Gas leaving the interstellar comet 3I/ATLAS moves at roughly half the speed of gas coming off Solar System comets at the same distance from the Sun. Anita Cochran of McDonald Observatory led the study.
The same spectra turned up molecules familiar from ordinary comets and iron and nickel emission far stronger than the team expected. Together they describe a visitor that looks like a comet in its chemistry and behaves unlike one in its motion, a pairing that Cochran’s team treats as two separate puzzles to be solved from the same set of exposures, taken on one December night with a telescope in West Texas.
Haser model fits behind the half-speed result
Cochran and co-authors Gregory Zeimann, Emmanuel Jehin, Hideyo Kawakita and Adam McKay posted the paper to arXiv on Sept. 1, 2026. In the abstract, they report that Haser model fits to the spectra “require the gas outflow velocity to be about half that of Solar System comets at the same heliocentric distance.” The Haser model is the standard way astronomers convert the brightness profile of a molecule along the coma into an expansion speed and a destruction rate.
The comparison is made at matched distance because outflow speed depends on how strongly sunlight heats the ice. At the roughly 2 astronomical units from the Sun where 3I/ATLAS sat on Dec. 9, a typical Solar System comet would be pushing gas outward faster than the data allow here.
Sci.News, which covered the paper after its publication in the Planetary Science Journal (volume 7, article 226), puts the comet at about 2.03 AU from the Sun and 1.83 AU from Earth during the observation. Neither the abstract nor that report states the slower speed in kilometers per second, so the half-speed ratio is the figure the authors themselves chose to publish.
VIRUS spectrograph and the 85,000 km reach
The instrument was VIRUS, an integral field spectrograph on the Hobby-Eberly Telescope. The team used a single integral field unit and pulled spectra at 1-arcsecond steps, tracing the coma out to about 85,000 km sunward of the brightest point. That long baseline is what makes the speed measurement possible: a molecule’s fade with distance from the nucleus is the raw material the Haser fit works on. The spectra showed emission from cyanogen, tricarbon, methylidyne and dicarbon, the same family of molecules seen in comets that formed around the Sun. Cochran and colleagues say, as quoted by Sci.News, that the team hopes to learn about “conditions in the natal planetary system of this target,” a reference to the unknown star where the comet originally formed.
Iron and nickel lines, and a possible CO2 brake
The abstract also flags “very strong” lines of neutral iron and nickel. The authors derived scale lengths for three nickel lines and four iron lines, and Sci.News reports the emission likely comes straight off the nucleus and is unusually strong next to Solar System comets and the earlier interstellar objects 1I/ʻOumuamua and 2I/Borisov. In the researchers’ words, 3I/ATLAS “was exceptional even compared to the other interstellar objects.”
For the slow outflow, the team’s suggestion, as relayed by Sci.News, is that heavy, carbon dioxide-rich gas may be dragging the speed down. The proposal is a suggestion about the cause, not a measured composition of the nucleus. NASA’s 3I/ATLAS page notes that the SPHEREx space telescope detected carbon dioxide, water and organic molecules in the coma during a December 2025 campaign, the same month as the Hobby-Eberly observation.
The metals speak to what the nucleus is made of; the outflow speed speaks to how gas is driven off it.
An object found in July 2025 on an unbound orbit
The ATLAS survey telescope at Rio Hurtado, Chile, found the comet on July 1, 2025, and the Minor Planet Center’s circular of July 2, 2025 gave it an eccentricity near 6.28, a hyperbolic path that no object bound to the Sun can follow. Bryce Bolin and colleagues, in a discovery-period paper in Monthly Notices Letters, put its speed at infinity at about 57 km/s and concluded the origin is interstellar.
NASA lists perihelion at about 1.4 AU on Oct. 30, 2025, just inside the orbit of Mars, and a closest approach to Earth of about 1.8 AU, with no threat to the planet. Modelers have treated its coma as unusual too: Eric Keto and Abraham Loeb described it in a coma model that resembles a hyperactive comet at 4 AU.
The one number the Cochran paper pins down is the ratio, roughly 0.5, between 3I/ATLAS and a Solar System comet at 2 AU, and the study does not claim to say where the comet formed.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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