The short answer to whether Earth’s Moon has a moon of its own is no. A celestial body orbiting a moon is known as a sub-satellite. Although the laws of physics do not forbid the existence of such objects, the conditions required for long-term orbital stability are extremely rare. In our Solar System, no moon has been observed to host a natural, permanent satellite. The search for sub-satellites focuses on understanding the complex gravitational dance between a moon, its host planet, and the Sun.
Why Stable Lunar Satellites Cannot Exist
The primary reason the Moon cannot retain a permanent satellite is the overwhelming gravitational influence of the Earth. Any potential sub-satellite would immediately become a participant in what astronomers call the restricted three-body problem, involving the Sun, Earth, and Moon. The immense gravitational pull from the Earth acts as a constant, disruptive force, preventing a small object from settling into a stable, long-term orbit around the Moon.
This instability is caused by tidal forces. The Earth’s gravity exerts a differential pull across the Moon and its potential satellite, distorting the sub-satellite’s orbit. This constant perturbation would destabilize the orbit entirely. The object would either be ejected from the Moon’s grasp into an orbit around the Earth, or its trajectory would decay, causing it to crash onto the lunar surface.
The Moon’s gravitational field is also not perfectly uniform, which further complicates stable orbits. Regions of enhanced gravity called mass concentrations, or “mascons,” exist beneath the lunar surface, particularly on the near side. These dense areas tug unevenly on any low-flying satellite, causing its orbit to wobble and change shape rapidly. Even artificial satellites in low-lunar orbit require frequent course corrections due to this uneven distribution of mass.
Defining the Moon’s Gravitational Reach
The physical boundary of the Moon’s gravitational dominance is quantified by the Hill Sphere. This concept defines the region of space where the Moon’s gravity is the main force acting on an object, despite the presence of the much larger Earth. The size of this sphere is calculated based on the masses of the Moon and Earth, and the distance separating them.
For our Moon, the calculated radius of its Hill Sphere is approximately 60,000 kilometers. This distance represents the maximum range at which an object could theoretically maintain an orbit around the Moon without being pulled away by the Earth. Since the Moon orbits the Earth at an average distance of about 384,400 kilometers, the Moon’s gravitational reach is limited relative to the space it occupies.
Even within this 60,000-kilometer boundary, achieving a stable orbit is challenging. The most secure orbits are found much closer to the Moon, typically within the inner half of the Hill Sphere radius. Orbits that move in the same direction as the Moon’s rotation (prograde orbits) are far more susceptible to destabilization by Earth’s gravity than those moving in the opposite direction (retrograde orbits). This narrow, dynamic region makes the long-term retention of a natural sub-satellite exceptionally unlikely.
Objects That Mimic Moons
Transient Natural Satellites
One phenomenon involves small asteroids that temporarily enter Earth’s gravitational field, becoming short-lived “mini-moons.” These objects, typically a few meters in diameter, orbit Earth for a few months or years before being flung back out into a solar orbit.
Quasi-Satellites
A related class of objects is the quasi-satellite, which is gravitationally linked to Earth but does not orbit the planet itself. Quasi-satellites, such as the asteroid Kamoʻoalewa, share the Earth’s orbital period around the Sun. Their path makes them appear to loop around Earth when viewed from our planet. These objects are not true moons because their orbits are primarily controlled by the Sun and they are located outside Earth’s Hill Sphere.
Kordylewski Clouds
A final example are the Kordylewski Clouds, sometimes referred to as “dust moons.” These are not solid objects but extremely faint concentrations of cosmic dust trapped at the L4 and L5 Lagrange points of the Earth-Moon system. These points are gravitationally stable regions 60 degrees ahead of and behind the Moon in its orbit. While confirmed to exist as clouds of particles, they do not constitute a solid satellite orbiting the Moon.