SPHEREx Pre-perihelion Mapping of H₂O, CO₂, and CO in Interstellar Object 3I/ATLAS

Year: 2026 DOI: 10.3847/2041-8213/ae4974

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 μm, lightcurves, spectra, and imaging of 3I were obtained. From these, robust detections of water gas emission at 2.7–2.8 μm and CO₂ gas at 4.23–4.27 μm plus tentative detections of ¹³CO₂ and CO gas were found. A slightly extended H₂O coma was detected, and a huge CO₂ atmosphere extending out to at least 4.2 × 10⁵ km was discovered. Gas production rates and 1σ errors for H₂O, ¹²CO₂, ¹³CO₂, and CO were Qgas = 3.2 × 10²⁶ ± 20%, 1.6 × 10²⁷ ± 10%, 1.3 × 10²⁵ ± 25%, and 1.0 × 10²⁶ ± 25%, respectively. Coaddition of all λ = 1.0–1.5 μm 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 μm is consistent with a nucleus of <2.5 km radius surrounded by a 100 times brighter coma. The 1.5–4.0 μm continuum structure shows a strong spectral feature commensurate with water ice absorption seen in Kuiper Belt objects and distant comets. The observed cometary behavior of 3I, including its preponderance of CO₂ emission, lack of CO output, small size, and predominance of large icy chunks of material in a flux-dominant coma, is similar to the behavior of short-period comet 103P/Hartley 2, the ”hyperactive, strongly thermally processed comet” flyby target of the NASA Deep Impact Extended mission in 2010. This correspondence suggests that interstellar objects can be significantly thermally processed before ejection into the interstellar medium, and by comparison to 1I and 2I, this processing can be widely variable in its physical outcome.