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 objects2,3,4,5,6,7. 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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Fig. 1: The geometry-reduced and colour-corrected r′-band photometry, Hr′, of 1I/‘Oumuamua cannot be well described by a model of simple rotation.
Fig. 2: Rotational colour variations.


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W.C.F., A.F., M.T.B. and P.L. acknowledge support from Science and Technology Facilities Council grant ST/P0003094/1. M.T.B. also acknowledges support from Science and Technology Facilities Council grant ST/L000709/1. The work by P.P. was supported by the Grant Agency of the Czech Republic (grant 17-00774S). C.S. is supported by a Science and Technology Facilities Council Ernest Rutherford Fellowship and grant ST/L004569/1.

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W.C.F. compiled the common filter dataset, re-reduced observations where necessary and led the analysis and writing of the manuscript. P.P., P. L. and I.S. performed the lightcurve modelling and assisted with writing. A.F. calculated damping timescale estimates and assisted with writing the paper. M.T.B. and C.S. assisted with interpretation of the lightcurve results and writing the paper.

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Correspondence to Wesley C. Fraser.

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Fraser, W.C., Pravec, P., Fitzsimmons, A. et al. The tumbling rotational state of 1I/‘Oumuamua. Nat Astron 2, 383–386 (2018).

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