Earth’s Moon is its only natural satellite. While planets can host multiple moons, our Moon does not possess its own. The intricate gravitational dynamics within our solar system prevent the formation and sustained orbit of “sub-moons” around Earth’s Moon.
The Moon’s Gravitational Neighborhood
The Moon’s gravitational environment is shaped by influences from both Earth and the Sun. The region where the Moon’s gravity dominates over Earth’s is known as its Hill sphere. This theoretical boundary defines the maximum distance at which a smaller object could orbit the Moon stably without being pulled away by Earth’s stronger gravity.
The Moon’s Hill sphere is relatively small, extending 60,000 kilometers (37,000 miles) from its center. This distance is less than one-sixth of the 384,399-kilometer distance between the Moon and Earth. Earth’s substantial mass and proximity significantly limit the Moon’s ability to retain its own celestial companions. Objects beyond this confined zone would fall under Earth’s primary gravitational influence, not the Moon.
Why Moons Don’t Have Moons
Our Moon cannot host its own natural satellites due to Earth’s overpowering gravitational influence. Any object attempting to orbit the Moon would still be subject to Earth’s powerful pull, which would continuously perturb its orbit. This constant tug would make any potential sub-moon’s orbit highly unstable, preventing it from maintaining a consistent path.
Tidal forces further complicate the stability of potential lunar moons. These differential gravitational forces would distort or quickly eject any small object trying to establish an orbit. The Moon’s own irregular gravitational field, characterized by dense regions called mascons, also contributes to orbital instability, causing the paths of orbiting objects to shift and decay over time. These combined gravitational effects create an environment too disruptive for long-term natural sub-satellites.
What Else Orbits the Moon
Although the Moon does not have natural moons, it is orbited by various artificial satellites. Since 1966, numerous spacecraft have been sent to orbit the Moon for scientific study and exploration. These artificial satellites, such as NASA’s Lunar Reconnaissance Orbiter (LRO) and the CAPSTONE mission, provide valuable data about the lunar surface and its environment.
These artificial orbiters, however, require periodic adjustments to maintain their paths due to the unstable gravitational conditions. Without constant maneuvering, their orbits would eventually decay, leading to a crash onto the lunar surface or ejection from lunar orbit. While small asteroids or space debris might temporarily enter the Moon’s gravitational influence, they are typically short-lived visitors, either escaping or colliding.