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3I/ATLAS Sheds Particles that Are Much Bigger Than Common Sunlight-Scattering Dust


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Images of the tightly-collimated anti-tail jet from 3I/ATLAS in the sunward direction (lower left), as processed through a Larson-Sekanina gradient filter on December 15, 2025 (top) by Toni Scarmato, and on December 13, 2025 (bottom) by Teerasak Thaluang.

Recent images of the interstellar object 3I/ATLAS show a tightly collimated anti-tail jet that extends out to at least 100,000 kilometers in length towards the Sun.

It is commonly assumed that the glow in this jet results from the scattering of sunlight by dust particles. Here we derive constraints on the characteristic size of these particles, assuming that they originate from a natural comet.

If these particles start with an initial velocity V, then they can reach a distance L before being stopped by the solar radiative deceleration A:

L=V²/(2A).

The solar radiation force on spherical particles with a radius of R larger than the wavelength of sunlight is of order the momentum flux carried by sunlight at a heliocentric distance r, namely: ~L/(c*4*pi*r²) where L is the solar luminosity and c is the speed of light, times the cross-sectional area of the particle, namely: (pi*R²).

At a heliocentric distance of r~2au, where the recent images were taken, the solar deceleration for a particle with solid density of 1g/cm³ is:

A~ (0.1 cm/s²)*(R/1micron)^{-1}.

Therefore, to reach L~100,000km, the initial velocity of these dust particles must be

V~(450m/s)*(R/1micron)^{-1/2}.

For sub-micron dust particles, this velocity exceeds the thermal speed of gas particles at that heliocentric distance and is therefore untenable. Such a velocity cannot be reached if 3I/ATLAS is a natural comet, even if the dust is perfectly dragged by outflowing gas from the sublimation of volatiles on the surface of an icy rock by sunlight. The solar deceleration scales inversely with particle radius and should be larger for smaller particles with a radius comparable to the characteristic wavelength of sunlight, ~0.5micron.

In conclusion, the observed length of the anti-tail generated by 3I/ATLAS can only be sustained by particles with a radius much larger than common dust particles which have the largest surface area per unit mass for scattering sunlight and usually dominate the glow around solar-system comets. 3I/ATLAS is therefore anomalous in the size of the particles that dominate the glow of its sunward jet.

The anti-tail glow of 3I/ATLAS is dominated by particles that are bigger than the typical dust particles that scatter sunlight most efficiently. This is required to explain why the sunward jet of 3I/ATLAS is much longer than observed in familiar comets. From its discovery, the observed anti-tail jet was inferred to be 10 times longer than it was wide — after correcting for its projection angle of 10 degrees in the Hubble image from July 21, 2025 (as reported here and analyzed here).

The tight collimation of the anti-tail jet of 3I/ATLAS might be the result of the release of large particles from a very small fraction of the surface of the nucleus.

In addition, there is also an upper limit on how big these anti-tail particles can be. Much larger particles have a smaller surface area per unit mass and cannot be easily dragged by gas to the required high-speed V.

The mass loss from the Sun-facing side of 3I/ATLAS was estimated from data obtained by the Webb telescope (as reported here) at a value of Mdot~150kg/s and could have grown by a factor of a few near perihelion. For our calculation, we will adopt an enhanced value of Mdot~500 kg/s for the post-perihelion anti-tail, observed over the past couple of months. Given this mass loss rate, the outflowing gas density D declines inversely with the square of the distance d from the nucleus of 3I/ATLAS, according to the relation:

D=Mdot/(O*V*d²)

where O is the conical solid angle of the outflowing sunward jet. The recent images of 3I/ATLAAS imply that the anti-tail jet is collimated to within a cone with an opening angular radius of about 8 degrees — corresponding to O=0.06. Using this value along with the derived expression for V, gives a jet mass density as a function of distance d:

D~(2x10^{-8}g/cm³)*(R/1micron)^{1/2}*(d/1km)^{-2}

The time required for drag to bring a solid-density particle of radius R to the velocity of the outflowing gas V is:

~8R/[V*(D/1g/cm³)] ~ (1s)*(R/1micron)*(d/1km)^{2}

This drag time scales as (R*O/Mdot)*d² and does not depend on the initial outflow speed of the gas V (which eventually is assumed to be matched by the particles through drag).

This drag time must be shorter than the jet dilution time over which the outflowing gas gets rarefied with distance d from the nucleus:

~(d/V)=(2s)*(d/1km)*(R/0.1micron)^{1/2}.

This inequality is only possible for particles that are sufficiently small, with:

(R/100micron) < (d/200m)^{-2}.

For the typical particle radius to be larger than 100 microns, the nucleus radius must be smaller than 200 meters. This is not allowed by the lower limit on the nucleus radius, as derived here from the Hubble image data. Adopting a more modest value of Mdot near perihelion would reduce the limit on the maximum particle size.

In conclusion, the radius of the particles in the outflow from 3I/ATLAS must be bigger than 1 micron in order for them to reach the observed length of the anti-tail jet and smaller than 100 microns in order for them to reach the required jet speed through drag on the outflowing gas. These requirements apply to a natural comet where the anti-tail is sourced by sublimated ice, but not to an exotic source — like the exhaust of a rocket that could endow the particles and gas with an arbitrary ejection speed.

Further constraints on the typical particle size could be derived through a direct measurement of the velocity V of the sunward jet by a spectroscopic measurement of its Doppler shift relative to the nucleus of 3I/ATLAS.

***

As we attempt to quantitatively characterize the interstellar object 3I/ATLAS, we must keep in mind what Erwin Schrödinger, a prominent founder of quantum mechanics, said about scientific insights:

“The task is not so much to see what no one has yet seen; but to think what nobody has yet thought, about that which everybody sees”.

***

This morning, before my routine 3-mile jog at sunrise, I received the following message:

“Dr. Loeb,

Thank you so much for all of your excellent 3I/ATLAS coverage! It has been an absolute thrill to follow along with each new piece of mind bendingly perplexing data we’ve been gathering on this interstellar object!

I’ve always been interested in what’s out there beyond the stars, but never have I been so captivated by any space object as I am by 3I/ATLAS. I love that it’s forcing people to ask difficult questions and rethink what’s possible in regards to how physics work in solar systems beyond our own.

Thank you for having the courage to share your hypothesis and thoughts on what 3I/ATLAS might be, and all of the different possibilities. Your ability to vividly imagine reminds me of this beautiful Einstein quote:

“Imagination is more important than knowledge. For knowledge is limited to all we now know and understand, while imagination embraces the entire world, and all there ever will be to know and understand.”

Take care and have a great New Year!

Respectfully,
Drew”

ABOUT THE AUTHOR

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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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