Interstellar comet 3I/ATLAS: discovery and physical description

Abstract

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, $e$ $\simeq$ 6.08 and velocity at infinity, v${}_{\infty }$ $\simeq$ 57 km/s, indicating an interstellar origin. \textbf{We obtained B,V, R, I, g, r, i, and z photometry with the Kottamia Astronomical Observatory 1.88-m telescope, the Palomar 200-inch telescope, and the Astrophysical Research Consortium 3.5-m telescope 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$0.05 mag, r-i=0.16$\pm$0.03 mag, i-z=-0.02$\pm$0.07 mag, and g-i=1.00$\pm$0.05 mag and a spectral slope of 16.0$\pm$1.9 $\%$/100 nm.} We calculate the dust cross-section within 10,000 km of the comet to be 184.6$\pm$4.6 km${}^2$, assuming an albedo of 0.10. 3I/ATLAS's coma has FWHM$\simeq$2.2 arcsec and A(0${}^{\circ }$)f$\rho$=280.8$\pm$3.2 cm. \textbf{We estimate that 3I/ATLAS's \textmu m-scale to mm-scale dust is ejected at $\sim$0.01-1 m/s, implying a dust production of $\sim$0.1 - 1.0 kg/s.

8 pages, 5 figures, 3 tables, in press at MNRAS:L, accepted 2025 July 15, received 2025 July 14, in original form 2025 July 7

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Cited by in corpus (18)

• Assessing interstellar comet 3I/ATLAS with the 10.4 m Gran Telescopio Canarias and the Two-meter Twin Telescope
• Temporal Evolution of the Third Interstellar Comet 3I/ATLAS: Spin, Color, Spectra and Dust Activity
• X-SHOOTER Spectrum of Comet 3I/ATLAS: Insights into a Distant Interstellar Visitor
• Very Large Telescope Observations of Interstellar Comet 3I/ATLAS. II. From Quiescence to Glow: Dramatic Rise of Ni i Emission and Incipient CN Outgassing at Large Heliocentric Distances*
• Interstellar Comet 3I/ATLAS: Evidence for Galactic Cosmic Ray Processing
• Simultaneous visible spectrophotometry of interstellar object 3I/ATLAS with Seimei/TriCCS
• Pre-discovery TESS Observations of Interstellar Object 3I/ATLAS
• The Volatile Inventory of 3I/ATLAS as seen with JWST/MIRI
• The Physics of Cometary Anti-tails as Observed in 3I/ATLAS
• Discovery of an icy and nitrogen-rich extrasolar planetesimal
• Effect of different Non-Gravitational accelerations on the trajectory of Interstellar Comet 3I/ATLAS
• Interstellar Object 3I/ATLAS Observed from Mars by China's Tianwen-1 Spacecraft
• Dynamical simulation of the Interstellar Comet 3I/ATLAS
• Pre-perihelion Volatile Evolution of Interstellar Comet 3I/ATLAS Indicating Significant Contribution from Extended Source in the Coma
• Narrowband Radio Technosignature Search toward 3I/ATLAS with FAST
• Icy Volatile Enhancements in Evolving Protoplanetary Disks
• The Inbound Gas and Dust Evolution of Interstellar Comet 3I/ATLAS from Optical Spectroscopy and Multi-band Photometry
• TSSC comet-centered data products from TESS 3I/ATLAS observations