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Rubin survey expected to find more of Earth's short-lived mini-moons
Space.com reports University of Helsinki researcher Grigori Fedorets told the Europlanet Science Congress that Vera C. Rubin's sky survey should catch more temporary Earth satellites, some close enough to study.
Temporary satellites of Earth — small asteroids captured briefly by the planet's gravity — could become easier to find and study as the Vera C. Rubin Observatory's decade-long sky survey proceeds, according to Space.com, citing a September talk by Grigori Fedorets of the University of Helsinki at the Europlanet Science Congress in The Hague1.
Fedorets said a typical mini-moon stays about nine months and completes roughly three orbits before returning to a solar orbit, Space.com reported1. His modeling suggests the largest such object around Earth at a given time is typically about 75 centimeters (30 inches) across, and that an asteroid about 3 meters (10 feet) wide should be captured every 10 to 20 years1.
Only two mini-moons have been confirmed, Space.com said: 2006 RH120 and 2020 CD3, both discovered by the Catalina Sky Survey in Arizona1. Two fireballs, over the Czech Republic in 2013 and Australia in 2016, were traced to objects that had been in geocentric orbits, and two other asteroids, 2022 NX1 and 2024 PT5, were captured only briefly without completing a full orbit, the report said1. Fedorets also noted a recently detected object, Sar2901, that could be a third mini-moon or an artificial object; Space.com said the Minor Planet Center expects it to be observable again around mid-to-late November1.
Mini-moons sit in a size range where models of near-Earth asteroids disagree with fireball records, Space.com reported, so more detections could test those models1. Some nearby small bodies may be lunar ejecta; Fedorets cited a 2025 study led by Theodore Kareta on 2024 PT5 as the strongest evidence yet that some of Earth's small neighbors resemble lunar rock, according to the article1.
Fedorets estimated Rubin's Legacy Survey of Space and Time should catch a mini-moon about once every two years on its own1. He and colleagues are setting up a system at Finland's Center for Scientific Computing to run Rubin alerts through the ANTARES broker and flag candidates, which could raise detections to two or three per year, Space.com said1. Regular discoveries could pave the way for exploration in the early 2030s and sampling missions later in the decade, Fedorets said, according to the report1.
