Earth has only one permanent, large natural satellite: the Moon (Luna). The answer to whether Earth has a second moon is technically no, based on the scientific criteria Luna meets. However, this definitive “no” fails to capture the complexity of objects that share Earth’s orbital neighborhood, leading to persistent public interest in a “second moon.” Our planet constantly interacts with smaller, less stable companions whose movements are intricately linked to our own. These objects fall into distinct categories, each with a unique gravitational relationship to the Earth-Sun system.
What Defines a Natural Satellite?
A true natural satellite, like Earth’s Moon, maintains a stable, long-term orbit primarily around its planet, rather than the Sun. The boundary for this gravitational dominance is the Hill sphere, a region where a planet’s gravity outweighs the Sun’s pull. For Earth, the Hill sphere extends approximately 1.5 million kilometers, creating a stable zone for orbiting bodies. An object must remain reliably within this sphere to be considered a moon; otherwise, the Sun’s gravity will eventually pull it away.
The Moon orbits Earth at an average distance of about 384,400 kilometers, well within this sphere of influence. A stable orbit also requires the satellite to have significant mass, ensuring its motion is not easily perturbed. Luna’s large mass and stable orbit make it a permanent fixture, distinguishing it from the small, temporary objects that occasionally enter Earth’s gravitational neighborhood.
The Co-Orbitals (Quasi-Moons)
The most persistent candidates for a “second moon” are co-orbitals, sometimes called quasi-satellites. These are small near-Earth asteroids that share Earth’s orbital period, meaning they take almost exactly one year to orbit the Sun. This 1:1 orbital resonance creates a complex, synchronized motion where the object appears to “follow” the Earth around the Sun.
One prominent example is the asteroid 3753 Cruithne, which follows a horseshoe-shaped path relative to Earth. While Cruithne’s orbit brings it periodically close to Earth, it never actually orbits our planet, maintaining its fundamental orbit around the Sun. Its complex path means that, from Earth’s perspective, it traces a looping pattern that encircles our planet.
Another significant quasi-satellite is 2016 HO3, which has been in a stable co-orbital relationship with Earth for about a century. This asteroid, estimated to be between 40 and 100 meters in size, follows an intricate path that alternately takes it sunward and ahead of Earth for half the year, then slightly farther from the Sun and behind us. The orbit of 2016 HO3 causes it to loop around Earth, but it remains gravitationally bound to the Sun, never venturing closer than about 9.1 million miles from our planet. These objects are not true moons because their orbits are fundamentally heliocentric, or centered on the Sun, even though their paths are intricately linked with Earth’s.
Transient Mini-Moons
A distinctly different group of companions are the transient mini-moons, which are asteroids temporarily captured by Earth’s gravity. Capture occurs when a passing near-Earth object enters Earth’s Hill sphere and is pulled into a temporary, unstable orbit. These objects are typically very small, often car-sized or smaller, and their capture is short-lived, lasting only a few months to a few years.
The first confirmed example was 2006 RH120, a tiny asteroid about two to three meters in diameter. It was captured into Earth orbit from July 2006 to July 2007 before escaping back into a solar orbit. More recently, 2020 CD3 was cataloged as another temporary captured object.
The temporary nature of these orbits is due to the combined, constantly shifting gravitational influence of the Sun, the Earth, and the Moon. This complexity makes their orbits highly unstable, and they are eventually ejected back into independent solar orbits by a gravitational slingshot effect. Although they are true satellites during their short time in orbit, their small size and fleeting existence prevent them from meeting the criteria for a permanent natural satellite.