Abstract

The first known interstellar object ’Oumuamua exhibited a nongravitational acceleration that appeared inconsistent with cometary outgassing, leaving radiation pressure as the most likely force. Barring the alien lightsail hypothesis, an ultra-low density due to a fractal structure might also explain the acceleration of ’Oumuamua by radiation pressure. In this Letter 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.