The discovery of 3I/ATLAS on July 1, 2025 added a third name to the short list of known interstellar objects, after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. Unlike its predecessors, however, new observations suggest 3I/ATLAS is anomalously big.
Astrometric data compiled by the Minor Planet Center from May 15 to September 23 revealed in the new study that its trajectory is consistent with pure gravitational motion.
Telescopes clearly show 3I/ATLAS shedding material as it moves through space. In most comets, this outgassing acts like tiny rocket thrusters, pushing the body off its predicted course. Yet in this case, no such push can be measured. The only way to explain that silence is if the nucleus is far larger and heavier than expected, massive enough to shrug off the recoil.
“Our analysis compared the evolution in the observed sky positions of 3I/ATLAS (Right Ascension [RA] and Declination [Dec]) to the trajectory expected from gravity alone. The optical astrometric data span 4 022 individual measurements from 227 observatories worldwide,” said Avi Loeb, Professor of Science at Harvard University and head of the Galileo Project.
The comparison to the expected sky positions provided a net upper limit of 0.028 arcseconds on sky position shifts during the 4.5 months between May 15 and September 23, 2025.
and mid-September 2025. Top and bottom panels show RA and Dec residuals respectively. Left panel: Binned residuals
averaged over 8 equal-duration time bins with error bars showing standard error of the mean. Right panel: Individual residual
measurements for all 4 022 observations. Credit: Harvard CfA/Authors
Webb Space Telescope observations from August 6 measured a mass loss rate of 150 kilograms per second and an outflow velocity of 440 m/s. If this material were sufficient to accelerate the nucleus, it would have been observable. But because no such acceleration occurred, the object must have a minimum mass of 33 billion tons, study authors claim.
At a solid density typical of comets, this corresponds to a nucleus with a diameter larger than 5 km (3 miles). That estimate is consistent with the upper bound of the 0.44–5.6 km (0.27–3.4 miles) size range derived earlier from Hubble Space Telescope imagery. It is also an order of magnitude larger than 2I/Borisov and at least 1 000 times bigger than ʻOumuamua.
The Hubble Space Telescope captured an optically thin anti-tail on July 21. This faint feature, pointing toward the Sun, persisted through July and most of August. Later, on August 27, Gemini South detected a faint tail extending away from the Sun.
Despite these changes, the brightest point of emission remained centered on the nucleus, reducing the chance that centroid shifts affected the astrometry.
Taken together, the observations show that the comet is indeed losing mass. But because this loss does not produce a detectable change in speed, it points to a nucleus that is unusually big.
Models of interstellar object ejection predict that smaller fragments, on the order of 0.1 km (330 feet), should be much more common than kilometer-scale (0.62 miles) bodies. By those estimates, astronomers should have detected hundreds of thousands of smaller ʻOumuamua-like objects before encountering one as large as 3I/ATLAS.
Yet, to date, only two smaller interstellar objects have been recorded.
Two upcoming encounters may provide us the true size of 3I/ATLAS, and clarify whether this object is an extreme outlier within known physics, or something beyond.
On October 3, 2025, the Mars Reconnaissance Orbiter’s HiRISE camera will image the object at a pixel scale of 30 km. The brightness of the nucleus within the brightest pixel, corrected for albedo, should refine estimates of surface area.
Later, on March 16, 2026, the Juno spacecraft will pass near the object in the vicinity of Jupiter. Instruments aboard Juno may be able to measure its brightness and possibly resolve details of its surface activity.
References:
1 Upper limit on the non-gravitational acceleration and lower limits on the nucleus mass and diameter of 3I/ATLAS – Cloete, Loeb & Veres – September 2025 – Harvard CfA – Draft version accessed September 25
2 News on 3I/ATLAS: Lack of Non-Gravitational Acceleration Implies an Anomalously Massive Object – Avi Loeb – September 24, 2025