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"Major Anomaly" As Interstellar Object 3I/Atlas Measured To Be Over 33
Billion Tons
Measuring the object's non-gravitational acceleration, the team believes
they found something "anomalous".
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Edited by Katy Evans

Interstellar object 3I/Atlas zips past a dense star field.
Interstellar object 3I/Atlas zips past a dense star field.

Image credit: International Gemini Observatory/NOIRLab/NSF/AURA/K. Meech (IfA/U. Hawaii). Image Processing: Jen Miller & Mahdi Zamani (NSF NOIRLab)

Anew study has attempted to pin down the properties of interstellar
comet 3I/Atlas, finding it is "anomalously massive" at around 33 billion
tons.

On July 1, 2025, astronomers spotted an object moving through the Solar
System at nearly twice the velocity of previous interstellar visitors rCyOumuamua and Comet Borisov. The object, which was confirmed to be an interstellar comet with its own dusty coma, and suspected to be far
larger than the previous two, with a then-estimated nucleus (the rocky
part of the comet, excluding its coma) of around 5.6 kilometers (3.5 miles).

Sizing comets is a tricky business, primarily because to do so, you need
to distinguish the comet from its coma. As comets approach the Sun in
their orbit and heat up, they outgas, losing gas and later (when they
are even closer to the Sun) dust, which forms their distinctive trail or
coma. This outgassing acts like a thruster, slightly altering the
trajectory, rotation, and speed of the comet.

That can complicate measurements, but it can also provide key clues. In
a new paper, which has not yet been peer reviewed, from Harvard's
Richard Cloete, Avi Loeb, and Peter Vere+i, the team looked at data
compiled by the Minor Planet Center between May 15 and September 23,
2025, from 227 observatories around the world, and compared the object's trajectory to what we would expect from gravitational acceleration alone
(i.e. acceleration caused by the Sun's mass as it approaches closer).


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The team's paper found that the non-gravitational acceleration was
pretty small, at below 15 meters per day squared. That's pretty tiny, considering that we have already seen significant outgassing by the
comet, including using the JWST, with a mass loss rate of around 150
kilograms (330 pounds) per second. To this team crunching the numbers,
that suggests that the object's nucleus is massive, resisting change to acceleration as the Sun-facing side outgasses.

The team estimates that the object weighs over 33 billion tons rCo or 33 trillion kilograms rCo with a nucleus diameter of 5 kilometers (3.1
miles). This is large for a comet, yes, but at 500 trillion tons, or
5|u1017 kilograms (500 quadrillion kilograms), C/2014 UN271 (Bernardinelli-Bernstein) still has it beat. But then again, it has the largest comet nucleus ever seen at 128 kilometers (80 miles) across.

So, where is the anomaly? According to Loeb, the mystery is why we
haven't spotted many more interstellar objects before we spotted one of
this size.

"3I/ATLAS is more massive than the other two interstellar objects, 1I/`Oumuamua and 2I/Borisov by 3rCo5 orders of magnitude, constituting a
major anomaly," Loeb said in a blog post. "Given the limited reservoir
of heavy elements, we should have discovered on the order of a hundred thousand interstellar objects on the 0.1-kilometer scale of 1I/`Oumuamua before finding 3I/ATLAS, yet we only detected two interstellar objects previously."

That's certainly intriguing, if the comet is confirmed to be of this
size. Loeb, as he is known to do, once again proposed the (highly
unlikely) possibility that it may be an alien spacecraft.

"The mass of 3I/ATLAS scales with its diameter cubed. If the nucleus
diameter of 3I/ATLAS will be 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," he added. "An
alternative technological origin could explain the rare alignment of the trajectory of 3I/ATLAS with the ecliptic plane (having a random chance
of 1 in 500, as discussed here), and the detection of nickel without
iron rCo as found in industrially-manufactured alloys."

These claims, as NASA has pointed out, shouldn't be taken too seriously,
with Loeb himself calling it a "pedagogical exercise" in his first paper suggesting it.

rCLIt looks like a comet. It does comet things. It very, very strongly resembles, in just about every way, the comets that we know,rCY Tom
Statler, NASArCOs lead scientist for Solar System small bodies, told The Guardian, responding to the claims.

rCLIt has some interesting properties that are a little bit different from
our solar system comets, but it behaves like a comet. And so the
evidence is overwhelmingly pointing to this object being a natural body. ItrCOs a comet.rCY


Nevertheless, it would be interesting if 3I/Atlas were much more massive
than the other spotted interstellar visitors, and this work suggests it
could be.

We should be able to get a better look at the object as it approaches,
with the potential to observe it using the HiRISE camera onboard the
Mars Reconnaissance Orbiter on October 3, 2025. Frustratingly, it will
be on the other side of the Sun during closest approach and will
disappear from view, popping back up again in December.

Additionally, we now have the Vera C. Rubin Observatory up and running.
Until 2025, astronomers have found around 20,000 new asteroids per year,
but when the observatory began working, it found 2,104 new asteroids in
just 10 hours of observations. With more data, and hopefully more
interstellar objects to look at, we may be able to place more
constraints on this puzzling object, and others like it.

The paper is posted to Harvard's website.

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ORIGINALLY PUBLISHED
3 hours ago
Written by James Felton

Edited
by
Katy Evans


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