The slow rate of rotation may been accounted for by a moon orbiting the asteroid , but a search of the NEAR images revealed none bigger than 10 km in diameter out to 20 times the radius of 253 Mathilde . The slow rate of rotation may be accounted for by a satellite orbiting the asteroid , but a search of the NEAR images revealed none larger than 10 km in diameter out to 20 times the radius of 253 Mathilde .