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The first interstellar object 'Oumuamua is discovered in 2017. When 'Oumuamuatravels in interstellar space, it keeps colliding with interstellar medium(ISM). Given a sufficiently long interaction time, its rotation state maychange significantly because of the angular momentum transfer with interstellarmedium. Using generated Gaussian random spheres with the dimension ratios 6:1:1and 5:5:1, this paper explores the ISM torque curve and proposes that ISMcollision may account for 'Oumuamua's tumbling with the simple constant-torqueanalytical method. The statistic results show that the asymptotic obliquitiesdistribute mostly at 0 and 180 degree and most cases spin down at theasymptotic obliquity, indicating the ISM collision effect is similar to theYORP effect with zero heat conductivity assumed. Given a long time ofdeceleration of the spin rate, an initial major-axis rotation may evolve intotumbling motion under ISM torque. Using a constant-torque analytical model, thetimescales of evolving into tumbling for the sample of 200 shapes are found torange from several Gyrs to ens of Gyrs, highly dependent on the chosen shape.The mean value is about 8.5 Gyrs for prolate shapes and 7.3 Gyrs for oblateshapes. Rotation of asteroids in the Oort cloud might be also dominated by theISM collision effect since the YORP effect is quite weak at such a longdistance from the Sun. Although this paper assumes an ideal mirror reflectionand a constant relative velocity of 'Oumuamua, the results still show theimportance of the ISM collision effect.

Interstellar Asteroid Rotation with the Mechanical Torque Produced by Interstellar Medium