A new report (accessible here) from the UVES spectrograph of the European Very Large Telescope in Chile provides data on the anomalous abundance of nickel and iron in the plume of gas surrounding the interstellar object 3I/ATLAS.
The presence of nickel and iron atoms in the spectrum of comets is unexpected because the surface temperature is too low to allow the sublimation of refractory minerals containing these metals. The interstellar comet 2I/Borisov showed nickel and iron with an abundance ratio that is similar to that observed in solar system comets. On average, the typical cometary ratio is an order of magnitude higher than the solar abundance ratio of nickel to iron.
The new data on 3I/ATLAS covers six time-bins at heliocentric distances ranging from 3.14 to 2.14 times the Earth-Sun separation (AU). Nickel was detected at all times, but iron was only detected at heliocentric distances smaller than 2.64 AU. Altogether, 3I/ATLAS shows a high production rate of nickel atoms as well as a high nickel to iron ratio, making it exceptional when compared to solar system comets and 2I/Borisov.
In conclusion, the authors write: “At the distances at which comets are observed, the temperature is far too low to vaporize silicate, sulfide, and metallic grains that contain nickel and iron atoms. Therefore, the presence of nickel and iron atoms in cometary coma is extremely puzzling… 3I/ATLAS, which is a C2-depleted comet, exhibits extreme properties in the early phases of its activity with regard to the production rates and abundance ratios of nickel and iron.”
Hopefully, we will learn much more about 3I/ATLAS in the coming days. Between October 1–7, 2025, NASA’s Mars Reconnaissance Orbiter and ESA’s Mars orbiters Mars Express and ExoMars will observe 3I/ATLAS as it passes within 29 million kilometers from Mars.
The highest resolution image so far was obtained by the Hubble Space Telescope on July 21, 2025, when 3I/ATLAS was at a distance of 570 million kilometers from the telescope. This is 20 times farther than the closest approach of 3I/ATLAS to Mars. As a result, the HiRISE camera on the Mars Reconnaissance Orbiter will obtain images with a much better spatial resolution of 30 kilometers per pixel. The brightest pixel in the HiRISE image would gauge the surface area of 3I/ATLAS and therefore its diameter. In a recent paper (accessible here), I derived that the diameter of 3I/ATLAS is larger than 5 kilometers — the width of Manhattan Island. The first recognized interstellar object, 1I/`Oumuamua, was pancake shaped and 0.1 kilometers in diameter — the size of a football field (see discussion here). Why is the third interstellar object 3I/ATLAS a million times more massive than the first one? The HiRISE image might exacerbate or weaken the discrepancy. Nature is sometimes more imaginative than script writers in Hollywood.
Science is fun because we can learn something new from evidence, and not from past narratives dictated by the “adults in the room”. My biggest reward arrived recently in the form of an email message from a former US Air Force pilot who wrote that his daughter wants to become a scientist after seeing me on television talking about 3I/ATLAS.
Early this morning, I was asked on “Fox & Friends” whether my gut feeling is that 3I/ATLAS is a natural comet or a technological artifact? I responded that 3I/ATLAS is most likely a natural comet, but we need to be prepared for a black swan event where among the interstellar rocks discovered by the NSF-DOE Rubin Observatory there might be a tennis ball thrown by a cosmic neighbor.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
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