Who Are Earth’s Mini-Moons and Quasi-Moons? Meet Our Planet’s Hidden Companions
Have you ever looked up at the Moon and wondered if it’s truly alone up there? What if we told you that Earth keeps other companions—tiny, secret moons that come and go while nobody’s watching?
Welcome, dear friends, to another article written specifically for you by FreeAstroScience.com, where we explain complex scientific principles in simple terms. Today we’re introducing you to mini-moons and quasi-moons: small asteroids that dance with our planet, sometimes for a few weeks, sometimes for generations. One of them may even be a lost piece of the Moon itself—and a spacecraft is hovering beside it right now.
Whoever you are and wherever you’re reading from, this story belongs to you too. Stay with us to the very end, and we promise you’ll never look at the night sky the same way again.
What Are Mini-Moons? Are They Real Moons?
Picture a hitchhiker catching a ride, then hopping off at the next town. That’s a mini-moon.
Mini-moons, or Temporarily Captured Objects (TCOs), are small asteroids—usually just a few meters wide—that get snagged by Earth’s gravity for a short while. They aren’t permanent like our Moon. They slip into our planet’s gravitational net, circle us for weeks or months, then break free and resume their own path around the Sun.
To qualify, an asteroid must enter Earth’s Hill sphere moving slower than the local escape velocity. It then completes at least one partial loop before the Sun and Moon tug it away. Most are so small and dark that we never spot them. In fact, astronomers have only ever observed four: 1991 VG, 2006 RH120, 2020 CD3, and 2024 PT5. Each one arrived quietly, stayed briefly, and left without a goodbye.
What Are Quasi-Moons? Why Do They Stick Around for Decades?
Quasi-moons play a longer game. They never actually orbit Earth. Their paths around the Sun match ours so closely that, from our window seat, they seem to loop around us. Astronomers call this a 1:1 mean-motion resonance: one lap around the Sun for them, one lap for us, year after year.
Think of two cars racing side by side on a circular track. One surges ahead, the other catches up. From inside either car, the neighbor seems to circle you. Neither ever leaves the track. That’s the illusion a quasi-moon creates.
With the discovery of 2025 PN7, roughly eight quasi-moons are now known. They’re larger and easier to track than mini-moons, and they can shadow us for decades or even centuries.
| Feature | Mini-Moons | Quasi-Moons |
|---|---|---|
| Gravitational binding | Temporarily bound to Earth | Not bound to Earth; they orbit the Sun |
| Orbit type | Partial orbits around Earth | 1:1 resonance with Earth around the Sun |
| Duration | Weeks to months (~9 months on average) | Decades to centuries |
| How many | 1–6 at any time; only 4 ever observed | About 8 known (as of 2026) |
| Detection difficulty | Extremely faint and fast | Larger, easier to track |
How Big Is Earth’s Gravitational Net? The Hill Sphere Explained
Earth’s gravity does more than hold onto our Moon. It carves out a huge zone of influence called the Hill sphere. Inside this bubble, Earth’s pull beats the Sun’s, so small objects can be temporarily captured. The bubble stretches out to about 1.47 million kilometers (roughly 0.0098 AU)—nearly four times the Moon’s distance from us.
rH ≈ a(1−e) × (m / 3M)1/3
Where: rH = Hill sphere radius, a = semi-major axis of Earth’s orbit, e = orbital eccentricity, m = mass of Earth, M = mass of the Sun.
What Speed Seals the Capture?
At Earth’s surface, escape velocity is 11.19 km/s. Farther out, that threshold drops. A passing asteroid becomes a mini-moon only if its speed relative to Earth falls below the local escape velocity at its distance. Slower than the limit? Captured. Faster? It just waves and moves on.
Is Earth Ever Really Alone? How Common Are Mini-Moons?
Here’s the part we love telling you. Earth is almost never alone.
Simulations by Fedorets, Granvik, and Jedicke (2017) suggest our planet hosts one to six mini-moons a few meters across at any given moment. They’re simply too small and faint to see without powerful telescopes. These quiet hitchhikers come and go while we sleep, making our cosmic neighborhood far livelier than it looks.
2024 PT5: What Did Our Latest Two-Month Guest Teach Us?
The most recent confirmed mini-moon is 2024 PT5. The ATLAS telescope in Sutherland, South Africa, discovered it on August 7, 2024. Earth’s gravity loosely held this roughly 10-meter rock from September 29 to November 25, 2024. Two months of cosmic company, then farewell.
What makes it special? Its spectrum resembles lunar rock. That hints it may be a chunk of the Moon, blasted off by an ancient impact. If confirmed, it would be the second near-Earth asteroid with a suspected lunar origin, after Kamoʻoalewa. Imagine that: a possible piece of our own Moon came home for a visit, circled us briefly, and drifted away again.
