Interstellar comet 3I/ATLAS: Discovery and physical description

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

Monthly Notices of the Royal Astronomical Society: Letters · 2025

Abstract

We present the characteristics of interstellar comet 3I/Asteroid Terrestrial-impact Last Alert System (ATLAS), discovered on 2025 July 1 by the ATLAS. The comet has eccentricity $\simeq$6.1 and velocity at infinity $\simeq$57 km s−1, indicating an interstellar origin. We obtained B, V, R, I, g, r, i, and z photometry at Kottamia Astronomical Observatory, Palomar Observatory, and Apache Point Observatory on 2025 July 2, 3, and 6. We measured colour indices B–V = 0.98 $\pm$ 0.23, V–R = 0.71 $\pm$ 0.09, R–I = 0.14 $\pm$ 0.10, g–r = 0.84 $\pm$ 0.05, r–i = 0.16$\pm$0.03, i–z = –0.02$\pm$0.07, g–i = 1.00$\pm$0.05, and a spectral slope of 16.0 $\pm$ 1.9 ${{\ \rm per\ cent}}$/100 nm. We estimate the comet’s dust cross-section within 10 000 km to be 184.6 $\pm$ 4.6 km$^2$, assuming a 0.1 albedo. 3I/ATLAS’s coma has full width at half-maximum $\simeq$ 2.2 arcsec and A(0$^\circ$)f$\rho$ = 280.8 $\pm$ 3.2 cm. We estimate 3I/ATLAS is ejecting $\mu$m to mm dust at $\sim$0.01–1 m s−1, with a mass-loss of $\sim$0.1–1.0 kg s−1.

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How to cite

Bolin, B. T., Belyakov, M., Fremling, C., Graham, M. J., Abdelaziz, A. M., Elhosseiny, E., Gray, C. L., Ingebretsen, C., Jewett, G., Lisse, C. M., Karpov, S., Kilic, M., Mašek, M., Molham, M., Roderick, D., Takey, A., Abron, L.-M., Coughlin, M. W., Hsieh, C.-H., et al. (2025). Interstellar comet 3I/ATLAS: discovery and physical description. Monthly Notices of the Royal Astronomical Society: Letters, 542(1), L139–L143. https://doi.org/10.1093/mnrasl/slaf078