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KEY FINDINGS

• When 3I/ATLAS passed closest to the Sun in October 2025, astronomers anticipated that the encounter could dramatically alter how the object behaves.
• Perihelion passages often reshape comets and interstellar visitors, sometimes changing their rotation, triggering new jets, or destabilizing their structure.
• According to new analysis published by Avi Loeb, 3I/ATLAS appears to have emerged from that encounter spinning at nearly the same pace — about once every seven hours.

Avi Loeb’s latest findings suggest the interstellar object’s spin remained stable through its closest solar encounter, raising new questions about its internal structure and jet behavior.

[USA HERALD] - In a recent Medium post, Loeb reports that post-perihelion observations of 3I/ATLAS indicate a rotation period of approximately 7.1 hours, based on two independent observational techniques. The findings are drawn from a new paper co-authored with astronomer Toni Scarmato and rely on both jet orientation measurements and brightness variability.

One method tracks periodic changes in the orientation of jets emitted from the object as it rotates. The second examines rhythmic variations in the object’s overall brightness, which includes light scattered by the surrounding coma and jet structures rather than the nucleus itself.

Using images captured by the Hubble Space Telescope between November 20 and December 27, 2025, the researchers measured subtle “wobbles” in the position angle of the anti-sunward jet. Those wobbles, shifting by roughly ±20 degrees, repeated with a period of about 7.2 hours.

Separately, brightness data collected between December 9 and December 22, 2025 from the MPC L92 observatory in Calabria, Italy showed periodic fluctuations of roughly ±30 percent. That signal yielded a nearly identical period of 7.136 hours.

According to Loeb, the close agreement between the two methods supports a post-perihelion rotation period of about seven hours, suggesting that the object’s spin rate did not undergo a major change during its closest approach to the Sun on October 29, 2025.

What did change, however, was how that rotation manifests observationally.

Jet Precession and an Unusual Alignment

Loeb notes that the jet does not remain fixed in direction but instead appears to precess around the rotation axis, producing both the observed jet wobble and the brightness variations. The average jet orientation points near 270 degrees on the sky, while the Sun–3I/ATLAS direction lies near 290 degrees.

This implies that the object’s rotation axis is aligned with the Sun–object axis to within roughly 10–20 degrees — an alignment Loeb describes as statistically unlikely to occur at random for an object that formed in interstellar space.

That alignment matters because earlier Hubble observations showed that less than one percent of the observed light from 3I/ATLAS comes from the solid nucleus. Most of the brightness originates in the surrounding coma. As a result, the seven-hour brightness cycle is not thought to reflect sunlight bouncing off a rotating solid body, but rather the changing geometry of dust and gas jets.

In Loeb’s interpretation, the rotating nucleus drives jets that sweep through space like a rotating sprinkler, periodically thickening or thinning the dust along Earth’s line of sight. This produces a rhythmic, jet-driven brightness signal — what he has previously described as a kind of “heartbeat” variability.

What the Seven-Hour Rotation Might Mean

Before perihelion, brightness data suggested a longer periodicity of roughly 16 to 17 hours. Loeb argues that the discrepancy does not necessarily indicate that the object sped up, but instead that perihelion may have activated additional jets or source regions.

If only one rotational pole was illuminated before perihelion, and both poles became active afterward, the brightness cycle could effectively double in frequency without a change in the underlying spin rate. In that scenario, the seven-hour signal reflects jet geometry rather than a fundamental change in rotation.

Crucially, Loeb reports that the jet precession period before perihelion and after perihelion are broadly consistent, reinforcing the idea that 3I/ATLAS retained its rotational stability despite intense solar heating.

As 3I/ATLAS approaches near-perfect opposition on January 22, 2026, Loeb notes that the viewing geometry will change again, with the anti-sunward jet fading from view and the sunward anti-tail pointing almost directly toward Earth. That alignment could provide another test of whether jet-driven modulation — rather than nucleus reflection — truly governs the object’s observed variability.

For now, the reported seven-hour rotation period adds another intriguing piece to the puzzle of how interstellar objects behave when they pass through our solar system — and how much they can reveal about conditions far beyond it.

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