By 
September 26th, 2025
This image of interstellar comet 3I/ATLAS was captured by the Hubble Space Telescope’s Wide Field Camera on 21 July 2025.
Image credit: NASA, ESA, D. Jewitt (UCLA); Image Processing: J. DePasquale (STScI)
By all accounts, that interstellar interloper — object 3I/ATLAS – is a big story. And it’s gotten bigger.
It turns out that this celestial wanderer is massive and large.
New research using data on the motion of 3I/ATLAS — as compiled by the Minor Planet Center at the Smithsonian Astrophysical Observatory – implies that 3I/ATLAS is more massive than 33 billion tons and its diameter is larger than 3 miles (5 kilometers).
3I/ATLAS will be near Mars on October 3, 2025. That would allow the super-powerful High Resolution Imaging Science Experiment (HiRISE) camera to snag imagery of the object.
On patrol – NASA’s Mars Reconnaissance Orbiter (MRO).
Image credit: NASA/JPL
All in all, exciting detective work is ahead on watching this weirdo of a world.
Major anomaly
The object’s lack of non-gravitational acceleration implies an anomalously massive object reports Richard Cloete of the Institute for Theory & Computation at Harvard University, along with Harvard’s Avi Loeb, as well as the Minor Planet Center’s Peter Vere.
Their collective work – “Non-Gravitational Acceleration and Lower Limits on the Nucleus Mass and Diameter of 3I/ATLAS” – is a September 24th draft version and is available here at: https://lweb.cfa.harvard.edu/~loeb/CLV.pdf
Avi Loeb
(Image Credit: Chris Michel, National Academy of Sciences, 2023)
Alternative technological origin
Avi Loeb is the head of the Galileo Project at Harvard, and never short on stirring up cosmic ponderings about the significance of 31/ATLAS.
The newly published research suggests that 3I/ATLAS is more massive than the other two interstellar objects, 1I/`Oumuamua and 2I/Borisov by 3–5 orders of magnitude, “constituting a major anomaly,” Loeb points out.
“A future detection of a major maneuver of 3I/ATLAS would suggest propulsion by a technologically manufactured engine,” suggests Loeb in a recent essay, available at:
Loeb points out that, if the nucleus diameter of 3I/ATLAS is found to be larger than 5 kilometers in the HiRISE image, “then an origin associated with the interstellar mass reservoir of rocky material will be untenable,” boosting the view by the scientist of an alternative technological origin.
3I/ATLAS as captured August 27 by the Gemini Multi-Object Spectrograph (GMOS) on Gemini South at Cerro Pachón in Chile.
Image credit: International Gemini Observatory/NOIRLab/NSF/AURA/Shadow the Scientist. Image Processing: J. Miller & M. Rodriguez (International Gemini Observatory/NSF NOIRLab), T.A. Rector (University of Alaska Anchorage/NSF NOIRLab), M. Zamani (NSF NOIRLab)
Stay curious
Is 3I/ATLAS an unusually massive comet with an unusual chemical composition on an unusually rare trajectory, Loeb says, or alien technology?
“In both cases, the object could shed CO2 and H2O ices from material that collected on its frozen surface as it plowed through interplanetary and interstellar space. We should not decide about the nature of 3I/ATLAS based on the chemical composition of its skin, for the same reason that we should not judge a book by its cover,” Loeb advises.
“Hopefully, we will know more in the coming weeks. Stay curious,” Loeb concludes. “As Galileo Galilei instructed us, scientific truth is revealed by data, not by authority.”
Posted in Space News
