Interstellar comet 3I/ATLAS: discovery and physical description
Bryce Bolin, Matthew Belyakov, C. Fremling, M. J. Graham, A.M. Abdelaziz, E. G. Elhosseiny, Candace Gray, Carl Ingebretsen, Gracyn Jewett, C. M. Lisse, С. Карпов, Mukremin Kilic, M. Mašek, Mona Molham, Diana Roderick, A. Takey, Laura-May Abron, M. W. Coughlin, Cheng-Han Hsieh, Keith Noll, Ian Wong
Abstract
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.