SPHEREx Pre-perihelion Mapping of H 2 O, CO 2 , and CO in Interstellar Object 3I/ATLAS

Abstract From 2025 August 1 to August 15 UT, the SPHEREx spacecraft observed interstellar object 3I/ATLAS. Using R = 40–130 spectrophotometry at λ = 0.7–5 μ m, lightcurves, spectra, and imaging of 3I were obtained. From these, robust detections of water gas emission at 2.7–2.8 μ m and CO 2 gas at 4.23–4.27 μ m plus tentative detections of 13 CO 2 and CO gas were found. A slightly extended H 2 O coma was detected, and a huge CO 2 atmosphere extending out to at least 4.2 × 10 5 km was discovered. Gas production rates and 1 σ errors for H 2 O, 12 CO 2 , 13 CO 2 , and CO were Q gas = 3.2 × 10 26 ± 20%, 1.6 × 10 27 ± 10%, 1.3 × 10 25 ± 25%, and 1.0 × 10 26 ± 25%, respectively. Coaddition of all λ = 1.0–1.5 μ m scattered light continuum images from 3I produced an image with high signal-to-noise ratio consistent with an unresolved source. The lightcurve of scattered light showed ≲15% variability over the observation period. The absolute brightness of 3I at 1.0–1.5 μ m is consistent with a nucleus of