Interstellar Object 3I/ATLAS’s Tail Appears To Have Changed Direction

On July 1, astronomers at the Asteroid Terrestrial-impact Last Alert System (ATLAS) spotted an object speeding through the Solar System on an escape trajectory. It was soon found to be an interstellar visitor – the third we have caught so far after 1I/’Oumuamua and 2I/Borisov. 

Since then, astronomers have been tracking 3I/ATLAS and have found a number of unusual features of the comet, which may be a 10-billion-year-old time capsule from an earlier age of the universe.

 

One unusual and almost unheard of feature seen on the object is an “anti-tail“, observed during July and August. 

As comets approach the Sun, the volatile ices within them sublimate (turn from solid to gas without an intermediate liquid phase), lifting gas and dust grains off the surface. As the comet travels, radiation pressure and the solar wind push the dust and gas away from the Sun’s direction, creating a distinctive cometary coma behind it.

Or at least, that’s what we see in most comets. In some comets, in addition to the tail and the ion tail, it is possible to see an “anti-tail”, pointing towards the Sun. Generally, these are an optical illusion, the result of our position, but very rarely, it can be a “true” anti-tail.

“Occasionally, a comet can be seen also showing a ‘third’ tail apparently pointing towards the Sun. This is known as an ‘anti-tail’ and is an optical illusion,” the European Space Agency explains of one type. “The tail is really pointing directly away from the Sun, but because of the Earth’s relative location in space, we see the wide tail, fanning out behind the comet and appearing to be on either side of the central region containing the nucleus.”

3I/ATLAS has this second type of anti-tail.

“Observations of interstellar comet 3I/ATLAS at 3.8 au show an elongated coma similar to a cometary tail but pointing in the direction of the Sun,” a preprint paper by Harvard astronomers Avi Loeb and Eric Keto explains, adding “this type of anti-tail, not a result of perspective, may not have been previously observed.”

However, as pointed out by Jason Wright, professor of astronomy and astrophysics at Penn State, these anti-tails have been observed before, dating back to at least 1974 with observations of Comet Kohoutek.

“With a rotating comet nucleus; ejecta from a spot can come off with heliocentric velocity that puts it either in front of or behind the nucleus – it does not matter which side it starts from,” Michael Busch, NSF Astronomy and Astrophysics Postdoctoral Fellow, explained on BlueSky.  “Small dust and ejected gas gets pushed out by radiation pressure and solar wind. But larger pieces of ejecta spread out along the orbit; both in front of and behind the nucleus. For solar system comets, this eventually gives a meteoroid stream around the entire orbit.”

While anti-tails are not unheard of, they are still rare and pretty interesting. And monitoring 3I/ATLAS as it approaches the Sun can tell us a lot about the object and the environment it came from.

“It is statistically unlikely that 3I has approached any star more closely than it will approach the Sun in 2025, at least since the (presumed) ejection from its parent protoplanetary disk,” the authors of a new paper, which has not yet been peer reviewed, explain. “Therefore, pre-perihelion observations present an opportunity to study the rise of activity on an object that has been held at interstellar temperatures (≲10 K) possibly for billions of years.”

Observing the comet before it went out of view from our perspective (though it couldn’t dodge our Mars orbiters), the team found that the anti-tail developed into a tail in September.

“The morphology of the comet changes from a Sun-facing dust fan in the early 2025 July observations, to one dominated by an antisolar dust tail at later dates,” the team explains, adding that independent spectroscopic observations indicate that carbon dioxide is the primary driver of activity on the object.

“We attribute the delayed emergence of the tail to the large size (effective radius 100 µm) and slow ejection (5 m s^-1) of the optically dominant dust particles, and their consequently sluggish response to solar radiation pressure.”

In a separate paper, Harvard astronomers Avi Loeb and Eric Keto suggest their own mechanism behind the slow evolution of anti-tail to tail.

“As 3I/ATLAS approaches the Sun, the exponential temperature dependence of the sublimation rate causes a continuous increase in the production rate of ice fragments and a sharp decline in their residence time in the outflow, The combined effects produce a peak in total scattering cross-section due to H2O ice grains at a distance of 3 to 4 au from the Sun,” Loeb explains in an accompanying blog post. “At closer heliocentric distances, the scattering becomes dominated by longer-lived refractory dust particles and larger volatile grains with survival times long enough to form a tail that stretches away from the Sun.”

Loeb points out that from July to October, the comet has lost around 2 million tons in mass, around 0.00005 percent of its total mass, using Loeb and colleagues’ estimate of 33 billion tons. 

As this is Loeb, who has a history of suggesting aliens as a possibility for natural phenomena, he goes on to suggest that this change from an anti-tail to a tail could be a sign that the object is technological in nature.

“My colleague, Adam Hibberd, pointed out that if the object is an alien spacecraft slowing down, and the anti-tail is braking thrust, then this change from anti-tail to tail would be entirely expected near perihelion,” he wrote. “In that case, the transition would constitute a technosignature in the form of an unexpected phenomenon indicative of controlled maneuvering, possibly with the intention of achieving a bound heliocentric orbit between Mars’s and Jupiter’s orbits.”

While pretty much all other astronomers, SETI, and NASA are confident that the object is natural, given its cometary behavior, we will soon be able to test this “alien mothership” scenario. In December, the object will be back in view of our telescopes. When it continues along its path out of the Solar System, the alien spacecraft idea will look very silly indeed. But hopefully we can take many more observations of the object before it departs, and learn more about this ancient and well-traveled comet.

The paper, which is yet to be peer-reviewed, is posted to the preprint server arXiv.