Interstellar comet 3I/ATLAS: discovery and physical descriptionOPEN ACCESS
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
Preprint 8 July 2025, submitted to MNRAS
“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◦)f𝜌 of 287.2±2.8 cm. Finally, we use the sunward extent of the coma to constrain the dust ejection speed, finding that μm-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.”
