3I/ATLAS has left the inner solar system: What Hubble, Webb and SPHEREx found
NASA says interstellar comet 3I/ATLAS is leaving the solar system for good, and Webb, Hubble and SPHEREx data have since pinned down its chemistry and the size of its nucleus.
Interstellar comet 3I/ATLAS passed the Sun on Oct. 30, 2025 and is now on its way out of the solar system for good, NASA says. Since then, Webb has detected methane in it and Hubble has picked out its nucleus, the solid core of the comet.
From a July 2025 discovery to a closest pass in December
The NASA-funded ATLAS survey telescope in Rio Hurtado, Chile, first reported the object to the Minor Planet Center on July 1, 2025. Archived images from ATLAS and from Caltech’s Zwicky Transient Facility showed it as early as June 14, 2025. It is the third interstellar object known, after 1I/’Oumuamua and 2I/Borisov, which is where the “3” in its name comes from.
NASA’s facts page puts the comet’s speed at about 137,000 mph (221,000 km/h) when it was found, still inside Jupiter’s orbit. Gravity from the Sun raised that to about 153,000 mph (246,000 km/h) at perihelion, its closest point to the Sun, on Oct. 30, 2025, when it was about 130 million miles (210 million km) from the Sun, just inside Mars’s orbit. Its closest approach to Earth came on Dec. 19, 2025, at about 1.8 au, or 170 million miles (270 million km). NASA states there was no danger to Earth.
The comet’s path is hyperbolic, which means it is moving too fast to be bound by the Sun, and NASA says it is leaving at the same speed it arrived.
A fleet of spacecraft watched one comet
NASA lists observations from an unusually wide set of instruments. Hubble imaged the comet on July 21, 2025, from about 277 million miles away, and again on Nov. 30, 2025. Webb’s Near-Infrared Spectrograph observed it on Aug. 6, 2025. Mars orbiters and rovers took part too: the Mars Reconnaissance Orbiter’s HiRISE camera imaged it on Oct. 2, 2025, from about 18.6 million miles away, and the Perseverance rover photographed it on Oct. 4. NASA’s Europa Clipper used its ultraviolet spectrograph on Nov. 6, 2025, and Parker Solar Probe imaged it near the Sun between Oct. 15 and Nov. 5.
NASA says the SPHEREx space telescope recorded infrared light from dust, water, organic molecules and carbon dioxide in the coma, the cloud of gas and dust around the nucleus, during a December 2025 campaign. MAVEN found hydrogen atoms around the comet, and TESS observed it in January 2026 to study its activity and rotation.
Methane, carbon dioxide and a nucleus about 1.3 km in radius
Two peer-reviewed results sharpen the picture. In a paper in The Astrophysical Journal Letters (volume 1001, L11, published April 8, 2026), Matthew Belyakov and colleagues used Webb’s Mid-Infrared Instrument on Dec. 15-16 and Dec. 27, 2025, when the comet was 2.20 and 2.54 au from the Sun. They report fluorescence from water, carbon dioxide, methane and atomic nickel, and describe the result as the first direct detection of methane in an interstellar object.
The team found the CO2 to water ratio strongly enhanced compared with typical solar system comets, and the methane to water ratio somewhat enriched. Overall outgassing fell significantly over the 12 days between the two epochs, with water dropping more steeply than the other gases. The authors interpret the methane as coming from unprocessed material below the surface, after the outer layers had lost theirs earlier.
On size, a paper by Man-To Hui, David Jewitt and colleagues, accepted by The Astrophysical Journal Letters, used Hubble images from December 2025 to January 2026 to detect the nucleus itself. Assuming a comet-like albedo (the share of light a surface reflects) of 0.04, they derive an effective radius of about 1.3 plus or minus 0.2 km. That sits inside the earlier NASA bounds of 440 meters to 5.6 km for the nucleus diameter, set from Hubble data of Aug. 20, 2025.
What the data still cannot settle
The size depends on an assumed albedo. The team’s measured quantity is the product of albedo and cross-section, about 0.22 plus or minus 0.07 square kilometers, so a different surface reflectivity would change the radius. The same paper reports that, if brightness changes come only from rotation, the nucleus must be elongated by at least 2:1, with a rotation period of about an hour or more.
The Hubble team also found that the comet faded faster after perihelion than it brightened beforehand, with an activity index of 4.5 plus or minus 0.3. Webb’s chemistry is likewise based on two observing dates, so how representative the methane ratios are of the whole nucleus is not known. NASA’s page says the comet formed in another star system and has drifted for “millions or even billions of years”, and gives no more precise age.
Hui and colleagues also infer that interstellar objects like this one probably went unseen before ‘Oumuamua was found, unless the population has a steep size distribution. That is a statistical inference from discovery numbers, not a measured census.
A comet with no return date
NASA says the comet began moving away from the Sun in December 2025 and will leave the solar system permanently, and its hyperbolic orbit means it will not come back. No further close approaches are scheduled. The remaining science will come from the archived data, which is why papers on the observations from late 2025 and early 2026 are still appearing.
