A moon’s moon needs a narrow orbital foothold
A moon can, in principle, have its own moon—a small body orbiting the larger moon rather than its planet. The challenge is that the small body is caught between two gravitational influences: the moon’s pull, which can hold it close, and the planet’s pull, which grows harder to ignore farther away.
The boundary between staying and escaping
Near the moon, its gravity dominates. But if the smaller body orbits too close, differences in gravity across its width can stretch it; in extreme cases, those tidal forces can break it apart or send debris crashing down. Farther out, the planet increasingly disrupts the orbit. Beyond a certain distance, the small body is more likely to orbit the planet directly than remain bound to the moon.
The viable region between these limits is not a fixed distance. It depends on the masses of the planet, moon, and smaller body, as well as the moon’s distance from its planet and the shape of the orbits. A moon farther from its planet generally has more room to hold on to a companion; a close-in moon is more strongly affected by the planet’s gravity.
“Possible” does not mean easy to keep
Even inside the safe zone, an orbit must remain stable over time. Gravitational tugs from the planet and other moons can gradually alter it, while a collision or a close encounter could eject the small body. So a moonmoon is physically possible, but only if it occupies a suitable orbit and avoids disturbances that push it across the boundary.
That is why orbital elbow room matters: a large moon in a favorable orbit could offer a stable place for a smaller companion, while another moon may have no lasting space for one at all.
