The question of whether Earth possesses a second moon has long captured public imagination. While the traditional answer is no, modern celestial mechanics reveals a complex population of space rocks that share a temporary or resonant relationship with Earth. Defining a “moon” requires distinguishing between objects truly gravitationally bound to a planet and those that merely appear to follow it. The scientific community has identified several types of co-orbitals, none of which meet the long-term, stable criteria set by our primary Moon.
Defining Earth’s Primary Moon and Stable Satellites
Earth’s one permanent natural satellite, the Moon, serves as the standard against which other potential companions are measured. It is locked in a stable orbit, maintained by a consistent balance between its orbital velocity and Earth’s gravitational pull. This orbit is dynamically stable, ensuring its consistent path over billions of years.
A natural satellite must operate within the gravitational sphere of influence of its planet, known as the Hill sphere. The Moon’s massive size gives it overwhelming gravitational dominance over any other object that might attempt to orbit Earth. Objects outside this sphere are governed primarily by the Sun’s gravity, which disqualifies many candidates often called “second moons.”
Quasi-Satellites and Resonant Orbiters
Co-orbital objects maintain a long-term relationship with Earth without being true satellites. These objects are in a 1:1 orbital resonance, meaning they take nearly the same amount of time to orbit the Sun as Earth does. A quasi-satellite appears to trace an oblong, looping path around our planet, but it is technically orbiting the Sun.
The most famous example is the asteroid 3753 Cruithne, sometimes inaccurately called Earth’s “second moon.” Cruithne is in a horseshoe orbit, where the asteroid appears to chase or be chased by Earth, but never crosses its orbital track. Its orbit is highly eccentric and remains well outside Earth’s Hill sphere, confirming it is not gravitationally bound.
Cruithne completes a full cycle of movement relative to Earth approximately every 770 years. Since the object primarily orbits the Sun, just like Earth, it cannot be classified as a moon. Several other objects, such as 2002 AA29, also share this 1:1 resonance, demonstrating that this configuration is a common feature of solar system dynamics.
Transiently Captured Orbiters (Mini-Moons)
Transiently captured orbiters, or “mini-moons,” are small asteroids that temporarily enter a true orbit around Earth. This occurs when a slow-moving near-Earth asteroid is captured by Earth’s gravity, typically lasting a few months to a few years. The orbits of these captured objects are highly unstable due to the chaotic gravitational influences of the Sun and the Moon.
The asteroid 2020 CD3 is a recent, confirmed example of a mini-moon, captured by Earth sometime between 2016 and 2017. Its temporary capture orbit was highly irregular and ended around May 2020, when the asteroid escaped back into a solar orbit. The mechanism is temporary satellite capture, which pulls in an asteroid that has an Earth-like heliocentric orbit and low relative velocity.
The first asteroid confirmed to undergo this process was 2006 RH120, which orbited Earth between 2006 and 2007 before moving on. These fleeting companions are constantly being captured and released. Earth likely has a temporary mini-moon at any given time, but none of them are permanent.
The Search for Kordylewski Clouds
The theoretical Kordylewski Clouds are accumulations of interplanetary dust located at the Earth-Moon Lagrange points L4 and L5. These points are gravitationally stable regions that form the apexes of equilateral triangles with the Earth and the Moon. Objects placed here tend to remain relatively fixed in position.
The Kordylewski Clouds are not solid objects but extremely faint, diffuse concentrations of dust, making them difficult to observe. Polish astronomer Kazimierz Kordylewski first reported observing these clouds in 1961, though their existence remained controversial. Their ephemeral nature means they do not meet the criteria for a traditional moon, as the dust particles are constantly being swept away and replaced by solar radiation pressure and other gravitational perturbations.
In 2018, Hungarian astronomers confirmed the existence of these dust clouds using specialized imaging polarimetry. The confirmation showed that the clouds are large, rapidly evolving structures of dust that scatter sunlight. Their existence confirms a prediction of celestial mechanics, but they are best described as dusty pseudo-satellites rather than a second moon.