The cigar-shaped artist’s impression of interstellar visitor 1I/’Oumuamua by Martin Kornmesser has already become quite iconic. Ironically, what was actually observed is shown here — obtained by co-adding 13 individual 100-second exposures (as ’Oumuamua is so faint) taken with the 4.2-m William Herschel Telescope on 29 October 2017: a (rather spherical) point source on a background of streaks created by moving stars, seen from a reference system locked on the rapidly moving ’Oumuamua in the centre of the frame. A powerful diagnostic to characterize such small bodies is their variation in brightness with time: their lightcurve. Fitting the lightcurve with a periodic curve gives the rotational period of the body. In some cases, however, a single rotation period cannot fit the whole time series, indicating that the body does not rotate around one of its principal axes: an excited rotation state called tumbling. This is what two independent studies published in this issue of Nature Astronomy, respectively led by Wesley Fraser and Michał Drahus, find for ’Oumuamua.

Credit: Alan Fitzsimmons, Queen’s University Belfast / Isaac Newton Group, La Palma

The two studies use different methodologies. Fraser et al. collected data from five different surveys spanning six nights. Drahus et al. used ~8 hours of observations taken in the course of two days from the Gemini North telescope. These two approaches are complementary: the former gives a longer time series but has more sources of uncertainty as each telescope suffers from different systematics; the latter covers a shorter time — an important factor when hunting for rotational irregularities — but the data come from the same telescope. In any case, they both reach the same conclusions. In addition to the tumbling, they also detect huge brightness variations per rotation, almost 2.5 magnitudes, larger than any other Solar System body and from which the inferred highly elongated shape of the object comes from.

But why is the tumbling property so interesting? Tumbling small bodies are an extreme minority in the Solar System: usually, damping mechanisms regularize the rotation. The tumbling state is thus a window on ’Oumuamua’s history: at a certain moment, something happened and induced a lasting perturbation. According to Drahus et al., the most probable cause is a violent collision. The other main mechanism to generate tumbling, sublimation torques, works only on active cometary-like bodies and ’Oumuamua didn’t show any activity when it flew close to the Sun. Fraser et al. suggest that ’Oumuamua must be quite rigid, otherwise it would have been dampened already, and the tumbling event may have happened in its home system.

Several other studies have already tried to extract more information from the tumbling state. Jonathan Katz suggests that ’Oumuamua remained in a region with a high density of similar objects for thousands of years after its formation (preprint at; 2018). According to Michael Belton et al., the excited state might have been generated by the same event that expelled it from its home system (preprint at; 2018), whereas Thiem Hoang et al. propose an alternative scenario where rotational torques induced by its high-speed travel in the interstellar medium could have induced rotational disruption (preprint at; 2018).

’Oumuamua is now too faint to be observed, but the prospects for detecting more interstellar objects to constrain their properties and history are promising, particularly with the upcoming Large Synoptic Survey Telescope (LSST).