The tumbling rotational state of 1I/'Oumuamua

The discovery1 of 1I/2017 U1 (1I/'Oumuamua) has provided the first glimpse of a planetesimal born in another planetary system. This interloper exhibits a variable colour within a range that is broadly consistent with local small bodies, such as the P- and D-type asteroids, Jupiter Trojans and dynamically excited Kuiper belt objects(2-7). 1I/'Oumuamua appears unusually elongated in shape, with an axial ratio exceeding 5: 1 (refs(1,4,5,8)). Rotation period estimates are inconsistent and varied, with reported values between 6.9 and 8.3 h (refs(4-6,9)). Here, we analyse all the available optical photometry data reported to date. No single rotation period can explain the exhibited brightness variations. Rather, 1I/'Oumuamua appears to be in an excited rotational state undergoing non-principal axis rotation, or tumbling. A satisfactory solution has apparent lightcurve frequencies of 0.135 and 0.126 h(-1) and implies a longest-to-shortest axis ratio of. 5: 1, although the available data are insufficient to uniquely constrain the true frequencies and shape. Assuming a body that responds to non-principal axis rotation in a similar manner to Solar System asteroids and comets, the timescale to damp 1I/'Oumuamua's tumbling is at least one billion years. 1I/'Oumuamua was probably set tumbling within its parent planetary system and will remain tumbling well after it has left ours.