2025 PN7: Who Is the Newest Quasi-Moon Dancing Near Earth?
In August 2025, the Pan-STARRS 1 telescope at Haleakalā Observatory in Hawaii spotted something new: 2025 PN7, a quasi-moon about 19 meters wide. Orbital analysis revealed a lovely surprise. This little rock has kept pace with Earth since the mid-1960s, and it should stay in step until around 2083. That’s nearly 120 years of silent companionship—most of it before we even knew it existed.
Its orbit is almost a twin of ours: a semi-major axis of about 1.003 AU, an eccentricity near 0.11, and a tilt of roughly 2 degrees. At magnitude ~26, it’s far too faint for amateur telescopes, which explains how it dodged us for six decades. Archival images later exposed it hiding in survey data as far back as 2014. The Dark Energy Survey even photographed it on November 17, 2018, years before anyone knew what that faint dot was.
Kamoʻoalewa: Why Is It Earth’s Most Faithful Companion?
Now meet the veteran. Kamoʻoalewa (469219, formerly 2016 HO3) is the most loyal quasi-moon we know. Pan-STARRS discovered it on April 27, 2016. It measures between 40 and 100 meters across—about the size of a city block—and it has shadowed Earth for roughly a century. Projections say it’ll stay with us for another 300 years. Its Hawaiian name means “oscillating celestial fragment,” and honestly, the name fits like a glove.
In 2021, a team led by Ben Sharkey found that Kamoʻoalewa’s spectrum closely matches space-weathered lunar silicates. That raised a bold idea: it may be a fragment of our own Moon, possibly ejected by the impact that carved the Giordano Bruno crater. Some newer studies point to Tycho instead. It also spins fast, completing a rotation every 28 minutes.
A word of honesty from us: the lunar origin remains a strong hypothesis, not a settled fact. Distant spectra can fool even careful observers, since space weathering changes how a surface reflects light. That uncertainty is exactly why a spacecraft is parked beside it right now.
Tianwen-2: What Is China’s Probe Doing at Kamoʻoalewa Right Now?
As we publish this (July 4, 2026), China’s Tianwen-2 probe is closing to within about 20 kilometers of Kamoʻoalewa—humanity’s first true close-up of a quasi-moon. The spacecraft launched on May 29, 2025 (Beijing time) aboard a Long March 3B rocket. After a 13-month cruise, it reached the asteroid’s vicinity around June 7, 2026. Charmingly, radio amateurs in Germany and the Netherlands confirmed the approach before any official announcement did.
How Do You Grab a Piece of a Spinning Rock?
Carefully. Very carefully. From July 2026 to early 2027, Tianwen-2 will map the surface with cameras, LiDAR, and radar. Then it will try to collect at least 100 grams of material—possibly 200 to 1,000 grams. Mission planners can choose among hovering suction, a touch-and-go pass, and a never-before-tried anchor-and-attach method using drill-tipped robotic arms. Remember, this rock rotates once every 28 minutes. Sampling it is like snatching a coin off a spinning record.
The samples should land near the Jiuquan Satellite Launch Center in November 2027. After the drop-off, the probe heads onward to comet 311P/PANSTARRS, arriving in 2035. Small rock, giant ambitions.
| Event | Date / Period |
|---|---|
| Launch (Long March 3B, Xichang) | May 29, 2025 |
| Rendezvous with Kamoʻoalewa | ~June 7, 2026 |
| Close approach to ~20 km, first detailed images | July 4, 2026 |
| Mapping, site selection & sample collection | July 2026 – April 2027 |
| Departure from Kamoʻoalewa | April 24, 2027 |
| Capsule release & landing near Jiuquan | November 29, 2027 |
| Extended mission: arrival at comet 311P/PANSTARRS | January 24, 2035 |
How Do We Find Objects the Size of a Bus in the Dark?
Spotting these tiny, coal-dark objects is like finding a black cat in a cellar—at midnight, with the lights off. Astronomers lean on three pillars:
- Automated wide-field telescopes. ATLAS, with four telescopes across Hawaii, Chile, and South Africa, scans the visible sky several times each night. Each unit pairs a 50-cm telescope with a 110-megapixel camera, catching objects down to magnitude 20.
- Deep surveys. Pan-STARRS in Hawaii uses two 1.8-meter telescopes and a 1.4-gigapixel camera to reach fainter targets, down to magnitude 22.5. It found both Kamoʻoalewa and 2025 PN7. Stars stay put between exposures; asteroids betray themselves by moving.
