Asked by: Liam Farmer, Birmingham
There is no reason why a moon could not have a moon. But for such a ‘sub-moon’ to survive for any length of time, it would require a stable orbit around its parent moon. This generally means that the sub-moon would have to be quite small and orbit quite close to the parent moon. The bigger the sub-moon is and the further it is from the parent moon, the more likely it will be influenced by the gravitational attraction of the parent planet. Tidal forces can also easily prevent a stable orbit being possible. So, moons of moons are possible but probably extremely rare.