Context. The Yarkovsky−O'Keefe−Radzievskii−Paddack (YORP) effect plays an important role in the rotational properties and evolution of asteroids. While the YORP effect induced by the macroscopic shape of the asteroid and by the presence of surface boulders has been well studied, no investigation has been performed yet regarding how craters with given properties influence this effect. Aims. We introduce and estimate the crater-induced YORP effect (CYORP), which arises from the concave structure of the crater, to investigate the magnitude of the resulting torques as a function of varying properties of the crater and the asteroid by a semi-analytical method. Methods. By using a simple spherical shape model of the crater and assuming zero thermal inertia, we calculated the total YORP torque due to the crater, which was averaged over the spin and orbital motions of the asteroid, accounting for self-sheltering and self-sheltering effects.Results. The general form of the CYORP torque can be expressed in terms of the crater radius R 0 and the asteroid radius R ast :, where W is an efficiency factor. We find that the typical values of W are about 0.04 and 0.025 for the spin and obliquity component, respectively, which indicates that the CYORP can be comparable to the normal YORP torque when the size of the crater is about one-tenth of the size of the asteroid, or equivalently when the crater/roughness covers one-tenth of the asteroid surface. Although the torque decreases with the crater size R 0 as ∼ R 2 0 , the combined contribution of all small craters can become non-negligible due to their large number when the commonly used power-law crater size distribution is considered. The CYORP torque of small concave structures, usually considered as surface roughness, is essential to the accurate calculation of the complete YORP torque. Under the CYORP effect that is produced by collisions, asteroids go through a random walk in spin rate and obliquity, with a YORP reset timescale typically of 0.4 Myr. This has strong implications for the rotational evolution and orbital evolution of asteroids.Conclusions. Craters and roughness on asteroid surfaces, which correspond to concave structures, can influence the YORP torques and therefore the rotational properties and evolution of asteroids. We suggest that the CYORP effect should be considered in the future investigation of the YORP effect on asteroids.

Context. Enigmatic dynamical and spectral properties of the first interstellar object (ISO), 1I/2017 U1 ('Oumuamua), led to many hypotheses, including a suggestion that it may be an "artificial" spacecraft with a thin radiation-pressure-driven light sail. Since similar discoveries by forthcoming instruments, such as the Vera Rubin telescope and the Chinese Space Station Telescope (CSST), are anticipated, a critical identification of key observable tests is warranted for the quantitative distinctions between various scenarios. Aims. We scrutinize the light-sail scenario by making comparisons between physical models and observational constraints. These analyses can be generalized for future surveys of 'Oumuamua-like objects. Methods. The light sail goes through a drift in interstellar space due to the magnetic field and gas atoms, which poses challenges to the navigation system. When the light sail enters the inner Solar System, the sideways radiation pressure leads to a considerable non-radial displacement. The immensely high dimensional ratio and the tumbling motion could cause a light curve with an extremely large amplitude and could even make the light sail invisible from time to time. These observational features allow us to examine the light-sail scenario of interstellar objects. Results. The drift of the freely rotating light sail in the interstellar medium is ∼ 100 au even if the travel distance is only 1 pc. The probability of the expected brightness modulation of the light sail matching with 'Oumuamua's observed variation amplitude (∼ 2.5 -3) is < 1.5%. In addition, the probability of the tumbling light sail being visible (brighter than V=27) in all 55 observations spread over two months after discovery is 0.4%. Radiation pressure could cause a larger displacement normal to the orbital plane for a light sail than that for 'Oumuamua. Also, the ratio of antisolar to sideways acceleration of 'Oumuamua deviates from that of the light sail by ∼1.5 σ. Conclusions. We suggest that 'Oumuamua is unlikely to be a light sail. The dynamics of an intruding light sail, if it exists, has distinct observational signatures, which can be quantitatively identified and analyzed with our methods in future surveys.