Earth almost always has more than one moon. The others are just very small, and they never stay long — but they could provide accessible and unique insights into the solar system.
Temporary satellites, or mini-moons, are small asteroids that wander into Earth’s neighborhood and get captured briefly by our planet’s gravity. A typical one stays about nine months and circles Earth roughly three times before escaping back into orbit around the sun, Grigori Fedorets of Finland's University of Helsinki said in a presentation at the Europlanet Science Congress (EPSC) 2026 in The Hague in September.
Fedorets' modeling suggests that the largest mini-moon around Earth at any given moment is typically only about 75 centimeters (30 inches) across. An asteroid about 3 meters (10 feet) wide should be captured every 10 to 20 years.
"This is not just a theoretical construct. There is observational evidence," Fedorets said. Only two mini-moons have been confirmed to date, however — 2006 RH120 and 2020 CD3, both discovered by the Catalina Sky Survey in Arizona. Two fireballs, seen over the Czech Republic in 2013 and Australia in 2016, were traced back to objects that had been in geocentric orbits. Two more asteroids, 2022 NX1 and 2024 PT5, were captured only briefly, without completing a full orbit.
Fedorets noted a potential third mini-moon — the recently detected Sar2901, which could also be an artificial object. Sar2901 has since faded from view but should be observable again around mid-to-late November, according to the Minor Planet Center.
Small rocks, big questions
Mini-moons are so small that scientists know little about them. For objects between 1 and 10 meters (3.3 to 33 feet) across, models of the near-Earth asteroid population disagree with the numbers suggested by records of fireballs in Earth's atmosphere. Mini-moons sit right in that gap, so finding more of them should show which picture is right.
They may also reveal how much of the material in Earth’s neighborhood traces its origin back to our moon, as some small asteroids may be chunks of the lunar surface blasted into space by impacts. During the temporary flyby of asteroid 2024 PT5, for example, scientists found its spectrum to be closer to lunar rock than to typical asteroids, according to a 2025 study led by Theodore Kareta, which Fedorets described as the strongest evidence yet for lunar ejecta among Earth's small neighbors.
Kamo’oalewa, a much larger asteroid and quasi-moon of Earth, at which China’s Tianwen-2 mission arrived in July, has been the subject of debate over possible lunar or asteroidal origins.
Rubin game-changer and budget sample returns
The transformative Vera C. Rubin Observatory's 10-year Legacy Survey of Space and Time (LSST) should find many more mini-moons, though further work will be needed. On its own, Fedorets estimates, the survey should catch a mini-moon about once every two years.
Fedorets and colleagues are setting up a dedicated system running on a virtual server at Finland's Center for Scientific Computing, to sift Rubin's alerts through the ANTARES real-time astronomical data broker to flag potential mini-moons for follow-up observations, which could raise detections to two or three per year.
Making regular discoveries could open doors for future missions, providing targets for relatively easy, low-cost asteroid sample-return missions. Mini-moons’ proximity to Earth, and their low relative speeds, means less rocket propellant is needed to get out to these diminutive targets and back to Earth.
Initially, Fedorets envisions cubesat tech demo missions, which would pave the way for mid-sized missions that could collect samples. He sees the Tianwen-2 low-energy sample mission as a potential stepping stone to resource utilization, while also noting that, at the higher end of the scale, there have been programs such as NASA’s cancelled Asteroid Redirect Mission (ARM) and plans by, for example, TransAstra to put a 100-ton asteroid into a stable orbit around Earth.
"This is not sci-fi," Fedorets said. "These people are actually thinking about this kind of stuff."
The short warning times and instability of the orbits of mini-moons dictates potential mission profiles. A spacecraft could wait in Earth-moon space, ready to head for a newly found mini-moon, similar to how ESA's Comet Interceptor will launch and then wait for a suitable comet to study. Asteroids could also be nudged into more stable orbits, allowing more extensive future missions.
"LSST is now online, and over the next 10 years, we are expected to find many more of these temporary moons," Fedorets said. Regular discoveries could pave the way for exploration in the early 2030s and sampling missions later in the decade.
