The tumbling rotational state of 1I/'Oumuamua

Wesley C. Fraser, Petr Pravec, Alan Fitzsimmons, Pedro Lacerda, Michele T. Bannister, Colin Snodgrass, Igor Smoli?

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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.

1. Meech, K. J., Weryk, R. & Micheli, M. A brief visit from a red and extremely elongated interstellar asteroid. Nature 552, 378?381 (2017). 2. Ye, Q.-Z., Zhang, Q., Kelley, M. S. P. & Brown, P. G. 1I/2017 U1 (?Oumuamua) is hot: imaging, spectroscopy, and search of meteor activity. Astrophys. J. Lett. 851, L5 (2017). 3. Fitzsimmons, A. et al. Spectroscopy and thermal modelling of the frst interstellar object 1I/2017 U1 ?Oumuamua. Nat. Astron. https://doi. org/10.1038/s41550-017-0361-4 (2018). 4. Bannister, M. T. et al. Col-OSSOS: colors of the interstellar planetesimal 1I/?Oumuamua. Astrophys. J. Lett. 851, L38 (2017). 5. Jewitt, D. et al. Interstellar Interloper 1I/2017 U1: observations from the NOT and WIYN telescopes. Astrophys. J. Lett. 850, L36 (2017). 6. Bolin, B. T. et al. APO time-resolved color photometry of highly elongated interstellar object 1I/?Oumuamua. Astrophys. J. Lett. 852, L2 (2018). 7. Masiero, J. Palomar optical spectrum of hyperbolic near-earth object A/2017 U1. Preprint at (2017). 8. Knight, M. M. et al. On the rotation period and shape of the hyperbolic asteroid 1I/?Oumuamua (2017 U1) from its lightcurve. Astrophys. J. Lett. 851, L31 (2017). 9. Feng, F. & Jones, H. R. A. ?Oumuamua as a messenger from the Local Association. Astrophys. J. Lett. 852, L27 (2018).
Original languageEnglish
Number of pages13
JournalNature Astronomy
Early online date9 Feb 2018
Publication statusPublished - 1 May 2018


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