nucleus. Continued monitoring is needed to test whether the observed chemistry and dust properties reflect the nucleus or surface processing.
A steady-looking coma
Researchers combined multiband images and optical spectra collected during an inbound campaign from late July to mid-September 2025. Faulkes monitoring ran from July 26 to September 11, while observations from Maidanak covered August 1 to August 29. The team standardized much of its photometry within a projected comet-centered radius of 10,000 kilometers.
Within that aperture, the complete-period SDSS color averages were g−r 0.644, r−i 0.265, i−z_s 0.089 and g−i 0.909 magnitudes, each reported with small measurement uncertainties. The g−i index was the reddest of the reported combinations. Across the monitored interval, the study found no strong secular color change.
The coma was not uniform with distance from the nucleus. Relative to a 20,000-kilometer reference, g−i and r−i were positive in the inner coma and fell toward zero farther out, while i−z_s showed no significant radial gradient. The authors say this pattern may reflect changes in the sizes of dust grains with distance, rather than a demonstrated physical cause.
Image enhancement showed a relatively stable sunward jet at a position angle of about 278 degrees and an antisolar tail at about 106 degrees. Both features persisted from early July through September; the authors associated the jet with a possible high-latitude active region near 75 degrees.
CN stood out in the spectra
The spectra became less red and flatter in August and September than in the strongest earlier July measurements. For example, a slope reported for July 29 was 30.9% per 1,000 angstroms, compared with 16.3% per 1,000 angstroms on August 29. The observations used different wavelength intervals, so the comparison is not a single uniform measurement.
Among the molecular features searched for, the CN band near 3,883.5 angstroms was the only clearly identified prominent emission. Its peak signal-to-noise ratio was about 4.5 and its integrated signal-to-noise ratio about 5.14. CO+ and N2+ stayed below the study’s robust-detection threshold, while a possible C3 feature was considered likely spurious.
A Haser-model analysis estimated the logarithmic CN production rate at log Q(CN) 25.6 ± 0.2 mol s−1. It placed three-sigma upper limits of 22.6 for C3 and 24.3 for C2. Those limits led the authors to describe 3I/ATLAS as carbon-chain depleted, although that classification partly rests on molecules that were not securely detected.
Dust clues, with a warning label
To interpret the colors and polarization, the researchers compared several models of light scattering by clumps of dust grains. A mixture made of 90% organic material and 10% pyroxene in a BAM2 aggregate model matched the reported colors, R-band polarization and radial profiles better than processed organic aggregates alone.
That mixture is a model result, not a direct measurement of the nucleus. The authors describe the inferred composition as a rough estimate because the optical properties, or refractive indices, assigned to organic matter and silicate minerals remain uncertain. A separate model found that a 10% water-ice component could affect colors, but that micrometer-scale mixing inside aggregates could make the effect negligible.
Taken together, the observations describe a dust-dominated coma with relatively stable dust properties over the monitored interval. Gas made only a small estimated contribution to the broadband light—about 2.1% in g and 1.3% in r within an aperture of roughly 10,000 kilometers. The study does not show whether these properties reflect the bulk nucleus or surface processing.
What comes next
The study covers one comet during its inbound, pre-perihelion interval. The authors say continued monitoring after perihelion is needed to see whether the chemical depletion and dust properties persist and to help distinguish the comet’s underlying composition from surface processing.
A technical issue affected the August 29 Nordic Optical Telescope spectrum: an incorrect grism was used, although the data were retained for estimating the spectral slope. The authors’ composition estimate therefore remains dependent on model assumptions and future observations.
Paper data and sources
Original title: The Inbound Gas and Dust Evolution of Interstellar Comet 3I/ATLAS from Optical Spectroscopy and Multi-band Photometry
Authors: Toni Santana-Ros, Alexey Sergeyev, Oleksandra Ivanova et al.
Journal/Repository: T. Santana-Ros et al 2026, ApJ, 1007, 182
Status: Peer-reviewed
First online: 2026-08-18
DOI: 10.3847/1538-4357/ae8dbb
Original paper · Full text