The team of NASA’s SPHEREx space observatory just reported tantalizing new data on the interstellar object 3I/ATLAS (accessible here). The observations were made between August 8–12, 2025 when 3I/ATLAS was at a distance from the Sun of 3.2 times the Earth-Sun separation (AU) and a distance from Earth of 2.6 AU.
The new observations reveal a cloud of carbon dioxide (CO2) around 3I/ATLAS corresponding to a mass loss rate of about 70 kilograms per second. No water (H2O) cloud was detected with an upper limit of 4.5 kilograms per second on the water mass loss rate. This is an order of magnitude below the previous claims of water detection with a mass loss rate of order 40 kilograms per second at a larger distance from the Sun of 3.5 AU. These early claims from two research teams were unsubstantiated by the reported data, as I argued in a previous essay (accessible here). The excellent SPHEREx report notes that “The lack of a bright water gas coma is puzzling as 3I/ATLAS was not too far outside the Solar system’s “water ice line” at 2.5 AU during the observations.”
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