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tokens: obliquity and angular velocity (Rubincam 2000 ; Vokrouhlický Capek 2002 ; Scheeres 2007 ), and even causing the tumbling motion (Vokrouhlický et al. 2007 ). Combined with the effect of internal energy dissipation, the YORP effect may lead to a new asymptotic tumbling state of asteroids (Breiter Murawiecka 2015 ). In Section 2 , the main equations and the shape model in the simulation are introduced. Highly elongated and disk-like irregular shapes are generated as pseudo ‘Oumuamua. In Section 3 , the description of the torque curve and statistic results are shown. The origin of tumbling motion of the interstellar asteroid ‘Oumuamua is discussed. A comparison is made between the ISM collision effect and the YORP effect for distant asteroids in the solar system. 2. Simulation 2.1. Equations In this study, the kinetic approach is used to analyze the collisions between the interstellar asteroid and gas, as first proposed by Hoang et al. ( 2018 ). For a surface element dS , The mass of particles colliding with the surface within a time interval dt isHere I set v as the translational velocity vector of the asteroid relative to surrounding interstellar gas. Although the asteroid is rotating, the rotational velocity is too small compared to the translational velocity. Here, n is the outward unit normal vector of the surface element dS . Several assumptions are made in the following derivation: (1) mirror reflection of particles and no sputtering on the surface, (2) the strongly supersonic regime of the body, and (3) the convexity of the surface. After colliding with the surface element, gas particles are reflected, with the velocity component perpendicular to the surface reversed. In this process, the momentum transferred from the particles to the asteroid is is the reflection coefficient, with γ r = 1 representing a mirror reflection. Plugging ( 6 ) into ( 7 ), we can obtain In this paper γ r is set to 1 because of the assumption (1). Note that the process of “sputtering” also produces a torque, which is calculated to be smaller by one order of magnitude than the ISM collision torque, which is shown in Appendix B . We should take caution in further research when neglecting this “sputtering” effect since the variation by just one order of magnitude could be removed by the uncertainty of the parameters. Assumption (2) discards the effect of the gas particles’ thermal motion. Let us consider interstellar gas in the warm phase with the temperature of 5000 K. Applying Maxwell’s velocity distribution, the averaged speed of the gas is , less than the assumed speed of the asteroid. The asteroid can be considered to be supersonic. In the hot region (e.g., above 20,000 K), the above equations are certainly invalid. Assumption (3) avoids the shadowing and secondary collisions of particles. But we could expect Equation ( 8 ) is still valid for moderately concave shapes. According to Golubov et al. ( 2016 ), for the YORP effect, the formulas proposed under the