- Orbit calculation. NASA’s Center for Near-Earth Object Studies (CNEOS) crunches the numbers. Its computers trace each object’s path and check whether its speed drops below the local escape velocity. If it does, temporary capture is confirmed.
| System | Specs / Location | Method | Role |
|---|---|---|---|
| ATLAS | 4 × 50 cm; Hawaii, Chile, South Africa | Wide-field, repeated exposures, real-time motion detection | Fast, small NEOs & mini-moons |
| Pan-STARRS | 2 × 1.8 m; Haleakalā, Hawaii | Deep survey, MOPS moving-object software | Fainter NEOs & quasi-moons |
| CNEOS (NASA JPL) | Computation center, California | Orbit determination, escape-velocity checks | Confirms captures, assesses impact risk |
Together, these systems turn faint blips of light into real discoveries. It’s a cosmic detective story, and every new mini-moon or quasi-moon adds a fresh chapter.
Why Should You Care About Rocks the Size of a House?
Fair question. These tiny companions reveal the fine print of gravity’s rulebook. Studying them sharpens our grasp of orbital mechanics, the history of lunar impacts, and even how to defend our planet from hazardous asteroids. Their predictable paths also make them ideal practice targets for spacecraft—and, one day, perhaps for asteroid mining.
There’s a gentler reason too. Missions like Tianwen-2 remind us that even the smallest rocks can carry clues to our origins. And knowing that Earth travels with quiet company—always, invisibly—brings a strange comfort. You’re never as alone as you think. Neither is your planet.
FreeAstroScience’s mission: we want to educate you never to turn off your mind, and to keep it active at all times—because the sleep of reason breeds monsters. Stay curious, question everything, and keep looking up.
Conclusion: Our Cosmic Neighborhood Is Never Empty
Let’s gather what we’ve learned. Earth’s story isn’t just about one big Moon. It’s about a whole cast of hidden companions: mini-moons like 2024 PT5 that visit for months, and quasi-moons like 2025 PN7 and Kamoʻoalewa that linger for generations. A gravitational bubble 1.47 million kilometers wide keeps catching passers-by. Telescopes in Hawaii, Chile, and South Africa keep spotting them. And right now, a spacecraft hovers beside one of them, ready to test whether it’s a lost piece of our Moon.
Whatever the samples reveal in 2027, we’ll learn something true about where we come from. Perhaps that’s the deeper lesson here: the sky holds company we cannot see, and knowledge finds what eyes cannot. So next time you look at the Moon, smile—it has friends up there, and so do you.
Come back to FreeAstroScience.com whenever you want to see the universe with fresh eyes. Keep your mind awake. There’s always more to discover.
FAQ: Mini-Moons and Quasi-Moons
- What is a mini-moon?
- A mini-moon is a small asteroid temporarily captured by Earth’s gravity. It circles our planet for weeks or months, then escapes back into its own orbit around the Sun.
- How many mini-moons does Earth have right now?
- Simulations suggest Earth usually hosts one to six meter-sized mini-moons at any moment, but they’re too faint to see without telescopes. Only four have ever been observed: 1991 VG, 2006 RH120, 2020 CD3, and 2024 PT5.
- What’s the difference between a mini-moon and a quasi-moon?
- Mini-moons are temporarily bound to Earth by gravity. Quasi-moons orbit the Sun in a 1:1 resonance with Earth, so they only appear to circle us—sometimes for decades or centuries.
- How are mini-moons and quasi-moons detected?
- Automated surveys like ATLAS and Pan-STARRS photograph the same sky patches repeatedly and flag moving points of light. NASA’s CNEOS then computes each orbit and checks whether the object’s speed falls below Earth’s local escape velocity.
- Why is the Tianwen-2 mission important?
- Tianwen-2 is China’s first asteroid sample-return mission. It reached Kamoʻoalewa in June 2026 and will return 100 to 1,000 grams of material to Earth in November 2027, testing whether the asteroid is a fragment of our own Moon.
Sources
- Fedorets, G., Granvik, M., & Jedicke, R. (2017). “Orbit and size distributions for asteroids temporarily captured by the Earth-Moon system.” Icarus, 285, 83–94.
- Sharkey, B. N. L., et al. (2021). “Lunar-like silicate material forms the Earth quasi-satellite (469219) 2016 HO3 Kamoʻoalewa.” Communications Earth & Environment, 2, 231.
- de la Fuente Marcos, C. & R. (2025). Discovery analysis of quasi-satellite 2025 PN7. Research Notes of the American Astronomical Society, September 2, 2025.
- NASA JPL Small-Body Database and Center for Near-Earth Object Studies (CNEOS); Minor Planet Center circulars.
- China National Space Administration (CNSA) Tianwen-2 mission updates; The Planetary Society mission coverage, June–July 2026.
- ATLAS Project and Pan-STARRS Observatory technical documentation.
- Sky & Telescope, EarthSky, and CNN Science reporting on 2025 PN7 (September–October 2025).
