The mysterious comet 3I/ATLAS appears to be extremely large, making it orders of magnitude more massive than two other confirmed interstellar objects observed in our solar system in years past, a new study suggests.
Based on a new analysis of the most precise tracking data collected on the object since its discovery in July, the interstellar comet appears to be “anomalously massive,” a finding that raises new questions regarding our expectations about interstellar objects that occasionally traverse our solar system.
The research was detailed in a new paper by Richard Cloete, Peter Vere, and Harvard astronomer Avi Loeb, which places new limits on the object’s apparent non-gravitational acceleration and overall mass.
The new findings are largely consistent with existing upper estimates for the unusual space object’s size, with some recent observations suggesting it could be up to three miles long.
An “Anomalously Massive” Interstellar Object
More than four months of optical measurements collected on 3I/ATLAS, compiled by the Minor Planet Center, were used by the team for their analysis, which compared 3I/ATLAS’s position to the trajectory that would be expected based on gravity alone.
Based on their results, the team found almost no measurable deviation from a gravitational path, which seems to point to the fact that the interstellar comet is experiencing some degree of non-gravitational acceleration, albeit a negligible amount.
Image Processing: J. Miller & M. Rodriguez (International Gemini Observatory/NSF NOIRLab), T.A. Rector (University of Alaska Anchorage/NSF NOIRLab), M. Zamani (NSF NOIRLab).
This finding would also seem to suggest that the object’s nucleus must possess a minimum diameter of roughly five kilometers (slightly more than three miles), which makes it several orders of magnitude more massive than the first two known interstellar objects, ʻOumuamua and Borisov.
Between May and September, more than 4,000 astrometric measurements related to the motion of 3I/ATLAS were collected from 227 observatories around the world. Based on this data, an upper limit on non-gravitational acceleration for the interstellar comet could be calculated, which was less than 15 meters per day squared.
According to the team’s study, this finding, with consideration for the principle of momentum conservation, suggests that gas jets produced on the surface of the object as it continues to be warmed by heat from the Sun should result in a measurable push. Based on current observations, however, the lack of detectable deviation exhibited by the object appears to imply that 3I/ATLAS is quite massive.
Webb Data Suggests a Massive Object
Additional data obtained from observations in early August by the James Webb Space Telescope reveal an estimated mass loss for 3I/ATLAS of around 150 kilograms per second, with a calculated gas outflow of about 440 meters per second. These values, when factored into momentum-balance equations, indicate that the comet’s nucleus likely possesses a minimum mass of 33 billion tons; this would mean that the comet would likely be at least five kilometers (slightly more than three miles) in diameter, if it is indeed composed of solid, ice-rich material.
The team’s newest estimate for the diameter of 3I/ATLAS is close to previously reported higher-range estimates for its size based on images obtained by the Hubble Space Telescope, with both estimates now appearing to indicate the object is larger than its two known predecessors.
Additionally, the team reports that the object’s nucleus has remained its brightest and most stable point in imagery collected since its discovery, which includes recent images obtained by Gemini South in late August, which first revealed the appearance of a conventional cometary tail.
This, the researchers say, offers additional support for the conclusion that 3I/ATLAS is extremely massive, since it appears to easily resist being deviated from its present course by outgassing from its Sun-facing side observed by astronomers over the last several weeks.
In an email to The Debrief, Avi Loeb said the fact that 3/ATLAS appears to be so much more massive than the first confirmed interstellar object, ‘Oumuamua, makes it very unusual, especially when considering astronomers’ expectations about the limited amount of rocky material that should proliferate throughout interstellar space.
“We should have detected an order of 100,000 ‘Oumuamuas before discovering an object as big as 3I/ATLAS,” Loeb told The Debrief, pointing to a paper he published in July, which initially suggested that 3I/ATLAS was either smaller or perhaps rarer than it looks. Loeb’s July paper, he explains, “showed that we should have found such an object less than once per ten thousand years.”
Although he and his colleagues place new constraints on the object’s size in their recent paper, the latest observations offer few additional clues about the interstellar object’s composition than what could be gleaned during earlier observations by NASA’s James Webb telescope and SPHEREx infrared space observatory.
