3I/ATLAS Update: Harvard's Loeb Reveals Pulsing Jets Resemble a Celestial Heartbeat

What if an object hurtling through our solar system, one already confirmed as an interstellar wanderer, started to pulse? Not slowly, but with the steady, predictable rhythm of a clock. Since its discovery, the light signature of the interstellar object 3I/ATLAS has displayed just such a phenomenon: a rhythmic variability with a period of 16.16 hours. This cosmic 'heartbeat' immediately presents a profound paradox, challenging standard explanations of cometary physics and forcing scientists to ask whether this object is simply an icy rock or something far more unusual.

Analysing the data requires rigorous, careful work, ensuring that we do not mistakenly attribute complex behaviour to a simple spin. This analysis is spearheaded by Avi Loeb, the Frank B. Baird, Jr., Professor of Science at Harvard University and former Chair of the Astronomy Department, who has consistently questioned whether unusual interstellar objects like this one might represent technological artefacts rather than natural comets.

Decoding the 16.16-Hour Pulsation of Comet 3I/ATLAS

Following its detection on July 1, 2025, the light from the interstellar object 3I/ATLAS showed a striking and sustained pulsating variability with a period of 16.16 hours. The spin period was first formally reported by T. Santana-Ross et al. in August 2025. The periodic variability was measured to have an amplitude of tens of percent, corresponding to approximately 0.3 mag in the light curve.

Initially, observers attempted to relate this significant variability to a rotating nucleus with an ellipsoidal shape, suggesting an axis ratio of about 0.8. However, a detailed analysis of the available data shows that the association of this dramatic light variation with the simple, solid shape of the nucleus is not warranted. The core problem lies in where the majority of the object's observed light originates.

The Coma Conundrum: Where the Light Comes From in 3I/ATLAS

Data from the Hubble Space Telescope image taken on July 21, 2025, revealed that the vast majority of the light observed from 3I/ATLAS originates from a glowing halo around it, known as a coma. This coma is transparent so that the nucleus can be seen through it. The fraction of the total light originating from the reflection of sunlight by the small, solid nucleus itself is unknown, because the nucleus size is not resolved in the Hubble image. However, the brightest pixel in the image presumably overlaps with the nucleus and contains a small fraction of the total light.

If, hypothetically, all the light originated from the solid surface of the nucleus, then the effective radius of the nucleus had to be Reff = 10 kilometres in visible light or Reff = 23 kilometres at a wavelength of 1-micrometer for a typical albedo of 4–5%. However, the actual radius of the nucleus must be much smaller since most of the light is coming from the coma.

If the actual radius of the nucleus is R, then the fraction of light that it reflects relative to the total (coma + nucleus) scales as the surface area, (R/R eff)^2. For example, the upper limit of R~ 2.8 kilometres inferred from the Hubble image, implies that the nucleus reflects less than a percent of the total light at a wavelength of 1-micrometer. The technical paradox is clear: If the nucleus is spinning over 16.16 hours, why was the periodic variability at a level of tens of percent in total light? The light must be coming from the larger coma structure, not the nucleus's reflective surface.

Natural or Technological? The Pulsing Jets of 3I/ATLAS

The only scientifically sound explanation for such large periodic light variations is that the extensive coma itself must be pulsating in brightness. Over the past month, images of 3I/ATLAS have indeed shown multiple collimated jets of material streaming away from the central body. The working hypothesis posits that if the mass loss in these jets is pulsed periodically, the resulting coma would display periodic variability in its scattering of sunlight.

In the context of a natural comet, this can arise from a sunward jet (an anti-tail) that is initiated only when a large pocket of ice on one side of the nucleus is facing the sun. As a result, the coma will get pumped up every time the ice pocket is facing the sun.

This mechanism literally resembles a heartbeat with a puff of gas and dust serving the role of a stream of 'blood' through the coma periodically over the rotation period of 16.16 hours. Assuming an outflow velocity of 440 metres per second as suggested in the Webb telescope report, the distance that sublimated volatiles can reach during 16.16 hours is 25,600 kilometres.

Crucially, however, the orientation of the jets provides the ultimate test. This 'heartbeat pattern' should have been apparent in a series of well-calibrated snapshots of the coma over several days, but none was systematically studied in the published literature. For a natural comet, the active jet must point towards the sun to be initiated by solar heating. Conversely, for a technological object, the direction of the pulsing jet could be arbitrary and not necessarily pointing towards the sun.

A movie showing the periodic brightening of the jets around 3I/ATLAS over several days can reveal whether the jets are natural or technological based on the orientation of the heartbeat pattern relative to the sun. At any event, it is clear that the reported periodicity over 16.16 hours is not directly associated with the shape of the nucleus but rather with the collimated jets coming from it out to much larger distances.

The rhythmic, 16.16 hour 'heartbeat' of Comet 3I/ATLAS presents a fascinating dichotomy: a natural icy pocket or a sign of technological origins. The answer hinges on the orientation of its pulsing jets relative to the sun — data currently lacking in the published literature.

This uncertainty underscores the urgency of detailed observation. To resolve this cosmic mystery and determine if we are observing exotic physics or simply an exotic artefact, the scientific community must mobilise to capture systematic, high-resolution 'movie' sequences of the comet's coma and jets over several days.