Earth frequently hosts “minimoons”—small asteroids captured by the planet’s gravity—but these objects often remain undetected due to their small size and rapid movement. While only a few have been confirmed, such as 2006 RH120 and 2020 CD3, researchers suggest these temporary satellites are continuously passing through Earth’s neighborhood, waiting for more advanced survey technology to bring them into clearer view.
How astronomers identify a minimoon
Minimoons are defined by how they interact with Earth’s gravitational field. According to research, there are two primary categories: temporarily captured orbiters, which complete at least one full revolution around the Earth, and temporarily captured flybys, which linger for days or months without finishing a loop. For an object to be captured, it must approach within approximately 4.5 million kilometers of Earth at a slow speed of about 3,500 kilometers per hour, causing its energy relative to the planet to tip into the negative.
Not every “second moon” is a natural rock. In 2002, an object initially identified as a minimoon was later revealed to be a discarded Apollo-era rocket booster.
Confirmed captures and their histories
Confirmed long-term captures are rare. The asteroid 2006 RH120 was gravitationally bound to Earth for about a year starting in mid-2006, during which it completed roughly four loops. Another notable example, 2020 CD3, was bound for at least two and a half years before it escaped in 2020. Both were discovered by the Catalina Sky Survey. In contrast, shorter-term visitors like 2024 PT5, which measured between 5 and 10 meters across, lingered near Earth from late September to late November 2024 without completing a full orbit.
The true scale of the minimoon population
The number of confirmed minimoons is likely a significant undercount of the actual population. Astronomer Robert Jedicke estimates that there is roughly one dishwasher-sized minimoon orbiting the Earth-Moon system at any given time. Because these objects are often only a few meters wide and move quickly against the night sky, they are too dim for standard observation. Most of the current population remains inferred through mathematical modeling rather than direct detection.
Pro Tip: Distinguishing between types
It is important to distinguish between true minimoons and other near-Earth objects. Quasi-satellites, such as 469219 Kamoʻoalewa, are often mislabeled as second moons. However, these objects co-orbit the Sun in step with Earth rather than being gravitationally bound to our planet. Some evidence suggests that both Kamoʻoalewa and 2024 PT5 might actually be lunar fragments ejected by ancient impacts.
Future prospects for detection
The gap between theoretical models and actual observations is expected to close as new survey technology comes online. The Vera C. Rubin Observatory is designed to be sensitive enough to detect these faint, fast-moving objects regularly. This shift could transform minimoons from rare curiosities into a well-catalogued population. Beyond simple record-keeping, these asteroids represent potential targets for low-cost robotic missions or sample return efforts due to their proximity and low relative velocity.
Frequently Asked Questions
- Are minimoons dangerous to Earth? No. Because they are typically only a few meters in size and are captured when moving slowly, they do not pose an impact threat.
- How long do these asteroids stay? It varies. Some are flybys that stay for only a few weeks, while others like 2020 CD3 can remain bound for years.
- Can I see a minimoon with a backyard telescope? Generally, no. Most are too dim and move too quickly to be seen without advanced professional-grade equipment.
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