NASA’s SPHEREx mission reveals that interstellar object 3I/ATLAS dramatically transformed into an active comet following its close approach to the Sun.

SPHEREx monitored 3I/ATLAS from December 8 to 15, 2025, shortly after its perihelion, the closest approach to the Sun.

“In December 2025, SPHEREx reobserved 3I/ATLAS post-perihelion and found it to be far more active than previously observed. August 2025 SPHEREx pre-perihelion observations show significant evidence that it has completely sublimated its ice, transforming into a comet,” explained astronomer Carrie Risse from Johns Hopkins University Applied Physics Laboratory.

New SPHEREx data indicates a gas- and dust-rich coma, revealing emissions of cyanide (CN), water (H₂O), organic compounds (CH), carbon dioxide (CO₂), and carbon monoxide (CO).

“The emergence of cyanide and organic matter indicates their presence in the aqueous phase,” said Avi Loeb, a Harvard University professor not involved in this study.

Researchers noted a sharp increase in water activity, accompanied by a significant surge in carbon monoxide emissions.

“The H₂O gas signature, previously nearly undetectable in August, is now 20 times stronger, indicating complete sublimation of water,” added the researchers.

“CO emission rates have increased by 20 times, as the CO/CO₂ ratio has risen 15-fold as well.”

SPHEREx images further demonstrate that the object’s gaseous envelope has expanded significantly.

“The gas comas observed by SPHEREx are distinctly resolved, extending from 1 foot to 3 feet in radius; all but the CN and CH organic comas appear round relative to the Sun and orbital velocity,” scientists noted.

“In contrast, the dust and organic matter images in the SPHEREx continuum exhibit a pear-shaped morphology, with the ‘pear stalk’ directed toward the Sun.”

“These varying shapes suggest that CN and CH are sourced from dust, while H₂O, CO₂, and CO gases are distributed symmetrically around the atomic nucleus.”

“No visible jets or anti-solar tail structures were detected.”

SPHEREx’s observations indicate fundamental changes in the physical state of 3I/ATLAS.

“December 2025’s observations align with a comet that is fully active and sublimating water ice,” the authors concluded.

The composition now resembles that of typical Solar System comets, with ice content comparable to most comets.

Researchers attribute these changes to the object’s trajectory through the inner solar system.

“By December, 3I/ATLAS had spent 3.5 months within the solar system’s ice line, causing all comet components and volatile CO₂ ice to become active,” they stated.

“Consequently, the bulk material of the cometary matrix evaporated, releasing its contents.”

“Further observations are anticipated before 3I/ATLAS passes through SPHEREx’s investigation pattern again in April 2026,” they concluded.

For more information, refer to their paper, submitted for publication in AAS Research Notes.

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CM Lisse et al. 2026. SPHEREx reobservations of interstellar object 3I/ATLAS in December 2025: increased post-perihelion activity, detection of coma dust, and new gaseous emissions. RNAAS submitted for publication. arXiv: 2601.06759