Comprehensive visible and near-infrared characterization of 3I/ATLAS during its encounter with the Solar System

Interstellar objects (ISOs) are planetary formation byproducts ejected from extrasolar star systems. To date, only three are known: 1I/'Oumuamua, 2I/Borisov, and 3I/ATLAS, which all showed vastly different physical properties. We describe the observational campaign led by our team to characterize the most recently discovered ISO, 3I/ATLAS, using a combination of visible- and infrared observations from Keck and Gemini, and relay observations from around the world as the comet passed through the solar system. We observed the evolution in the comet’s activity mechanisms and volatile production over a ~9-month period as the comet reached and left perihelion. Our observations provided high-signal-to-noise visible- and near-infrared spectroscopy, enabling constraints on surface composition, dust properties, and potential volatile-driven activity. Multi-band photometry and spectroscopy were used to derive color indices, spectral slopes, and limits on absorption features diagnostic of ices and silicates. Our observations provide an example of an observational framework for future interstellar object discoveries in the Vera C. Rubin era, where early identification and immediate follow-up will be essential to fully characterize these rare messengers from other planetary systems.