Movie illustrating how SPHEREx observation tracked the 3I/ATLAS object. Movie frames are centered on the object and scan in wavelength across the CO2 line. The object moved appreciably throughout the observations. SPHEREx wide spectral mapping capability enables the detection of a very extended CO2 coma. Credit: NASA/JPL-Caltech.

The bottom two panels show SPHEREx 0.75-5.0 μm 2-pixel radius aperture spectrophotometry (left) and solar spectrum corrected reflectance (right) for 3I. Red arrows denote possible significant background contamination due to known background sources. The arrows start at the measured flux+1σ. For the reflectance, the NASA-IRTF/SpeX 0.7-2.5 μm spectrum (green) is overlaid. The two sets of measurements are consistent, and characterized by a slightly rising slope from 0.7-1.0 μm, a flat regime from 1.0-1.5 μm, a shallow absorption at 1.5 and 2.1 μm, and a strong falloff starting at 2.4 μm. The SPHEREx spectrophotometry stays low from 2.5-5.0 μm, with a huge uptick at 4.3 μm due to CO2 gas emission. Also overlaid for comparison is the average “cliff” KBO spectrum of Pinellia-Alonso et al. 2025 (pink). The top row shows, from left to right (1) Stacked 1.0-1.5 μm SPHEREx geometry- and Solar flux-corrected image of 3I. There is no significant extension found in the stacked image beyond a stellar point source in a SPHEREx 6.15” pixel. (2) SPHEREx 3.0/4.26/4.7 μm H2O/CO2/CO gas coma. The object is undetected in H2O and CO. By contrast, a bright CO2 coma that extends out to at least 30-pixel (=348,000 km) was found. Credit: NASA/JPL-Caltech.