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^2,3,4,5,6,7. 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 ^4,5,6,9). 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.


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