Comprehensive Visible and Near-infrared Keck and Gemini Monitoring and Characterization of Interstellar Comet 3I/ATLAS during its Encounter with the Solar System

We present 14 optical imaging and eight optical spectroscopic epochs of interstellar comet 3I/ATLAS from Keck/LRIS and Gemini/GMOS, two near-infrared (NIR) imaging epochs from Keck/MOSFIRE, and four NIR spectroscopic epochs from Gemini/Flamingos-2 and GNIRS, obtained between 2025 July and 2026 May. Mean GMOS colors of $g-r=0.732\pm0.029$, $r-i=0.237\pm0.025$, and $i-z=0.109\pm0.022$ mag show no significant change across perihelion. Phase-corrected red-band activity follows $Afρ\propto r_h^{-2.10\pm0.14}$ inbound and $r_h^{-3.34\pm0.43}$ outbound. Transverse-width and sunward-plume speeds imply dust losses of $2.7\times10^9$ and $1.0\times10^{10}$ kg over 2025 July 3--2026 May 15 for 1-$μ$m--1-mm grains, equivalent to global layers of 0.26 and 0.99 m under the adopted grain and nucleus assumptions. The concentrated coma, compact sunward feature, and antisolar tail seen on 2025 August 27 broadened by 2025 November 26; extended dust persisted through 2026 February 17. C$_2$/CN nearly doubled from $0.257\pm0.014$ on 2025 September 14--15 to $0.481\pm0.080$ on 2025 November 26, while C$_3$/CN fell from $0.327\pm0.024$ on 2025 August 28 to $0.214\pm0.009$ on 2025 September 14--15. MOSFIRE NIR imaging on 2025 July 11 gives $Y-J=0.466\pm0.143$, $J-H=0.239\pm0.052$, $H-K_s=0.454\pm0.150$, and $J-K_s=0.693\pm0.202$ mag. Spectra give $J-H=0.24$ mag on 2025 September 5--6 and $J-H=0.27$--0.28 and $J-K_s=0.21$--0.22 mag on 2025 December 1--2 over the retained passband intervals. Model-calibrated imaging on 2026 February 6 gives near-solar $J-K_s=0.303\pm0.051$ mag. No water-ice absorption is robust to the tested continuum and calibration choices in the NIR spectra. These observations provide a lasting record of the comet's evolution through the Solar System and a benchmark for studying and monitoring future interstellar objects.