3I/ATLAS, the third interstellar object detected passing through our Solar System, is tumbling. Hubble Space Telescope images reveal that its jet structure — three persistent features visible in Larson-Sekanina filtered images — wobbles with a period of 7.20 hours. Independent photometry of the coma confirms the period at 7.136 hours. Two different measurement methods, same answer: the nucleus is not spinning smoothly around a single axis.
Scarmato and Loeb interpret this as non-principal-axis rotation — the nucleus precessing and nutating with an angular excursion of about 20 degrees. Think of it not as a top spinning on a table but as a top wobbling on its way down. The rotation axis wanders, carrying the jets with it, producing the periodic variation in jet position angles that HST captured over nearly a month of post-perihelion observation.
The measurement is straightforward. The interpretation raises questions. Non-principal-axis rotation in a small body means the rotation has not yet damped into simple spin around the axis of maximum moment of inertia. For Solar System comets, this damping takes thousands to millions of years depending on size and shape. A tumbling interstellar comet either arrived in a tumbling state — carrying information about whatever ejected it from its home system — or was recently torqued into tumbling by its own outgassing jets during the solar encounter.
The second explanation is more conservative. 3I/ATLAS is actively outgassing — the jets are the evidence — and asymmetric mass loss is exactly the kind of torque that can excite non-principal-axis rotation. The 20-degree wobble amplitude and 7.2-hour period constrain the nucleus shape and mass distribution, but extracting those constraints requires modeling the jet geometry and outgassing rates, which is model-dependent.
What is not model-dependent is that the thing is tumbling, and it came from another star. This is the third interstellar object we have measured in any detail. 1I/'Oumuamua was anomalous in almost every respect. 2I/Borisov was reassuringly normal — a comet that looked like a comet. 3I/ATLAS falls between: cometary activity, but with a tumbling nucleus and three persistent jets. The sample size is too small for statistics, but the variance within three objects is already interesting.