Interstellar comet 3I/ATLAS: discovery and physical description

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 6.1 and velocity at infinity 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 ± 0.23, V–R = 0.71 ± 0.09, R–I = 0.14 ± 0.10, g–r = 0.84 ± 0.05, r–i = 0.16±0.03, i–z = –0.02±0.07, g–i = 1.00±0.05, and a spectral slope of 16.0 ± 1.9 \ per\ cent/100 nm. We estimate the comet’s dust cross-section within 10 000 km to be 184.6 ± 4.6 km², assuming a 0.1 albedo. 3I/ATLAS’s coma has full width at half-maximum 2.2 arcsec and A(0^∘)fρ = 280.8 ± 3.2 cm. We estimate 3I/ATLAS is ejecting μm to mm dust at ~0.01–1 m s−1, with a mass-loss of ~0.1–1.0 kg s−1.