“The only hint about the composition is from the JWST and SPHEREx spectroscopy which indicated CO2 (87%), CO (9%) and H2O (4%) mass fractions,” Loeb told The Debrief. “VLT spectroscopy indicated rapidly rising trace amounts of cyanide and nickel without iron.”
“The lower limit that I derived in the new paper on the mass and size does not constrain the composition,” Loeb said.
Overall, based on the recent astrometric data Loeb and his colleagues analyzed for their study, Loeb says that the object’s surprisingly large mass makes it all the more intriguing.
“Given that massive objects are rare, the alignment of the trajectory of 3I/ATLAS with the orbital plane of the planets around the Sun (to within 5 degrees) is even more tantalizing,” Loeb told The Debrief. “Why would the first massive object possess this alignment?”
Controversial Possibilities
In recent weeks, Loeb has frequently suggested an intriguing—and controversial—possibility: that 3I/ATLAS might be something more complex than just a comet. As support for this unconventional view, the Harvard astronomer points not only to the object’s unusual mass but also to its trajectory, which brings it close to Jupiter and Mars, as well as the jet-like glow the object exhibited during observations in July and August, and notably, the detection of nickel and iron in spectrographic observations by the Webb Telescope and SPHEREx missions.
“The detection of nickel without iron is only known to exist in industrially produced alloys, made through the artificial carbonyl chemical pathway,” Loeb told The Debrief. “Each of these anomalies has a small probability given our knowledge base on solar system comets and the interstellar comet 2I/Borisov.”
“If you combine all of them, the probability is extremely small for it to belong to their class,” Loeb maintains.
Such anomalies, he contends, leave open the possibility that while 3I/ATLAS may exhibit several comet-like attributes, it could in fact be something else entirely. For Loeb, this might even include the remote chance that astronomers are seeing an object of technological origin.
However, most experts are hesitant to entertain such ideas. Tom Statler, NASA’s Lead Scientist for Solar System Small Bodies in the Planetary Science Division at NASA’s Washington Headquarters, recently told The Guardian that while 3I/ATLAS does exhibit several interesting properties, “it behaves like a comet,” adding that “the evidence is overwhelmingly pointing to this object being a natural body.”
“It’s a comet,” Statler affirmed.
Lingering Questions
While the debate over 3I/ATLAS is likely to continue as the object makes its closest approach to Mars on October 3, current observations have raised several intriguing questions.
For instance, such a large nucleus introduces a puzzle in terms of our expectations about interstellar object populations. If comets like ‘Oumuamua are more common than previously expected, and they are on what may be the smaller end of the interstellar object scale, this would seem to imply that astronomers should have detected as many as tens of thousands of them before detecting a three-mile-long object like 3I/ATLAS.
Loeb and his colleagues suggest that this may indicate, at very least, that current models of planetary system debris are far from complete. Additional observations in the days ahead—namely those that will be possible using the HiRISE camera on board NASA’s Mars Reconnaissance Orbiter—could provide crucial new data on the 3I/ATLAS’s surface area. Additionally, next March, the object will pass close enough to Jupiter that additional data may be obtained using NASA’s Juno spacecraft.
If these flybys collect the data needed to place additional constraints on the object’s size and other characteristics in the days ahead, it could raise further questions for astronomers that strain our present understanding of the interstellar reservoir of rocky material that is believed to exist out there.
For Avi Loeb, the prospect of obtaining such new information is an exciting one.
“I can’t wait for this data!” Loeb told The Debrief.
“The way to tell the difference between a dogmatist and a curious scientist is by flooding them with data,” Loeb added.
Despite the controversy that 3I/ATLAS has managed to generate since its discovery, one thing that almost everyone agrees on is that it defies many of our expectations about how such objects should behave, and thereby causes us to have to reconsider our preconceptions about the kinds of objects that populate the vastness of interstellar space, as well as how many of them may be lurking out there in the cold, silent darkness between stars.
The team’s new paper, “Upper Limit on the Non-Gravitational Acceleration and Lower Limits on the Nucleus Mass and Diameter of 3I/ATLAS,” is currently available online on the website of the Harvard & Smithsonian Center for Astrophysics.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.