Earth has one large, permanent natural satellite, known as the Moon. This companion has been the definitive natural satellite of our planet, orbiting Earth in a stable path. While the Moon is the only true answer to the question of whether Earth has a permanent moon, the complete answer must also consider a variety of other transient objects. These celestial bodies are sometimes temporarily captured or orbit the Sun in a way that makes them appear to be Earth’s companions.
Earth’s Permanent Companion (The Moon)
The Moon is unique in the Solar System due to its size relative to its parent planet. With a diameter of about 3,475 kilometers, the Moon is more than one-quarter the width of Earth, a ratio that is larger than any other planet-moon system in our neighborhood. This size means the Earth-Moon pair is sometimes described as a double-planet system, as the two bodies orbit a common gravitational center, called the barycenter, which lies between their cores. The Moon’s mass exerts a gravitational influence on Earth, driving ocean tides and stabilizing our planet’s axial tilt over astronomical timescales.
The Moon follows a fixed, stable orbit around Earth at an average distance of approximately 384,400 kilometers. Its formation story, known as the Giant Impact Hypothesis, suggests this permanence began around 4.5 billion years ago. This theory posits that a Mars-sized protoplanet collided with the proto-Earth. The resulting debris cloud coalesced to form the Moon, giving it a composition similar to Earth’s mantle but with a deficiency of iron, which helps explain its lower density.
This origin and subsequent gravitational relationship underscore the Moon’s status as a permanent fixture of the Earth system. The stability of its orbit ensures that it remains firmly within Earth’s Hill sphere, the region of space where Earth’s gravity dominates the Sun’s.
The Criteria for Lunar Status
A celestial body must meet specific technical requirements to be classified as a true “moon” or natural satellite. The primary condition is that the object must be gravitationally bound to the planet and orbit it in a sustained, stable trajectory. This orbital relationship must be firm enough that the planet’s gravitational pull is the primary force dictating the object’s path, overcoming the influence of the Sun.
For a natural satellite to be considered permanent, its orbit must exist within the planet’s Hill sphere. The Hill sphere is the region where the planet’s gravity is the dominant factor for objects in orbit around it. Objects outside this boundary are primarily under the Sun’s gravitational control. The long-term stability of the orbit over millions or billions of years is what separates a true moon from a transient companion.
Temporary Satellites and Quasi-Moons
Earth does have other occasional, smaller companions. These objects fall into two categories, neither of which meets the standard for permanent lunar status.
The first category involves temporary captured objects, often called “mini-moons.” These are small asteroids, typically only a few meters in diameter, that briefly enter Earth’s Hill sphere and are temporarily pulled into orbit by our planet’s gravity. Their orbits are highly unstable and short-lived, lasting only a few months or at most a few years before the object either escapes back into a solar orbit or burns up in the atmosphere. For example, the asteroid 2020 CD3 was a temporary satellite for a period between 2018 and 2020 before departing.
The second category is quasi-satellites, which are asteroids that appear to orbit Earth but are not technically true moons. These objects are in a co-orbital configuration with Earth, meaning they orbit the Sun and share the same orbital period as our planet. When viewed from Earth’s perspective, a quasi-satellite appears to loop around our planet in a complex, elongated path. However, because they orbit the Sun and lie outside the Earth’s Hill sphere, they are fundamentally controlled by the Sun’s gravity. Asteroids like 469219 Kamoʻoalewa and 3753 Cruithne are examples of quasi-satellites that maintain this synchronized, temporary relationship with Earth.