Interstellar Comet 3I/ATLAS Bursts with Gas After Sun Flyby

Comet 3I/ATLAS, an object from outside our solar system, showed a big jump in activity right after its closest pass by the Sun on October 29, 2025. Telescopes spotted strong releases of water vapor, carbon dioxide, carbon monoxide, and organic molecules in December 2025, marking a clear change from its earlier, quieter state.

Background

This comet, named 3I/ATLAS, came from interstellar space, meaning it does not orbit our Sun like most comets do. Telescopes first spotted it on July 1, 2025, in Chile through the ATLAS survey. Early views from NASA's Transiting Exoplanet Survey Satellite, or TESS, caught it active as far out as 6.4 AU from the Sun back in May 2025. That distance is about six times farther than Earth from the Sun, and the activity there pointed to ices more volatile than water driving the glow.

As it headed inward, more telescopes turned to it. The James Webb Space Telescope, or JWST, checked it out on August 6, 2025, using its infrared tools. Hubble joined in November 2025 with ultraviolet scans to measure gas makeup and ratios of elements like sulfur to oxygen. Ground telescopes, including one on Mount Abu in India and Gemini South in Chile, tracked changes in its tail and coma, the fuzzy cloud around it.

The comet reached perihelion, its closest point to the Sun, at 1.36 AU on October 29, 2025, at 11:44 UT. That put it between Earth and Mars orbits, where it hit speeds of 68 km/s. For a time around then, from early October to early November, it hid behind the Sun from Earth's view due to solar conjunction on October 21. Spacecraft near Mars, like NASA's Mars Reconnaissance Orbiter and MAVEN, plus Europe's ExoMars and UAE's Hope, snapped pictures as it passed by on October 3.

Before that close approach, the comet stayed steady without outbursts. Its tail grew to about 56,000 km long by late August, and scans showed rising levels of nickel and cyanide in the coma. Images from August revealed a wobbling jet of gas and dust, tied to the nucleus spinning every 15.5 hours or so.

Key Details

The real shift came after perihelion. NASA's SPHEREx mission observed it in December 2025 and found it far more active than before. Gas emissions jumped, with water vapor and carbon monoxide each up by a factor of 20 compared to pre-perihelion checks. Strong signals appeared from CN, a cyanide radical, organic C-H bonds, CO2, and CO.

Gas and Dust Changes

Analysis points to full sublimation of ices, where solid turns straight to gas from heat. Pre-perihelion, large icy dust grains dominated, too cold for much beyond CO2 to evaporate. By December, after 3.5 months inside the solar system's ice line—the spot where volatiles like water and CO freeze—the whole mix activated. Bulk material from the comet boiled off, freeing all its contents.

Earlier data from Swift Observatory in ultraviolet light found hydroxyl, a sign of water breaking down under sunlight, even at greater distances. JWST noted a coma heavy on CO2 gas with little water gas but plenty of water ice. The nucleus itself contributed little to the glow; the coma drove most of it.

A brightening spell hit just before it went behind the Sun, and that trend held through perihelion, as seen by solar imagers in space. Three mini-jets spaced 120 degrees apart drew notice, though no firm link to anything artificial has formed.

"This change from pre-perihelion observations makes sense because, in August 2025, 3I's behavior was dominated by large icy dust grain emission, with the icy grains too cold to sublimate anything more volatile than CO2 fully. By December 2025, though, the ISO had spent ~3.5 months inside the solar system's ice line, and all of the cometary constituents, not just the highly volatile CO2 and CO ice portions, were active." – SPHEREx study authors

Post-perihelion, it reappeared before sunrise in November, dimming as it climbed higher in morning skies through Virgo and Leo. Brightness fell below magnitude 12 by December's end.

What This Means

The surge tells scientists about how interstellar objects react in our solar system. Crossing the ice line warmed it enough to unlock deeper ices, unlike its home system where conditions differ. Water vapor at afar distances suggests volatiles drive activity without needing Earth's close heat.

Findings hint at the comet's makeup: rich in CO, CO2, and organics from another star. That offers a window into chemistry elsewhere, as these visitors carry untouched samples from distant places. Steady activity without big flares sets it apart from some local comets.

Observations continue as it heads out, bound for interstellar space again. JWST plans more looks, and ground teams track its fade. Data on dust like refractory coma grains and new gas types will build the picture of this rare guest. Past flybys by spacecraft like Psyche added close views, aiding the full story.

The event marks 2025 as a banner year for comets, with 3I/ATLAS as the star interstellar draw. Its path and changes help map how such wanderers behave near stars, informing hunts for similar ones.