Interstellar

Interstellar Comet 3I/ATLAS: Discovery And Physical Description

By Keith Cowing

Status Report

astro-ph.GA

July 11, 2025

Filed under 3I\ATLAS, astro-ph.GA, astrochemistry, astrogeology, astronomy, Comet, imaging, Interstellar

Interstellar Comet 3I/ATLAS: Discovery And Physical Description — The g-r vs. r-i colours of 3I/ATLAS from our measurements compared with solar system objects and other ISOs (Meech et al. 2017; Bolin et al. 2020). We use the scheme of Ivezić et al. (2002) where blue dots correspond to C-complex asteroids, red dots correspond to S-Complex asteroids, and green dots correspond to V-complex asteroids. We also plot the g-r vs. r-i colours of active comets from Solontoi et al. (2012) and trans-Neptunian objects from Ofek (2012). — astro-ph.IM

We describe the physical characteristics of interstellar comet 3I\ATLAS, discovered on 2025 July 1 by the Asteroid Terrestrial-impact Last Alert System.

The comet has eccentricity, e ≃ 6.08 and velocity at infinity, v∞ ≃ 57 km/s, indicating an interstellar origin. We obtained g, r, i photometry with the Palomar 200-inch Next Generation Palomar Spectrograph on 2025 July 3. We measured colour indices g-r = 0.43±0.02 mag, r-i = 0.16±0.02 mag, and g-i = 0.59±0.03 mag and a spectral slope of 1.3±0.9 %/100 nm.

We calculate the dust cross-section within 10,000 km of the comet to be 230.0±5.2 km², assuming an albedo of 0.10. The FWHM of 3I\ATLAS’s detection is ∼2.2 arcsec as measured in our r-band images and has A(0^∘)f_ρ of 287.2±2.8 cm. Finally, we use the sunward extent of the coma to constrain the dust ejection speed, finding that um-scale to mm-scale particles have ∼0.01-1 m/s, implying the comet’s dust mass-loss rate is ∼0.1 – 1.0 kg/s.

Bryce T. Bolin, Matthew Belyakov, Christoffer Fremling, Matthew J. Graham, Candace L. Gray, Carl Ingebretsen, Gracyn Jewett, Mukremin Kilic, Carey M. Lisse, Diana Roderick, Ahmed. M. Abdelaziz, Laura-May Abron, Michael W. Coughlin, Eslam Elhosseiny, Cheng-Han Hsieh, Martin Mašek, Mona Molham, Ali Takey, Keith S. Noll, Ian Wong

Comments: 8 page, 4, figures, 2 tables, submitted to MNRAS:L
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2507.05252 [astro-ph.EP] (or arXiv:2507.05252v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2507.05252
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Submission history
From: Bryce Bolin
[v1] Mon, 7 Jul 2025 17:58:08 UTC (2,109 KB)
https://arxiv.org/abs/2507.05252
Astrobiology, Astronomy, Astrochemistry, Astrogeology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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