https://hal-insu.archives-ouvertes.fr/insu-02348185Flekkøy, Eirik G.Eirik G.FlekkøyPoreLab [Oslo] - Department of Physics [Oslo] - Faculty of Mathematics and Natural Sciences [Oslo] - UiO - University of Oslo - NTNU - Norwegian University of Science and Technology [Oslo] - NTNU - Norwegian University of Science and TechnologyLuu, Jane X.Jane X.LuuCEED - Centre for Earth Evolution and Dynamics [Oslo] - Department of Geosciences [Oslo] - Faculty of Mathematics and Natural Sciences [Oslo] - UiO - University of OsloInstitute of Theoretical Astrophysics [Oslo] - UiO - University of OsloToussaint, RenaudRenaudToussaintIPGS - Institut de physique du globe de Strasbourg - UNISTRA - Université de Strasbourg - INSU - CNRS - Institut national des sciences de l'Univers - CNRS - Centre National de la Recherche ScientifiquePoreLab [Oslo] - Department of Physics [Oslo] - Faculty of Mathematics and Natural Sciences [Oslo] - UiO - University of Oslo - NTNU - Norwegian University of Science and Technology [Oslo] - NTNU - Norwegian University of Science and TechnologyThe interstellar object 'Oumuamua as a fractal dust aggregateHAL CCSD2019[PHYS.ASTR.EP] Physics [physics]/Astrophysics [astro-ph]/Earth and Planetary Astrophysics [astro-ph.EP][PHYS.ASTR] Physics [physics]/Astrophysics [astro-ph]Toussaint, Renaud2019-11-05 12:46:252021-11-03 06:49:262019-11-05 12:46:25enPreprints, Working Papers, ...1The first known interstellar object 'Oumuamua exhibited a nongravitational acceleration that appeared inconsistent with cometary outgassing, leaving radiation pressure as the most likely force. Bar the alien lightsail hypothesis, an ultra-low density due to a fractal structure might also explain the acceleration of 'Oumuamua by radiation pressure (Moro-Martin 2019). In this paper we report a decrease in 'Oumuamua's rotation period based on ground-based observations, and show that this spin-down can be explained by the YORP effect if 'Oumuamua is indeed a fractal body with the ultra-low density of $10^{-2}$ kg m$^{-3}$. We also investigate the mechanical consequences of 'Oumuamua as a fractal body subjected to rotational and tidal forces, and show that a fractal structure can survive these mechanical forces.