The interstellar comet 3I/ATLAS has captured attention over the past eight months, particularly after its closest approach to the Sun. Following this event, the comet reemerged in December 2025, revealing a dramatic transformation that has been documented in a new pre-print paper by Carey Lisse and colleagues from Johns Hopkins University.
Observational Breakthroughs with SPHEREx
The team utilized NASA’s SPHEREx observatory, which launched in March 2025. While its primary mission is to map the entire sky in near-infrared every six months, it also tracks “targets of opportunity” like 3I/ATLAS. During an initial observational run in August 2025, SPHEREx noted minimal activity from the comet, with only minor outgassing and negligible water release. This lack of activity was attributed to cosmic rays that had bombarded the comet for billions of years, creating a protective shell devoid of volatile ices.
Significant Changes Post-Perihelion
However, the situation changed dramatically as the comet approached the Sun. In December 2025, SPHEREx observed a surge in activity, indicating that the thermal energy from the Sun had penetrated the comet’s crust. Water production soared to nearly 40 times the levels recorded in August, while carbon dioxide production increased to 80 times the earlier measurements. The ratio of carbon monoxide to carbon dioxide also shifted to 2.5, aligning more closely with typical carbon-monoxide-dominant comets found in our solar system.
Outgassing of Organics and Cyanide
In addition to water and carbon dioxide, the comet exhibited a rich array of organic molecules, including methanol, formaldehyde, methane, and ethane. Notably, a new spectral line at 0.925 µm indicated the outgassing of cyanide, a feature absent in the earlier observations. The authors suggest that the levels of outgassing observed imply that the comet is evaporating as it approaches the Sun, marking a unique moment in its multi-billion-year existence.
Future Observations and Implications
The outgassing pattern appeared “pear-shaped,” with the stem pointing toward the Sun and no anti-sunward tail, likely due to the size of the particles involved. These observations suggest that the particles forming the comet’s halo are too large to be influenced by radiation pressure. While it is expected that 3I/ATLAS will not completely disintegrate before leaving the solar system, it will continue to be monitored by SPHEREx, which has one final observational window in April 2026.
This ongoing study of 3I/ATLAS presents a rare opportunity to observe the behavior of an interstellar comet as it interacts with solar energy, providing insights into its composition and evolution.
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