Earth regularly picks up small asteroids that loop around our planet for weeks, months, or even years before drifting away, and these temporary companions are often called “second moons” or “mini-moons.” The permanent moon we see every night remains Earth’s only long-term natural satellite, but the space around our planet is far busier than most people realize. Several categories of small rocky visitors share Earth’s neighborhood in different ways, from objects genuinely captured into orbit to asteroids that merely shadow our planet along its path around the Sun.
What Counts as a Second Moon
The phrase “Earth’s second moon” gets used loosely, so researchers have drawn sharper lines. The most literal version is a temporarily captured orbiter, or TCO: a small asteroid that enters the region of Earth’s gravitational dominance and completes at least one full loop around our planet before escaping back into orbit around the Sun. A related category is the temporarily captured flyby, or TCF: an object that enters that same gravitational zone and is briefly bound to Earth but leaves before finishing a complete orbit.1Icarus. Orbit and size distributions for asteroids temporarily captured by the Earth-Moon system Both types are genuine, if fleeting, natural satellites. They arrive unannounced from the population of near-Earth asteroids, stay for a while, and then the Sun’s gravity wins them back.
Then there are quasi-satellites, asteroids that orbit the Sun at nearly the same distance and speed as Earth, so they appear to circle our planet when viewed from a geocentric perspective. They are not gravitationally bound to Earth at all; they are really orbiting the Sun in a resonance that keeps them nearby. And further out, Earth Trojans sit in stable gravitational sweet spots along Earth’s orbit. None of these are “moons” in the way most people mean the word, but all of them contribute to the popular idea that Earth has more than one companion.
The Mini-Moons We Have Actually Seen
Only a handful of temporarily captured objects have been confirmed by observation. The first was 2006 RH120, a small asteroid roughly a few meters across that was captured into a geocentric orbit from about July 2006 to July 2007.2Monthly Notices of the Royal Astronomical Society. On the orbital evolution of meteoroid 2020 CD3, a temporarily captured orbiter of the Earth–Moon system It completed several wobbly loops around Earth before the Sun pulled it away. At the time, it was a novelty. Researchers suspected more objects like it existed but were simply too small and faint to spot.
That suspicion was confirmed about a decade later with the discovery of 2020 CD3. This object was even smaller, roughly 0.9 meters in diameter based on spectral observations with the Keck telescope, making it one of the tiniest asteroids ever studied in detail.3The Astrophysical Journal Letters. Characterization of Temporarily Captured Minimoon 2020 CD3 by Keck Time-resolved Spectrophotometry Its capture duration was unexpectedly long. Calculations suggest it entered Earth orbit around 2016, give or take a couple of years, and was not discovered until February 2020, shortly before it escaped back to a Sun-centered orbit in May of that year.4Monthly Notices of the Royal Astronomical Society. On the orbital evolution of meteoroid 2020 CD3, a temporarily captured orbiter of the Earth–Moon system That roughly four-year stay was longer than models had predicted for a typical mini-moon, though it aligns well with simulations of mini-moons that have close encounters with the Moon itself, which can nudge them into longer-lived orbits.5The Astronomical Journal. Establishing Earth’s Minimoon Population through Characterization of Asteroid 2020 CD3
The most recent headline-grabber was 2024 PT5. This small asteroid transitioned onto a geocentric orbit in late September 2024 and returned to a heliocentric path by mid-November, a visit lasting roughly two months.6Research Notes of the AAS. A Two-month Mini-moon: 2024 PT5 Captured by Earth from September to November7Odessa Astronomical Publications. DETERMINING THE ORBIT OF THE TEMPORARY EARTH SATELLITE OF ASTEROID 2024 PT5 Spectral analysis showed it has a basaltic composition, similar to certain volcanic rocks, and it follows a horseshoe-type path relative to Earth, meaning it will periodically return to our neighborhood.8Astronomy & Astrophysics. Basaltic quasi-mini-moon: Characterizing 2024 PT5 with the 10.4 m Gran Telescopio Canarias and the Two-meter Twin Telescope Despite widespread media coverage calling it Earth’s “new moon,” it was far too small and dim to see without a professional telescope.
Kamoʻoalewa, Earth’s Steadiest Companion
If any object deserves the informal title of Earth’s second moon, the quasi-satellite 469219 Kamoʻoalewa is probably the strongest candidate. Discovered in 2016, it is currently considered the most stable quasi-satellite of Earth, meaning it has been locked in its co-orbital dance with our planet longer than any other known object of this type.9The Astronomical Journal. Peculiar Orbital Characteristics of Earth Quasi-Satellite 469219 Kamo’oalewa: Implications for the Yarkovsky Detection and Orbital Uncertainty Propagation It never comes closer than about 38 times the Earth-Moon distance, so calling it a “moon” stretches the definition considerably. It orbits the Sun, not Earth. But from our vantage point, it appears to make a slow, looping circuit around us each year.
