The question of whether Earth possesses two moons is common, fueled by recent astronomical discoveries. The simple answer is that Earth has only one permanent, gravitationally bound natural satellite: the Moon. Complexity arises because various other celestial objects are temporarily captured or share Earth’s orbital path around the Sun. Understanding the dynamics of these objects requires a precise definition of what constitutes a true moon.
Defining a Natural Satellite
A natural satellite, or moon, is scientifically defined by the stability and nature of its orbit around a parent body. To be classified as a moon, an object must be gravitationally bound to the planet and orbit it over long timescales. This requires the orbit to be stable against gravitational perturbations from the Sun and other planets.
The key to this classification is the celestial body’s orbit remaining securely within the planet’s Hill sphere. The Hill sphere represents the region where a planet’s gravity is the dominant force influencing smaller objects. Objects with a permanent, regular orbit within this zone are considered true natural satellites.
Earth’s Primary Moon
Earth’s one permanent natural satellite is simply called the Moon, serving as the definitive example of a stable orbit. Its nearly circular path around our planet has remained stable for billions of years, meeting all scientific standards for a true moon. The Moon is massive enough that its gravitational pull significantly affects Earth, most visibly by controlling the ocean tides.
The Moon also stabilizes Earth’s axial tilt, preventing dramatic shifts that would cause extreme climate variations over geological timescales. This long-term, stable relationship and the Moon’s substantial size—roughly one-quarter the diameter of Earth—set it apart from all other objects in our planet’s vicinity.
Transient Mini-Moons
The objects most frequently mistaken for a second moon are transient mini-moons, which are small asteroids temporarily captured by Earth’s gravity. These captures occur when a passing near-Earth asteroid slows down enough to be pulled into a highly unstable orbit within the Earth-Moon system. Their orbits are chaotic, constantly perturbed by the combined gravitational forces of the Earth, the Moon, and the Sun.
These mini-moons typically remain in orbit for only a few months to a few years before the Sun’s gravity eventually flings them back into a solar orbit. One well-known example is the small asteroid 2020 CD3, estimated to be 6 to 12 feet (2 to 3.5 meters) across. This object was captured sometime between 2016 and 2018, discovered in early 2020, and subsequently ejected from Earth’s orbit by March of that year.
The existence of 2020 CD3 and previous examples like 2006 RH120 demonstrate that Earth regularly and temporarily captures such objects. Their short-lived presence and highly erratic paths, which can vary wildly in distance from Earth, contrast sharply with the stable, long-term orbit of the Moon.
Quasi-Satellites and Co-Orbitals
A separate category of object that complicates the “two moons” question involves quasi-satellites and co-orbitals. These bodies are not gravitationally bound to Earth, but they share the same orbital period around the Sun as Earth does, making them appear to follow our planet. A quasi-satellite’s path is primarily dictated by the Sun’s gravity.
These co-orbital objects maintain their relationship with Earth through a 1:1 orbital resonance, meaning they complete one orbit around the Sun in the same time as Earth. From Earth’s perspective, objects like the asteroid 3753 Cruithne appear to trace a complex, kidney-bean or horseshoe-shaped path. Cruithne’s orbit is far outside Earth’s Hill sphere, confirming it is a solar orbiter that periodically approaches our planet, not a true satellite.
Another notable example is the quasi-satellite Kamoʻoalewa (469219, 2016 HO3), which is the most stable and well-studied of Earth’s quasi-satellites. It is estimated to be about 150 feet (46 meters) in diameter and remains relatively close to Earth, though it is still orbiting the Sun. These co-orbitals highlight the complex gravitational dynamics in our solar neighborhood, but they do not meet the criteria to be considered a second moon.