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If 3I/ATLAS is a Comet, then Its Anti-Tail Jet Should Not Include Streaming Gas Beyond ~5,000…


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If 3I/ATLAS is a Comet, then Its Anti-Tail Jet Should Not Include Streaming Gas Beyond ~5,000 kilometers

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False-color image of 3I/ATLAS (top panel), taken on December 27, 2025 by a 0.2-meter telescope in Belgium. The field of view is 14.4 by 23.3 arcminutes, corresponding to 1.1 by 1.8 million kilometers. Another image from the same telescope on December 19, 2025 which was processed through the Larson-Sekanina rotational gradient filter (bottom panel), shows a prominent anti-tail jet towards the Sun, as indicated by the yellow line in the inset. (Image credit: Alfons Diepvens)

During the past two months after perihelion, the anti-tail jet from the interstellar object 3I/ATLAS was observed to extend out to a distance of several hundred million kilometers towards the Sun. In a previous calculation reported here and here, I showed that dust particles with a radius of order 10 micron could be dragged to a velocity of hundreds of meters per second by gas near their launch base and then reach a scale of several hundred million kilometers before being decelerated by solar radiation pressure. Will the outflowing gas accompany these dust particles out to the same distance or get pushed back more forcefully?

Whereas the main pushback on dust stems from radiation pressure by sunlight, the main pushback on gas originates from the solar wind. Before being slowed-down, the mass density of the outflowing gas in the jet, D_j, declines inversely with the square of distance, d:

D_j=Mdot/(2*pi*V*d²),

where Mdot and V are the mass loss rate and velocity of the sunward jet. The total mass loss was estimated from data obtained by the Webb telescope before perihelion (as reported here) at a value of Mdot~150 kg/s and most likely grew by a factor of a few near perihelion. I therefore adopt an enhanced value of Mdot~500 kg/s for the post-perihelion anti-tail.

In the context of 3I/ATLAS being a natural comet, volatiles from ices heated by sunlight yield a maximum outflow speed of the gas that is comparable to the thermal speed of the dominant constituent of CO2 molecules, V=0.2 km/s, as dictated by the surface temperature of ~200 degrees Kelvin at the current heliocentric distance of 3I/ATLAS of about twice the Earth-Sun separation (AU).

This gives the gas mass density of the anti-tail as a function of distance from 3I/ATLAS:

D_j~(1.6x10^{-17} g/cm³)/(d/5,000 km)²

The solar wind carries about 3x10^{-14} solar masses per year per 4*pi steradian and flows at a speed of about v~500 km/s, providing a wind mass density at a heliocentric distance of ~2 AU:

D_w~3x10^{-24} g/cm³.

The jetted gas is expected to be stopped at a distance from 3I/ATLAS where its ram pressure (1/2)D_j*V² is balanced by the ram-pressure of the solar wind, (1/2)D_w*v². This occurs at a stopping distance:

d_s~5,000 km.

Interestingly, this is roughly the traverse radius of the glowing halo (coma) around the nucleus of 3I/ATLAS in the images of 3I/ATLAS (including the Hubble Space Telescope images listed here). As the solar wind sweeps up the gas, it carries it together with sub-micron dust particles away from the Sun along the tail of 3I/ATLAS. However, the large dust particles above 10-microns continue to stream along the anti-tail out to a scale that is ~10 times longer in the direction of the Sun. On that scale, the jet is expected to be gas-free if 3I/ATLAS is a natural comet where gas is launched at a speed dictated by the sublimation of CO2 ice on the surface of a natural rock and limited to ~200 meters per second.

This result presents a clean test for the nature of 3I/ATLAS: if it is a natural comet, then the anti-tail jet should not include streaming gas beyond a distance of 5,000 kilometers from the nucleus. At distances much larger than 5,000 kilometers from 3I/ATLAS as a comet, the anti-tail should be composed primarily of a stream of 10-micron dust particles with no streaming gas.

However, if the launch speed of the anti-tail jet is set by a technological thruster, then the extent of the streaming gas could reach much larger scales towards the Sun:

1. For chemical-propellant thrusters with an exhaust speed of V=5 km/s, the streaming gas could extend out to d=25,000 kilometers.

2. For ion thrusters with an ejection speed of V=90 km/s, the streaming gas could extend out to d=100,000 kilometers.

The existence of streaming gas along the anti-tail can be tested through tagging a molecular tracer like CO2 or CO along the axis of the anti-tail jet and plotting the spatial profile of the tracer relative to scattered sunlight from dust inside the jet.

Here’s hoping that such data will be collected by ground-based telescopes, like Keck, VLT or ALMA, or by space observatories like SPHEREx or the Webb telescope.

Science is a learning experience. The best way to learn is by observing nature rather than forcing it to a popular narrative.

ABOUT THE AUTHOR

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(Image Credit: Chris Michel, National Academy of Sciences, 2023)

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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