Periodic light variability in interstellar object 3I/ATLAS linked to jet activity

The interstellar object 3I/ATLAS, first detected on July 1, 2025, exhibited light that varied periodically with a cycle of 16.16 hours, according to observations reported by Harvard Professor Avi Loeb.

This variation had an amplitude measured in tens of percent. Initial analyses associated this variability with the shape of the nucleus, which was estimated to have an axis ratio of approximately 0.8.

However, this association is not supported by the observational data.

Subsequent imaging from the Hubble Space Telescope on July 21, 2025, showed that most of the observed light originates from a coma surrounding the nucleus rather than from the solid surface itself.

The periodicity in light is therefore linked to jet activity rather than the nucleus shape.

Observations from Hubble

The Hubble Space Telescope image revealed that 3I/ATLAS is surrounded by a transparent coma, allowing partial visibility of the nucleus through the surrounding material.

The nucleus itself was not resolved in the image, so the fraction of light reflected directly from the nucleus is uncertain.

Estimates based on the brightest pixel, assumed to overlap the nucleus, indicate that it contains only a small portion of the total observed light.

Calculations assuming all light originates from the solid surface suggest an effective radius of 10 kilometers in visible light or 23 kilometers at a wavelength of 1 micrometer, assuming a standard albedo of 4–5 percent.

Observations indicate the actual radius of the nucleus is smaller, with an upper limit of approximately 2.8 kilometers based on the Hubble image.

This implies the nucleus reflects less than one percent of the total light at a wavelength of 1 micrometer.

Periodic variability and jet activity

The 16.16-hour light variation observed in 3I/ATLAS cannot be explained solely by the rotation of the nucleus due to its limited contribution to total brightness.

Over the period of observation, multiple jets were detected emanating from the nucleus.

If mass loss in these jets occurs in a pulsed manner, the scattering of sunlight by the coma would show periodic variations corresponding to the jet activity.

In the case of a natural comet, periodic jets can form when a localized ice pocket on the nucleus is exposed to the Sun, producing a periodic increase in outgassing.

Using an outflow velocity of 440 meters per second, as reported by the Webb Telescope, the sublimated material could travel up to 25,600 kilometers over the 16.16-hour rotation period.

Implications for cometary and technological models

Periodic brightening in the coma, or a “heartbeat” pattern, could be monitored through a series of well-calibrated images over several days to evaluate the jet activity.

For natural comets, these jets are expected to align with solar illumination. In contrast, for a technological object, the jet directions could be arbitrary and not necessarily aligned with the Sun.

Observing the orientation and repetition of the periodic brightening in the jets can provide data to distinguish between natural and technological processes.

Regardless of the nature of the object, the observed 16.16-hour periodicity is attributed to collimated jets extending far beyond the nucleus, rather than the nucleus’s ellipsoidal shape.

Analysis of the Hubble imaging data, combined with ground-based observations and measurements from the Webb Telescope on the speed and reach of the jets, supports the conclusion that the majority of light variation in 3I/ATLAS arises from periodic outgassing and jet activity rather than structural features of the nucleus.

Stay tuned for more updates.