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 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. 160. 03, i–z = –0. 020. 07, g–i = 1. 000. 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^) f = 280. 8 3. 2 cm. We estimate 3I/ATLAS is ejecting m to mm dust at 0. 01–1 m s−1, with a mass-loss of 0. 1–1. 0 kg s−1.
Bolin et al. (2025) studied this question.