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The interstellar object 31/ATLAS continues to puzzle astronomers, as a newly identified anti-tail anomaly raises fresh questions about its physical nature and possible origins. According to Harvard University astronomer Avi Loeb, the unusual glow of the sunward-facing jet of 31/ATLAS is dominated by particles far larger than those typically seen in ordinary comets.

These findings challenge long-standing assumptions about how cometary dust behaves.

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Anomalous Particle Sizes in 31/ATLAS

In a recent post on his personal blog, Avi Loeb explained that 31/ATLAS is “characterized by an anomalous size of the particles that dominate the glow of its jet facing the Sun.” Unlike familiar comets, whose brightness is driven by tiny dust grains that efficiently scatter sunlight, the anti-tail of 31/ATLAS appears to be composed of much larger particles.

Loeb emphasized that this distinction is critical. “Strictly speaking,” he wrote, “the anti-tail structure of 31/ATLAS is dominated by particles larger than typical dust grains, which are usually the most efficient at scattering sunlight.” This alone sets the object apart from nearly every comet previously observed within our own solar system.

Readers can explore Loeb’s ongoing analysis at his official blog, Science and Technology: Notes from the Frontier on Medium:
https://avi-loeb.medium.com/

Why the Anti-Tail Defies Expectations

The anti-tail of 31/ATLAS is especially intriguing because it points toward the Sun rather than away from it. Under normal circumstances, solar radiation pressure and the solar wind push dust and gas outward, forming a tail that extends in the opposite direction.

“This is uncharacteristic for comets,” Loeb noted, adding that the phenomenon was not discussed during a NASA conference, despite its implications. NASA’s general background on comet behavior underscores how unusual this is:
https://science.nasa.gov/solar-system/comets/

The most recent observations show a tightly collimated anti-tail jet extending at least 400,000 kilometers toward the Sun. From its discovery, the jet was inferred to be roughly ten times longer than it is wide, with a projection angle of about 10 degrees in images analyzed from the Hubble Space Telescope:
https://hubblesite.org/

An Ancient Visitor from Another Star System

The interstellar object 31/ATLAS was first detected on July 1 and later identified as a comet originating from another stellar system. Its coma—the cloud of gas and dust surrounding its nucleus—measures approximately 24 kilometers in diameter. 

Computer modeling suggests an age exceeding 7.5 billion years, making it roughly three billion years older than the Sun and potentially the oldest comet ever observed.

Such antiquity may help explain some of its strange properties, but it does not fully account for the unusual particle sizes required to sustain the observed anti-tail glow.

Constraints on Particle Size and Jet Formation

According to Loeb’s analysis, the observed length of the anti-tail can only be maintained by particles with radii much larger than common dust grains. However, there is also a physical upper limit. If the particles were significantly larger, they would possess too little surface area per unit mass to be accelerated by gas to the high speeds required.

“The tight configuration of the anti-tail jet,” Loeb suggested, “might be the result of the release of large particles from a small fraction of the surface of the nucleus.” This suggests localized and possibly episodic activity on 31/ATLAS, rather than uniform outgassing.

From Color Changes to Artificial Light?

Adding to the intrigue, Loeb reported in September that 31/ATLAS had undergone “anomalous evolution,” including a noticeable color change. Earlier, in August, he suggested that the object might emit its own light “similar to car headlights,” a phenomenon for which no natural explanation has yet been confirmed.

He went so far as to speculate that 31/ATLAS could be an artificial object powered by a strong energy source capable of producing light visible from millions of miles away. This extraordinary claim continues to spark debate within the scientific community.