Interstellar Comet 3I/ATLAS Shows Its Spectral Secrets Through Palomar and Apache Point Observations

Introduction

On July 1, 2025, astronomers discovered 3I/ATLAS, the third known interstellar object to enter our solar system. It was known to come from outside our solar system by its hyperbolic trajectory and high orbit eccentricity (e ≈ 6.15), 3I/ATLAS is a definite interstellar visitor. Following its discovery, many researchers and observatories postponed their observatory schedules to initiate comprehensive observations to study the composition and activity of 3I/ATLAS. Notably, spectrophotometric analyses conducted using the famous Mount Palomar 200-inch telescope and Apache Point Observatory have provided valuable insights into the comet's characteristics (1).

Artist’s depiction of 3I/ATLAS above Palomar observatory © Rebi -chronicles-stock.adobe.com

Changing Observational Schedules

Palomar 200-Inch Telescope

On July 3, 2025, astronomers observed 3I/ATLAS using the Next Generation Palomar Spectrograph (NGPS8), which operates in simultaneous two-channel mode covering the R (580–780 nm) and I (760–1040 nm) bands. The instrument's image slicer captures light outside the center slit onto two secondary slits, allowing for efficient light collection. Observations were made under seeing conditions measured by determining the full width at half maximum (FWHM) of the spectral trace in the spatial direction. Spectra of the G2V solar analog HIP088235 were also obtained to correct for telluric (Earth atmosphere interferences) features and slope (1).

Apache Point Observatory

Observations were made on August 6, 2025, and were conducted at Apache Point Observatory using the ARCTIC camera to obtain g/r/i/z-band (specific color filters: green, red, near-infrared) images. Additionally, the KOSMOS spectrometer was employed with the blue grism (combines a grating and a prism, hence "grism") to cover the 380–660 nm spectral range with a spectral resolution of approximately 500 (Feature resolution = 500/wavelength measured in nm). Similar to the Palomar observations, spectra of the solar analog HIP088235 were obtained for the Earth’s atmosphere correction (1).

Spectral Analysis and Findings

The combined spectrophotometric data from both observatories revealed a red spectral slope of 19%/100 nm in the 420–700 nm range and a more neutral 6%/100 nm slope over 700–1000 nm. These measurements indicate that 3I/ATLAS exhibits a redder spectrum compared to typical solar system comets and asteroids (1–5). Notably, no significant emission features, such as those from C₂ or CO+, were detected, suggesting a lack of detectable gas activity at the time of observation (1).

The absence of emission features may be attributed to the comet's distance from the Sun at the time of observation (approximately 4.3 au), where water ice sublimation is minimal. This observation contrasts with the behavior of 2I/Borisov, which exhibited activity beyond the water-ice sublimation line . The extended surface brightness profile observed suggests that 3I/ATLAS is an active object, though the exact mechanisms driving this activity remain unclear (1).

Implications and Future Observations

The findings from these spectrophotometric observations contribute to the growing body of knowledge about interstellar objects (1–5). The unique spectral characteristics of 3I/ATLAS provide insights into the composition and activity of bodies from other star systems. As the comet approaches perihelion in October 2025, further observations are planned to monitor its activity and composition. These future studies will be crucial in understanding the behavior of interstellar comets and their potential to inform theories about the formation and evolution of planetary systems (1–5).

References

(1) Belyakov, M.; Fremling, C.; Graham, M. J.; Bolin, B. T.; Kilic, M.; Jewett, G.; Lisse, C. M.; Ingebretsen, C.; Davis, M. R.; Wong, I. Palomar and Apache Point Spectrophotometry of Interstellar Comet 3I/ATLAS. Res. Notes AAS 2025, 7 (7), 123. DOI: 10.3847/2515-5172/adf059

(2) Tonry, J.; Denneau, L.; Alarcon, M.; Clocchiatti, A.; Erasmus, N.; Fitzsimmons, A.; Licandro, J.; Meech, K.; Siverd, R.; Weiland, H. ATLAS Photometry of Interstellar Object 3I/ATLAS. arXiv 2025, arXiv:2509.05562. DOI: 10.48550/arXiv.2509.05562

(3) Jewitt, D.; Seligman, D. Z.; Moro-Martín, A. Interstellar Objects. Contemp. Phys. 2022, 63 (3), 200–232. DOI: 10.1080/00107514.2023.2203976

(4) Meech, K. J.; Weryk, R.; Micheli, M.; Kleyna, J. T.; Hainaut, O.; Jedicke, R.; Magnier, E.; Denneau, L.; Farnocchia, D.; Micheli, M.; et al. A Brief Visit from a Red and Extremely Elongated Interstellar Asteroid. Nature 2017, 552, 378–381. DOI: 10.1038/nature25020

(5) Trilling, D. E.; Mommert, M.; Hora, J. L.; Farnocchia, D.; Chodas, P.; Giorgini, J.; Smith, H. A.; Carey, S.; Lisse, C. M.; Werner, M.; McNeill, A. Spitzer Observations of Interstellar Object 1I/‘Oumuamua. Astron. J. 2018, 156 (6), 261. DOI: 10.3847/1538-3881/aae88f