Using spectral data from the Hobby-Eberly Telescope in Texas, astronomers have traced gas from the interstellar comet 3I/ATLAS, only the third object of its kind ever observed, out to more than 85,000 km from its nucleus. The data suggest the gas flows at about half the speed typical of solar system comets, and the spectrum shows unusually strong signals of iron and nickel.
The spatial distribution of cyanogen around 3I/ATLAS. Image credit: Cochran et al., doi: 10.3847/PSJ/aea086.
3I/ATLAS was discovered by the NASA-funded ATLAS survey telescope in Rio Hurtado, Chile, on July 1, 2025.
Also known as C/2025 N1 (ATLAS) and A11pl3Z, the interstellar comet arrived from the direction of the constellation Sagittarius.
On October 30, 2025, the object reached perihelion, its closest approach to the Sun.
“By studying 3I/ATLAS, we hope to learn about conditions in the natal planetary system of this target and/or its long interstellar journey,” said Dr. Anita Cochran from McDonald Observatory and her colleagues.
Dr. Cochran and co-authors observed 3I/ATLAS on December 9, 2025, with the VIRUS spectrograph on the Hobby-Eberly Telescope.
The comet was then about 2.03 AU (astronomical units) from the Sun and 1.83 AU from Earth, moving away from the Sun after its closest approach.
The spectra showed the familiar molecules of solar system comets: cyanogen, tricarbon, methylidyne and dicarbon.
But when the astronomers modeled how those molecules are distributed with distance from the nucleus, standard assumptions did not fit.
The data matched only if the gas flows outward at about half the speed seen in typical comets at the same distance from the Sun.
According to the team, one possible explanation is that heavy carbon dioxide-rich gas is slowing the outflow.
The comet spectrum also showed very strong emission lines of neutral iron and nickel, which are likely coming directly off the nucleus in their metallic form.
“The presence of very strong iron and nickel lines in spectra of 3I/ATLAS is an important difference from solar system comets,” the researchers said.
“3I/ATLAS was exceptional even compared to the other interstellar objects, 1I/’Oumuamua and 2I/Borisov, due to its exceptionally high iron and nickel.”
“Wandering around in the interstellar medium should result in the body having high material strength, with iron and nickel metal on its surface.”
“However, the surface temperatures at the heliocentric distances that 3I was observed are too low to vaporize iron and nickel in metallic form.”
“If comets are leftovers from the formation of most planetary systems, and a high fraction are ejected from their formation nebula, there should be many such objects wandering around the Galaxy that can potentially cross through our Solar System,” they added.
“Our Sun, like most planetary systems, probably formed in a cluster of infant stellar systems.”
“The leftover debris of all those stars can, therefore, be influenced by nearby passing stellar systems.”
“As the young stars orbit around the center of the Galaxy, the debris they have ejected will sometimes continue wandering in space, devoid of a star to which they are attached.”
“Thus, unlike a comet in our Solar System that can be influenced by the gravity and radiation of our Sun, these other interstellar comet-like objects may be going into and out of insolation from stars they pass.”
The team’s paper appears in the Planetary Science Journal.
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Anita L. Cochran et al. 2026. IFU Observations of Comet 3I/ATLAS = C/2025 N1 (ATLAS) Using the Hobby-Eberly Telescope with the VIRUS Instrument. Planet. Sci. J 7, 226; doi: 10.3847/PSJ/aea086
