Earth Has a “Second Moon” (Sort Of): The Science of Mini-Moons

For most of recorded history, Earth was thought to have a single natural satellite, the Moon. Modern astronomical surveys have revealed that Earth’s orbital neighbourhood is more dynamic than previously assumed. Small asteroids sometimes become temporary companions, and other objects share Earth’s orbit in unusual ways that make them appear moonlike from our perspective. These bodies are commonly described as mini-moons or quasi-moons, although their orbital behaviour differs fundamentally from that of our permanent Moon.

What Counts as a Moon?

In planetary science, a true moon is an object gravitationally bound to a planet that orbits it continuously. Earth’s Moon fits this definition and has remained in orbit for billions of years. Mini-moons and quasi-moons do not meet that strict definition, but they interact with Earth’s gravity in scientifically important ways.

A mini-moon is a small near-Earth asteroid that becomes temporarily captured by Earth’s gravity. During this capture phase, the object completes at least one orbit around Earth before escaping back into a solar orbit. A quasi-moon, by contrast, is not gravitationally bound to Earth. Instead, it shares a one-to-one orbital resonance with Earth around the Sun, creating the illusion that it orbits Earth. The Planetary Society describes quasi-moons as bodies that complete one orbit around the Sun in nearly the same time as Earth does, yet follow slightly different paths that cause them to trace looping patterns relative to Earth.

Mini-Moons: Temporary Gravitational Captures

One recent example of a mini-moon is 2024 PT5, a small asteroid discovered by the Asteroid Terrestrial-impact Last Alert System. According to research published in Research Notes of the American Astronomical Society, 2024 PT5 became temporarily captured by Earth’s gravity between late September and late November 2024 before returning to a heliocentric orbit. Astronomers have documented similar events in the past. Objects such as 2006 RH120 and 2020 CD3 were also temporarily captured for periods ranging from months to years. These bodies are typically only a few meters across and extremely faint, making detection difficult.

The mechanism behind such captures involves Earth’s Hill sphere, the region of space where Earth’s gravity can dominate the Sun’s for slow-moving objects. If an asteroid approaches Earth with sufficiently low relative velocity and within the correct distance, gravitational interactions can briefly trap it into orbit. Perturbations from the Sun and the Moon eventually destabilise the orbit, allowing the object to escape. Astronomer Grigori Fedorets has noted in interviews that mini-moons are usually small and difficult to detect, yet they provide valuable data about the near-Earth object population because their proximity allows detailed observation.

Quasi-Moons: Resonant Companions

Unlike mini-moons, quasi-moons are not captured by Earth’s gravity. Instead, they move in resonance with Earth around the Sun. A well-studied example is 469219 Kamoʻoalewa, which follows a stable resonant orbit that keeps it near Earth for extended periods. From Earth’s vantage point, Kamoʻoalewa appears to circle our planet in a looping path. However, detailed orbital calculations show that it is primarily bound to the Sun rather than to Earth. The motion arises from a delicate gravitational balance within the Sun-Earth system.

Dynamical simulations conducted by researchers, including Carlos de la Fuente Marcos and Raúl de la Fuente Marco,s demonstrate how quasi-moons can remain in this configuration for decades or even centuries before gravitational perturbations alter their paths.

Why These Objects Matter

Mini-moons and quasi-moons are not merely astronomical curiosities. Their study improves understanding of orbital mechanics, gravitational interactions, and the distribution of small near-Earth objects. Because mini-moons approach Earth at relatively low speeds, they offer opportunities for future spacecraft missions to study asteroid composition up close.

These objects also refine models of impact risk assessment. Understanding how near-Earth asteroids move through Earth’s gravitational sphere helps scientists improve predictions about potential Earth-crossing trajectories.

A Dynamic Celestial Environment

The discovery of temporary mini-moons and long-term quasi-moons has reshaped our understanding of Earth’s immediate cosmic surroundings. While Earth still has only one permanent Moon, it occasionally hosts transient visitors and shares orbital space with resonant companions.

These findings highlight that planetary systems are not static. Instead, they involve continuous gravitational interactions that produce temporary configurations and subtle orbital dances. Through careful observation and mathematical modelling, astronomers continue to uncover how small bodies move within the Solar System, revealing that Earth’s neighbourhood is more active and complex than once believed.