A new paper describes another possible "quasi-moon" of Earth, and the interloping asteroid may have been following our planet around for decades, undetected.
Quasi-moons, the Planetary Society states, are “like a gravitational sleight of hand.” They are asteroids, which — from our point of view on Earth — appear to be orbiting our planet like our permanent moon does. However, they actually orbit the sun, only temporarily moving through the solar system alongside our planet.
If the status of the newly detected asteroid, called 2025 PN7, is confirmed, it would not be the only object seemingly behaving as a moon of Earth; there are seven other known quasi-moons in Earth-like orbits, and they are "full of surprises," said study co-author Carlos de la Fuente Marcos of the Complutense University of Madrid.
Of these quasi-moons, 2025 PN7 is the "smallest and the least stable known quasi-satellite of Earth," de la Fuente Marcos told Live Science in an email.
The newly discovered asteroid is only 62 feet (19 meters) wide — slightly smaller than the meteor that exploded over Chelyabinsk, Russia, in 2013. The asteroid is officially classified as having a brightness of magnitude 26, meaning it is visible only through good telescopes. (The lower the magnitude, the brighter the object. For comparison, most naked-eye stars are magnitude 6 or lower, and the bright star Sirius is roughly magnitude -1.5.)
The potential quasi-moon finding was published Sept. 2 in the journal Research Notes of the American Astronomical Society (AAS). The journal, which is not peer-reviewed, aims to allow authors to "promptly" share items of interest with the astronomical community, with papers "moderated by an editor for appropriateness and format before publication" in lieu of peer review. That approach allowed study authors Carlos de la Fuente Marcos and Raúl de la Fuente Marcos, also of the Complutense University of Madrid, to publish their findings rapidly.
An official Aug. 29 circular about 2025 PN7 from the International Astronomical Union shows data about the object dating back only to July 30, in observations by the Haleakalā Observatory's Pan-STARRS1 telescope in Maui, Hawaii.
The quasi-moon designation was first proposed for 2025 PN7 by French journalist and amateur astronomer Adrien Coffinet, who posted Aug. 30 on the Minor Planet Mailing List about how his calculations appeared to show that was the case.
"2025 PN7 seems to be a quasi-satellite of the Earth for the next 60 years," Coffinet wrote. Another in the group said it appears, from the object’s orbit, to have been flying nearby us already for about seven decades.
So, why didn't astronomers notice 2025 PN7 before now? "It is small, faint, and its visibility windows from Earth are rather unfavorable, so it is not surprising that it went unnoticed for that long," de la Fuente Marcos said.
More quasi-moons may be lurking out there. The Vera C. Rubin Observatory, which recently became operational and can scan for objects like this, "may uncover many more like 2025 PN7," de la Fuente Marcos added.
