3I/Atlas Jets Pulse in a Ghostly Heartbeat: Glowing Coma, Not Solid Core, Dominates Light in Hubble Image

The darkness of interstellar space was pierced on 1 July 2025, when astronomers detected a new visitor from the void. Known as 3I/ATLAS, this interstellar object immediately presented a riddle that has left scientists scrutinising their data. Unlike the steady shine of typical asteroids, the light from this object displayed a rhythmic, pulsating variability.

Every 16.16 hours, the object's brightness shifts significantly, with an amplitude changing by tens of percent. Initial observers were quick to attribute this to the object's physical rotation. They theorised they were watching an elongated nucleus, with an axis ratio of about 0.8, tumbling end-over-end through the cosmos.

Determining Why the Ellipsoidal Shape Theory Is Unwarranted

However, a deeper analysis suggests that relating this variability merely to the shape of the nucleus is a mistake. The data points to a more complex phenomenon than a simple spinning rock. The periodicity is undeniable, but its cause appears to be far more ethereal.

Based on an image captured by the Hubble Space Telescope on 21 July 2025, the majority of the light we see is not reflecting off a hard surface. Instead, it originates from a glowing halo surrounding the object, known as a coma. This coma is remarkably transparent, allowing the nucleus to be visible through the haze.

Calculating the True Radius of the Nucleus

The presence of this coma complicates our understanding of the object's size. Because the nucleus is not fully resolved in the Hubble image, we cannot be certain what fraction of the total light comes from sunlight reflecting off the solid core. We can presume, however, that the brightest pixel in the image likely overlaps with the nucleus.

Even so, this solid centre contains only a small fraction of the total luminosity. If we assumed all the light came from a solid surface with a typical albedo of 4–5%, the mathematics would require a much larger object. The effective radius would need to be 10 kilometres in visible light or 23 kilometres at a wavelength of 1-micrometer.

The Discrepancy Between Light Intensity and Surface Area

The reality of the situation contradicts these theoretical dimensions. The actual radius of the nucleus must be significantly smaller because the coma dominates the light profile. If we denote the actual radius as R, the light reflected by the nucleus relative to the total brightness scales as the square of the surface area ratio, (R/R_eff)².

Analysis inferred from the Hubble image places an upper limit on the radius at approximately 2.8 kilometres. This implies that the nucleus reflects less than one percent of the total light at a wavelength of 1-micrometer. This creates a baffling contradiction regarding the object's spin. If the nucleus is indeed spinning over a 16.16-hour period, how could such a tiny contributor cause the total light to vary by tens of percent?

Periodic Eruptions Mimic a Galactic Heartbeat

The answer likely lies in the activity observed over the past month, where images of 3I/ATLAS revealed multiple jets. If the mass loss within these jets is pulsed rather than continuous, the resulting coma would naturally display periodic variability. The sunlight scattering off the gas and dust would rise and fall in rhythm with the eruptions.

In the context of a natural comet, this creates a fascinating mechanism. A sunward jet, often called an anti-tail, could be initiated only when a specific large pocket of ice on the nucleus faces the Sun. As the object rotates, this ice pocket warms up and releases material.

Consequently, the coma gets 'pumped up,' every time the ice faces the solar heat. This process resembles a heartbeat, with a sudden puff of gas and dust acting as a stream of 'blood,' circulating through the coma periodically. This cycle repeats precisely over the rotation period of 16.16 hours.

Distinguishing Between Natural Jets and Technological Signals

The scale of this output is vast. Assuming an outflow velocity of 440 metres per second, as suggested in the Webb telescope report, the sublimated volatiles can travel 25,600 kilometres during a single rotation. Such a heartbeat pattern should have been obvious in well-calibrated snapshots taken over several days.

Unfortunately, no such series was systematically studied in the published literature. This leaves room for speculation regarding the object's origins. For a technological object, the pulsing jet would not need to align with the Sun and could point in an arbitrary direction.

A movie documenting the periodic brightening of the jets over several days could solve this puzzle. By observing the orientation of this 'heartbeat' relative to the Sun, we could determine if the jets are natural. At any event, the reported 16.16-hour periodicity is clearly not due to the nucleus shape, but the collimated jets extending into the dark.