What makes Kamoʻoalewa especially interesting is where it seems to have come from. Observations with large telescopes revealed that its reflectance spectrum is unusually red, steeper than typical stony asteroids. The best spectral match researchers found was with fine-grained lunar soil samples brought back by Apollo 14.10Communications Earth & Environment. Lunar-like silicate material forms the Earth quasi-satellite (469219) 2016 HO3 Kamoʻoalewa A later study compared the spectrum to data from China’s Yutu-1 rover and Chang’e-5 lander on the lunar surface, as well as laboratory measurements of Chang’e-5 soil samples, and found strong agreement. That analysis concluded Kamoʻoalewa likely originated from the Moon, possibly from its nearside, blasted off by an ancient impact.11The Innovation. Lunar origin of Earth quasi-satellite Kamoʻoalewa
If confirmed, this would make Kamoʻoalewa a piece of our own Moon wandering nearby, a remarkable scenario that has attracted mission planners. China’s Tianwen-2 mission aims to visit Kamoʻoalewa, collect a sample, and return it to Earth before continuing on to explore a main-belt comet.12Celestial Mechanics and Dynamical Astronomy. Co-orbital asteroids of terrestrial planets affected by the von Zeipel–Lidov–Kozai mechanism A returned sample would settle the lunar origin question definitively and provide the first material from a quasi-satellite.
Earth Trojans at the Lagrange Points
Beyond mini-moons and quasi-satellites, Earth also has Trojan asteroids. These sit near the L4 and L5 Lagrange points, positions along Earth’s orbit where the combined gravity of the Sun and Earth creates zones of relative stability. Jupiter has thousands of known Trojans. Earth, being much less massive, has far fewer, and the ones it does have are transient.
The first confirmed Earth Trojan was 2010 TK7, found librating around the L4 point (60 degrees ahead of Earth in its orbit). The second, 2020 XL5, was identified at the same Lagrange point. Dynamical modeling showed that 2020 XL5 has only been an Earth Trojan since roughly the fifteenth century and has about a 99.5% chance of leaving the L4 region within the next 10,000 years, primarily because close approaches to Venus destabilize its orbit.13The Astrophysical Journal Letters. The Second Earth Trojan 2020 XL5 So these are not permanent fixtures of our orbital environment either. They are passing through on astronomical timescales, even though a 10,000-year stay sounds long to us.
Dust Clouds at L4 and L5
Even fainter and harder to observe than rocky Trojans are the Kordylewski dust clouds, diffuse concentrations of interplanetary dust particles that have been reported near the L4 and L5 Lagrange points of the Earth-Moon system (not the Sun-Earth system, a distinction that matters). These were first claimed in the 1960s by the Polish astronomer Kazimierz Kordylewski, and their existence has been debated ever since because they are extremely faint and easily confused with other sources of scattered light.
Simulations have explored why observers historically reported seeing the L5 cloud more often than the L4 cloud. Modeling shows that the total number of dust particles trapped near L5 tends to be slightly larger than at L4, with the imbalance ranging from less than one percent to nearly nine percent depending on the conditions.14Icarus. Why was the Kordylewski dust cloud observed more frequently at the L5 Lagrange point than at L4? Whether this difference is large enough to explain the observational asymmetry is still an open question. The clouds, if they exist as coherent structures, are so tenuous that calling them “moons” would be extremely generous, but they represent yet another way material can be loosely associated with the Earth-Moon system.
Telling Rocks from Rockets
One persistent challenge in mini-moon science is distinguishing a genuine natural asteroid from a piece of human-made space junk. Derelict spacecraft, spent rocket stages, and debris in high orbits or cislunar space can look a lot like small asteroids when first spotted by a survey telescope. Spectral characterization can help separate the two categories, since rocket body paint, solar panels, and metal alloys reflect light differently from silicate rock. But the distinction is not always clean, because phase angle effects, meaning the way brightness changes as the Sun-object-observer geometry shifts through the night and across seasons, can mimic or obscure compositional signatures on artificial objects.15The Planetary Science Journal. Visible Spectral Atlas of Geostationary Satellites from Tucson, AZ for Differentiating Between Natural and Artificial Objects
This is not a theoretical concern. When 2020 CD3 was first spotted, one of the early questions was whether it might be a piece of space hardware rather than a natural body. Its small size put it right in the range where the two populations overlap. The Keck spectral observations that pinned down its composition and rotation ultimately confirmed it was a natural silicate-rich object.16The Astrophysical Journal Letters. Characterization of Temporarily Captured Minimoon 2020 CD3 by Keck Time-resolved Spectrophotometry As cislunar space gets busier with commercial and government missions, the need for rapid spectral follow-up of newly discovered objects will only grow.
How Many Mini-Moons Are Out There
Statistical models suggest that at any given time, Earth probably has at least one or two temporarily captured objects in the one-meter size range, and a larger but harder-to-detect population of smaller pebble-sized visitors. We have found so few because they are tiny, faint, and move quickly against the background stars, making them easy for survey telescopes to miss or dismiss as noise.
