Tumbling motion of 1I/'Oumuamua and its implications for the body's distant past

Models of the Solar System's evolution show that almost all the primitive material leftover from the formation of the planets was ejected to the interstellar space as a result of dynamical instabilities(1). Accordingly, minor bodies should also be ejected from other planetary systems and should be abundant in the interstellar space(2), giving hope for their direct detection and detailed characterization as they penetrate through the Solar System(3,4). These expectations materialized on 19 October 2017 ut with the Panoramic Survey Telescope and Rapid Response System's discovery of 1I/'Oumuamua(5). Here, we report homogeneous photometric observations of this body from Gemini North, which densely cover a total of 8.06 h over two nights. A combined ultra-deep image of 1I/'Oumuamua shows no signs of cometary activity, confirming the results from other, less sensitive searches(6-9). Our data also show an enormous range of rotational brightness variations of 2.6 +/- 0.2 mag, larger than ever observed in the population of small Solar System objects, suggesting a very elongated shape of the body. Most significantly, the light curve does not repeat exactly from one rotation cycle to another and its double-peaked periodicity of 7.56 +/- 0.01 h from our data is inconsistent with earlier determinations(6,7,10-12). These are clear signs of a tumbling motion, a remarkable characteristic of 1I/'Oumuamua's rotation that is consistent with a collision in the distant past. Bearing marks of a violent history, this first-known interstellar visitor tells us that collisional evolution of minor body populations in other planetary systems might be common.