3I/ATLAS Exhibits Non-Gravitational Acceleration Near Sun, Deepening Interstellar Mystery

Unseen Forces: Interstellar Object 3I/ATLAS Defies Gravity Near Perihelion

A groundbreaking observation made in late October 2025 has provided the first definitive evidence that the third confirmed interstellar object, 3I/ATLAS, is being influenced by forces other than the Sun’s gravity. As the object reached its closest point to the Sun (perihelion), researchers detected a measurable non-gravitational acceleration, a finding that strongly suggests 3I/ATLAS is a volatile, comet-like body, despite its lack of a visible coma or tail.

This discovery, documented by researchers including kwalsh4a and Marshall Eubanks, utilized a stack of 134 images captured by the PUNCH WFI3 satellite on October 29, 2025. The detection of this unexpected thrust adds 3I/ATLAS to the exclusive list of interstellar visitors—alongside the enigmatic ‘Oumuamua—that exhibit behavior challenging conventional astronomical classification.

The Anatomy of the Discovery: Observing the Unseen Thrust

When astronomers track celestial bodies, they rely on Newtonian mechanics, calculating trajectories based purely on gravitational interactions. If an object deviates from this predicted path, it indicates the presence of a non-gravitational force. For solar system comets, this force is well understood: it’s the result of sublimation.

Sublimation occurs when solar heating causes frozen volatile materials (ices like water, carbon dioxide, or carbon monoxide) beneath the surface to turn directly into gas. This gas streams away from the nucleus, creating a jet-like effect—a natural rocket thrust—that subtly pushes the object off its purely gravitational course. This process forms the characteristic coma and tail of a comet.

The 3I/ATLAS Anomaly

The observation of 3I/ATLAS near its perihelion on October 29, 2025, revealed a clear acceleration that could not be explained by gravity alone. This finding is significant for several reasons:

• Confirmation of Volatiles: The acceleration confirms that 3I/ATLAS contains volatile materials that are actively sublimating under solar heat, classifying it dynamically as a comet, even if visually it appears asteroidal.
• Precision Measurement: The PUNCH WFI3 satellite provided the necessary precision to detect this subtle but measurable deviation from its gravitational orbit.
• Lack of Coma: Crucially, 3I/ATLAS did not display the bright, visible coma or tail typically associated with active comets. This suggests that the volatile material is either deeply buried, or the outgassing is highly localized and directed, or perhaps involves less common, heavier molecules that don’t produce a large, visible dust cloud.

This discrepancy—acceleration without visible outgassing—is the core mystery that links 3I/ATLAS back to the most famous interstellar visitor, ‘Oumuamua.

The Interstellar Trio: Comparing the Visitors

3I/ATLAS is the third confirmed object known to have originated outside our solar system, following 1I/’Oumuamua (2017) and 2I/Borisov (2019). Each object has offered unique insights, but 3I/ATLAS helps solidify a pattern of unusual behavior among these galactic travelers.

The Shadow of ‘Oumuamua

‘Oumuamua’s trajectory was famously altered by a non-gravitational force, yet it showed no evidence of outgassing, leading to intense speculation about its composition and origin—including the controversial hypothesis that it might be an artificial object propelled by solar radiation pressure.

While 2I/Borisov behaved like a textbook comet, 3I/ATLAS now presents a middle ground: it shows the dynamic signature of a comet (the acceleration) but shares the visual characteristic of ‘Oumuamua (the missing coma).

This suggests that the mechanism driving the acceleration in both ‘Oumuamua and 3I/ATLAS might be related to the release of volatiles that are either invisible to standard optical telescopes or are released in such small quantities that they do not form a detectable dust cloud.

According to analysis of the acceleration magnitude, the force acting on 3I/ATLAS is consistent with what would be expected from the outgassing of common cometary ices. The challenge remains explaining why this activity is not producing a visible signature.

The Role of the PUNCH Mission

The detection of this subtle acceleration highlights the increasing capability of specialized space instruments. The Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission, which provided the crucial WFI3 data, is primarily designed to study the solar wind and the Sun’s corona. However, its wide field of view and sensitive detectors make it an ideal tool for tracking objects that pass close to the Sun, where the glare usually obscures ground-based observations.

Potential Explanations for the Missing Coma

If the acceleration is indeed caused by sublimation, astronomers are left with a few leading hypotheses to explain the absence of a visible coma:

1. Heavy Volatiles: The outgassing might involve molecules heavier than water ice (H₂O), such as nitrogen or carbon monoxide, which sublimate at lower temperatures and might not carry enough dust to create a bright, visible cloud.
2. Small Nucleus: If the nucleus of 3I/ATLAS is exceptionally small, the total amount of gas released might be insufficient to form a large, detectable coma, even if the relative acceleration is significant.
3. Dust Depletion: The object may have been stripped of its surface dust during its long journey through interstellar space, meaning the escaping gas jets carry very little reflective material.
4. Directed Venting: The volatile release could be highly concentrated in a few small, narrow jets, rather than a broad, diffuse cloud, making the overall coma faint.

Key Takeaways: What This Means for Interstellar Science

The confirmation of non-gravitational acceleration in 3I/ATLAS is a critical piece of evidence in the ongoing effort to understand the nature of objects ejected from other star systems. It reinforces the idea that many interstellar visitors are likely volatile, comet-like remnants.

• Cometary Nature Confirmed: Dynamically, 3I/ATLAS behaves like a comet, strengthening the hypothesis that the interstellar medium is populated by icy planetesimals.
• The ‘Oumuamua Link: The shared characteristic of non-gravitational acceleration without a visible coma suggests a common, yet still poorly understood, physical mechanism at play for these two objects.
• New Observational Techniques: The successful use of the PUNCH satellite demonstrates the value of leveraging solar observation instruments to track fast-moving objects near the Sun.

Conclusion

The detection of the non-gravitational thrust on 3I/ATLAS in late 2025 provides compelling evidence that this object is actively shedding material as it leaves our solar system. While the underlying physics—why the outgassing is ‘dark’—remains a subject of intensive study, this finding pushes astronomers closer to defining the typical characteristics of objects born around distant stars. The interstellar mystery continues, now with a third, highly intriguing data point.