That is expected to change with the Vera C. Rubin Observatory’s Legacy Survey of Space and Time, or LSST, which will scan the entire visible sky repeatedly with unprecedented depth. Modeling of the survey’s detection capabilities predicts it will find roughly three to six lunar-origin asteroids (objects larger than about five meters that were blasted off the Moon) per year, with existing surveys like Pan-STARRS and ATLAS finding less than one per year each.17The Astrophysical Journal. Detectability of Lunar-origin Asteroids in the LSST Era Mini-moons more broadly should also be found at higher rates, though because they are observable for a limited window before escaping Earth’s orbit, rapid follow-up observations will be crucial to characterize them before they disappear.18Research Notes of the AAS. Year 1 of the Legacy Survey of Space and Time (LSST): Recommendations for Template Production to Enable Solar System Small Body Transient and Time Domain Science
Why Anyone Cares About Temporary Moons
Mini-moons and Earth co-orbital asteroids are not just curiosities. Their proximity to Earth makes them among the easiest objects in the solar system to reach with a spacecraft, sometimes requiring less fuel than a trip to the Moon. That low energy cost makes them attractive targets for testing technologies like in-situ resource utilization, the idea of mining or processing asteroid material in space rather than launching everything from Earth’s surface. Objects like 2024 PT5, which recurrently pass through Earth’s neighborhood on horseshoe orbits, are particularly well suited because they keep coming back.19Astronomy & Astrophysics. Basaltic quasi-mini-moon: Characterizing 2024 PT5 with the 10.4 m Gran Telescopio Canarias and the Two-meter Twin Telescope
There is also a planetary defense angle. Understanding how small asteroids get captured into Earth orbit teaches us about the delivery pathways for objects that could eventually hit us. Simulations have shown that some small stony asteroids can be captured into temporary orbits by grazing Earth’s atmosphere, skipping off the upper air like a stone on water. A captured object in this scenario could re-enter the atmosphere multiple times before a final impact, producing a fireball and shock wave with each pass.20Monthly Notices of the Royal Astronomical Society. On the capture of small stony asteroids into the Earth’s orbit by atmospheric grazing No confirmed event of this kind has been identified in the historical record, partly because the time between successive atmospheric entries and the geographic spread of the impact zones would make the connection hard to recognize. But knowing that such trajectories are physically possible changes how we think about unusual fireball reports.
Co-orbitals Around Other Planets
Earth is not unique in having a crowd of small companions. Co-orbital asteroids have been identified around Venus and Mars as well, and numerical studies going back decades have argued for a steady-state flux of objects temporarily captured into co-orbital configurations with all the terrestrial planets.21Icarus. A Numerical Survey of Transient Co-orbitals of the Terrestrial Planets Recent work has expanded Venus’s known co-orbital population to around 20 objects, putting it on par with Earth and Mars.22Research Notes of the AAS. New Transient Co-orbital Asteroids of Venus
The dynamics differ from planet to planet. Some Earth co-orbitals experience long-term gravitational effects from the interplay of their orbital tilt and eccentricity that can cause their orbits to oscillate dramatically over thousands of years, a behavior also seen for at least one Mars co-orbital. Venus’s co-orbital population, by contrast, does not appear to show the same pattern strongly.23Celestial Mechanics and Dynamical Astronomy. Co-orbital asteroids of terrestrial planets affected by the von Zeipel–Lidov–Kozai mechanism The key takeaway is that temporary moons and co-orbital companions are not a quirk of Earth. They are a normal feature of how small bodies interact with planets, and the better our telescopes get, the more of them we find everywhere.
When the Next One Might Arrive
Because mini-moon captures are driven by the chaotic interplay of Sun, Earth, and Moon gravity acting on a vast population of small near-Earth asteroids, predicting exactly when the next one will show up is not really possible. What we can say is that the population models and the pace of recent discoveries both point toward captures happening regularly. 2006 RH120 was found by chance. 2020 CD3 was found by a survey specifically designed to catch fast-moving objects. 2024 PT5 was spotted almost routinely. As survey capabilities improve, the gap between discoveries should shrink.
Objects on horseshoe orbits, like 2024 PT5, are especially interesting because their returns are somewhat predictable. They swing past Earth, get nudged slightly, drift away for a few years or decades, and then come back for another close encounter that may or may not result in a temporary capture. The asteroid 3753 Cruithne, long misreported in popular media as “Earth’s second moon” after its discovery in 1986, follows a complex horseshoe-and-bean-shaped path relative to Earth that will persist for tens of thousands of years, though it never comes close enough to be captured into an actual geocentric orbit.24Icarus. A Numerical Survey of Transient Co-orbitals of the Terrestrial Planets The distinction between “orbiting the Sun near Earth” and “orbiting Earth” keeps getting muddled in news coverage, which is how the idea of a permanent second moon persists even though no such object exists.