What is a quasi-moon?
A quasi-moon is a curious kind of space object that shares a path with Earth around the Sun but does not orbit Earth in the way the Moon does. Instead of being permanently bound by Earth’s gravity, a quasi-moon follows a route that closely tracks our planet’s journey around the Sun. For years or even decades, it appears to drift alongside Earth, appearing to loop near our world without ever becoming a true satellite. In practice, its motion is governed primarily by the Sun’s gravity, with Earth only playing a minor role in shaping its delicate dance.
The case for a quasi-moon: what makes 2023 FW13 special
Astronomers recently confirmed that 2023 FW13, a small asteroid roughly 15 to 20 metres across, has been moving in step with Earth since at least the 1960s. Its journey was uncovered by combining decades of telescope observations from facilities in Hawaii and Arizona. The object is faint and dark, reflecting very little sunlight, which helped it hide in the glare of the night sky for a long time until modern sky surveys grew sensitive enough to notice it. 2023 FW13 stands out because it has remained closely associated with Earth for more than half a century, making it one of the most stable quasi-moons ever observed.
How scientists spotted a silent companion
Modern sky surveys such as Pan-STARRS in Hawaii and the Canada-France-Hawaii Telescope have transformed our ability to detect tiny, distant objects. 2023 FW13’s faintness meant it escaped notice for decades, but improved imaging and repeated tracking allowed researchers to reconstruct its long-term path. By tracing its motion backward in time, scientists demonstrated that the asteroid’s orbit has remained unusually synchronized with Earth, even as it travels across the broader Solar System.
How a quasi-moon moves: the horseshoe orbit
Unlike the Moon’s relatively neat, circular orbit around Earth, quasi-moons follow a more complex trajectory that traces a horseshoe-shaped path relative to our planet’s orbit around the Sun. The object can swing ahead of Earth, then fall behind as both bodies orbit the Sun together. In some cases, the quasi-moon can drift as far as 14 million kilometres from Earth—roughly 40 times farther than the actual Moon—yet stay dynamically connected through the Sun’s gravity. This configuration is why these objects are described as “quasi-moons” rather than true satellites: they are not gravitationally bound in the same tight way as the Moon, but they share a traveling companionship with Earth.
Why quasi-moons matter to science
Studying quasi-moons like 2023 FW13 helps scientists test ideas about how gravity works on small bodies in the Solar System. Their orbits act as natural probes of dynamical processes that shape how objects move when they’re not simply bound to a planet. By analyzing how these bodies drift, whip around, or escape, researchers refine models of planetary gravity, solar radiation effects, and the long-term evolution of near-Earth space. In addition, quasi-moons can offer clues about the early history of our planet and the sources of small bodies that drift through Earth’s neighborhood.
What we learn—and what we still question
Each newly tracked quasi-moon adds a data point to the bigger question: where do such objects come from, and how long do they linger near Earth? 2023 FW13’s enduring presence suggests that some companions can remain in Earth’s vicinity for centuries or longer, challenging previous assumptions about the fleeting nature of quasi-satellites. Ongoing monitoring will clarify whether these bodies are native dust or fragments from past collisions, and whether similar objects exist on unseen, longer-lived orbits.
Are there others? The broader landscape
Aside from 2023 FW13, another well-known quasi-moon is Kamoʻoalewa, a name that reflects its persistent, though not gravitationally bound, proximity to Earth. Together, these objects reveal that Earth’s neighborhood is more crowded than once thought, with a variety of small bodies sharing our solar journey in subtle, enduring ways. Most quasi-moons—unlike our Moon—do not stay forever; their orbits can evolve, drift away, or be perturbed by planetary gravity over time. The discovery of long-lasting quasi-moons expands our understanding of how small bodies migrate through the inner Solar System.
What comes next for the study of quasi-moons
Scientists plan to keep tracking 2023 FW13 and any newly identified quasi-moons to observe changes in their orbits as gravitational interactions unfold. Improvements in telescope sensitivity and survey cadence will help detect fainter objects that would otherwise remain invisible. With more data, researchers hope to uncover clues about the origins of these companions and refine techniques for detecting potentially hazardous space rocks that could pose a threat to Earth in the future. These silent companions thus contribute to a richer, more nuanced map of Earth’s cosmic surroundings.
The final word: a 60-year, quiet cosmic friendship
Quasi-moons like 2023 FW13 are small, elusive travelers that remind us Earth’s orbit is not as solitary as it might seem. They drift in and out of our celestial neighborhood, governed by the Sun’s gravity, offering a unique laboratory for testing gravity, orbital dynamics, and the history of our planetary system. As astronomers continue to watch these faint wanderers, we gain fresh insight into the delicate balance that shapes the Solar System we call home.
