Earth has one permanent Moon, but it is rarely alone. Small asteroids drift into our planet's gravitational grip, loop around it for a while, and then drift out again. Astronomers call them temporary natural satellites, or more casually, mini-moons. They are almost certainly up there right now. The problem is that they are tiny, faint and fast, and in decades of searching only a handful have been caught.
That could change soon. In an abstract for the Europlanet Science Congress (EPSC 2026), Grigori Fedorets argues that the Legacy Survey of Space and Time (LSST) at the Vera C. Rubin Observatory should begin turning these objects up at a steady rate. He also makes the case that they are some of the easiest targets in the solar system for a spacecraft to reach.
A very small, very temporary population
The numbers in Fedorets' abstract show why mini-moons are so hard to find. According to the abstract, the largest temporary moon orbiting Earth at any given time is only about 75 centimeters across. An object around 3 meters across is captured about once every 10 years (Space.com's account of his talk says every 10 to 20 years). A typical capture lasts around nine months, during which the object makes about three revolutions around Earth before it escapes back into a solar orbit.
So the target is a dim rock under a meter wide, moving quickly across the sky near Earth, that will be gone within the year. It is not surprising the tally is short. The abstract notes that only two have been found so far, both by the Catalina Sky Survey.
Those two, as Space.com reports, are 2006 RH120 and 2020 CD3. Two other objects, 2022 NX1 and 2024 PT5, were only briefly captured, so they sit at the edge of the definition. There is also a possible third mini-moon, designated Sar2901, which may turn out to be artificial rather than a natural rock. Per the Minor Planet Center, Space.com says it should be observable again in mid-to-late November, which could help settle what it is.
What Rubin changes
Rubin's survey is designed to sweep the sky over and over, and that cadence is what a fleeting, faint target needs. Fedorets' abstract estimates that LSST should discover a temporary captured asteroid every one to four years.
Space.com's reporting puts a finer point on that range. Rubin on its own should catch roughly one mini-moon about every two years, by Fedorets' estimate. A dedicated detection system running on Rubin's alerts through the ANTARES alert broker could raise the yield to two or three per year. That would be a meaningful jump, from a population known through a handful of detections to one that can be studied with a steadier supply of new examples.
The difference between those estimates comes down to software as much as glass. The abstract notes that, because of the narrow observational windows, additional observations and sieving through LSST alerts will be required to characterize these objects beyond the LSST baseline. Whether a mini-moon gets flagged as a mini-moon, and not lost among the many fast-moving objects in the data stream, depends on someone looking specifically for it.
The 2024 PT5 question: where do they come from?
One of the most interesting threads in this research is origin. The default assumption is that mini-moons are ordinary near-Earth asteroids that happened to pass by at the right speed and angle. But at least one recent visitor looks like it came from much closer to home.
In a paper posted to arXiv, Kareta and colleagues examined 2024 PT5, one of the briefly captured objects. They obtained spectra on 2024 August 16 using the Lowell Discovery Telescope and NASA's Infrared Telescope Facility (IRTF). The spectra matched lunar samples and were pyroxene-rich, unlike asteroids of comparable spectral redness, which are olivine-rich. The team also considered whether PT5 might be a piece of rocket hardware, a reasonable worry for anything on an Earth-like orbit. They found the solar radiation pressure on its trajectory was orders of magnitude lower than expected for an artificial object.
Their conclusion: 2024 PT5 is ejecta from an impact on the Moon. It is the second near-Earth asteroid suggested to have come from the Moon. Fedorets' abstract calls temporary satellites a key population for distinguishing between a main-belt and a lunar origin for asteroids in Earth's immediate vicinity, so more discoveries should help show how common lunar ejecta are among them. If a meaningful fraction of these objects are pieces of the Moon, then some of them could offer a way to study lunar material without going to the Moon.
Why It Matters
The scientific appeal is clear, but the most practical argument in Fedorets' abstract is about access. Because mini-moons spend a relatively long time in Earth's vicinity and have low delta-v, the measure of how much velocity change a mission needs, they are within easier reach than most asteroids. The abstract calls them outstanding test mission targets for asteroid mining, and says they offer chances to train for navigation around individual boulders, of the kind seen floating near asteroid Bennu during the OSIRIS-REx visit.
That makes them a natural proving ground. Rendezvousing with a small, poorly characterized rock is hard, and Space.com reports that mini-moons' proximity and low relative speeds mean less propellant is needed to reach them and return. According to Space.com, Fedorets envisions cubesat technology demonstrations first, then mid-sized sample-collecting missions, with regular discoveries potentially paving the way for exploration in the early 2030s and sampling missions later in the decade.
The catch is timing. A typical mini-moon stays for about nine months, so any mission has to be ready to move quickly once a good target is found. Space.com describes one profile in which a spacecraft waits in Earth-Moon space, ready to head for a newly found mini-moon. That is why the discovery rate matters so much. With one object every few years, planning a mission around a mini-moon is a gamble. With two or three a year, as a dedicated system might deliver, it starts to look like a more viable mission strategy.
There is also a scientific payoff beyond missions. Space.com notes that for objects between 1 and 10 meters across, models of the near-Earth asteroid population disagree with the numbers suggested by fireball records, and mini-moons sit right in that gap. The abstract makes the same point, saying LSST could help narrow the size-frequency gap at 1 to 10 meters between asteroid survey and bolide data.
What to watch
The nearest test comes in mid-to-late November, when Sar2901 is expected to be observable again. Follow-up there could show whether it is a third natural mini-moon or another piece of space hardware. Beyond that, the real measure of these predictions will be whether Rubin's alert stream starts producing new temporary moons on the schedule Fedorets and others expect. If it does, Earth's small, short-lived companions could move from oddities to a regular target for science, and possibly for spacecraft.
