Anomalous Post-Perihelion Activity of Interstellar Comet 3I/ATLAS: Insights from SPHEREx Infrared Spectroscopy

The interstellar visitor known as 3I/ATLAS, currently traversing our solar system, has exhibited highly unusual behavior by significantly ramping up its activity long after its closest approach to the Sun. This unexpected surge, documented in December 2025, challenges conventional models of cometary sublimation for objects moving away from solar warmth. Utilizing data from NASA’s SPHEREx mission, researchers have conducted a comprehensive analysis of the comet's emissions, revealing the presence of ancient, preserved materials that originated in a distant stellar system.

Although 3I/ATLAS reached its perihelion—the point of maximum proximity to the Sun—on October 29, 2025, its peak gas and dust activity did not occur until December 2025. This delay suggests a significant level of thermal inertia within the object. Infrared measurements from the SPHEREx space observatory, which is designed for all-sky near-infrared surveys, identified a massive release of volatile compounds. The comet's coma, or its tenuous gaseous envelope, was found to be rich in water ice (H₂O) vapor, carbon dioxide (CO₂), carbon monoxide (CO), and a variety of complex organic molecules, including methanol, methane, and cyanides.

The intense sublimation of water ice, which is estimated to constitute approximately one-third of the nucleus's mass, resulted in a dramatic increase in the comet's luminosity and the formation of a distinct, pear-shaped dust tail. This tail is created as solar radiation exerts pressure on the dust, though observations also noted the ejection of relatively large grains and rocky particles that are too heavy to be carried far by radiation pressure. Scientists, including Dr. Carey Lisse of the Johns Hopkins University Applied Physics Laboratory, who led the study, propose that 3I/ATLAS is encased in a thick crust modified by cosmic radiation over billions of years in interstellar space.

As solar energy gradually warmed this external shell, the heat eventually penetrated to the hidden, pristine ices located deep beneath the surface. This process triggered an explosive discharge of primordial material, including carbon-rich compounds, soot, and silicate dust, which had remained shielded from the environment for geological eons. This chemical mixture provides a rare and invaluable sample of the fundamental building blocks that existed in a planetary system entirely different from our own.

3I/ATLAS stands as only the third confirmed interstellar object ever detected, following the historic visits of 1I/ʻOumuamua and 2I/Borisov. Notably, 3I/ATLAS possesses the highest recorded orbital eccentricity at 6.139. The comet was first identified on July 1, 2025, by the ATLAS telescope system. A detailed study of its chemical composition as recorded in December 2025 was subsequently published in the journal Research Notes of the AAS in February 2026.

The SPHEREx team, which includes researcher Phil Korngut from the California Institute of Technology, intends to maintain a close watch on the object through April 2026 as it continues its journey out of the solar system. These ongoing observations offer a unique opportunity for a comparative analysis of prebiotic materials across the Milky Way galaxy. The data gathered from 3I/ATLAS will likely serve as a benchmark for understanding the chemical diversity of interstellar migrants for years to come.