Asteroid 2025 PN7 confirmed as Earth’s smallest, most unstable quasi-moon
A study published this month in Research Notes of the American Astronomical Society confirmed asteroid 2025 PN7 as Earth’s smallest and most unstable quasi-moon. Detected just over a month ago, it may have traveled with Earth for about 70 years and is expected to remain in a quasi-satellite orbit for another 128.
An artist’s illustration of the asteroid Kamo’oalewa, the target of China’s Tianwen 2 asteroid-sampling mission. Some astronomers think Kamo’oalewa may be a blasted-off chunk of Earth’s moon. Credit: Addy Graham/University of Arizona
Pan-STARRS1 at Haleakalā Observatory, Maui, first spotted 2025 PN7 at 14:44 UTC on August 2. The International Astronomical Union (IAU) issued a circular on August 29, noting observations dating back only to July 30.
French amateur astronomer Adrien Coffinet was the first to recognize its unusual orbit. On August 30, he posted to the Minor Planet Mailing List that calculations suggested the asteroid was a quasi-satellite of Earth. He predicted it would remain in this state for about 60 years, and that it may have been following Earth for roughly seven decades already.
Carlos and Raúl de la Fuente Marcos of the Complutense University of Madrid analyzed its orbit in a paper published in September in Research Notes of the American Astronomical Society. Their study confirmed 2025 PN7’s classification as a quasi-satellite, an object in orbital resonance with Earth but not gravitationally bound.
Orbital properties and uniqueness
2025 PN7 is an Apollo-type asteroid with an absolute magnitude of 26.4, corresponding to a diameter of about 19 m (62 feet). This makes it slightly smaller than the meteor that exploded over Chelyabinsk, Russia, in 2013. With an apparent brightness of magnitude 26, it can only be observed through large research telescopes.
The orbital elements published in the study are:
- Semimajor axis: 1.0030 AU
- Eccentricity: 0.1075
- Inclination: low, keeping it close to Earth’s orbital plane
These values place the asteroid within the Arjuna asteroid population, a class of Earth-like orbits clustered near 1 AU. Arjunas include both temporary minimoons and longer-lasting quasi-satellites.
Modeling shows that 2025 PN7 will remain in a quasi-satellite state for about 128 years, compared to 381 years for the well-known quasi-moon Kamoʻoalewa. In orbital maps, it appears close to Cardea (2004 GU9), another quasi-satellite.
Some quasi-moons, including 2025 PN7, are influenced by the Yarkovsky effect, a thermal force caused by the uneven emission of heat from rotating bodies. This subtle effect can gradually alter their orbits, contributing to instability over time.
Study co-author Carlos de la Fuente Marcos described it as “the smallest and the least stable known quasi-satellite of Earth.”
Historical background
The first asteroid discovered in an Earth-like orbit, 1991 VG, sparked speculation that it might be an interstellar probe. Decades of research have since shown that these objects are natural and belong to the Arjuna class of Earth-like asteroids.
Known quasi-satellites of Earth include:
- 164207 Cardea (2004 GU9)
- 469219 Kamoʻoalewa (2016 HO3)
- 277810 (2006 FV35)
- 2013 LX28
- 2014 OL339
- 2023 FW13
The discovery of 2025 PN7 brings the total number of confirmed Earth quasi-moons to seven.
Quasi-moons, minimoons, and ghost moons
Quasi-moons orbit the Sun but remain in a 1:1 mean-motion resonance with Earth. From Earth’s perspective, they appear to loop around our planet, though they are not gravitationally bound.
This distinguishes them from minimoons, which are temporarily captured into true Earth orbit for months or years. Only four minimoons have ever been confirmed, including 2020 CD3, and some candidates turned out to be artificial objects. The Gaia spacecraft and discarded rocket stages from Chang’e 2 and Lunar Prospector were once mistaken for minimoons.
A third category is that of ghost moons, or Kordylewski clouds, dust concentrations that may occupy stable Lagrange points ahead of or behind Earth. First reported in the 1960s and tentatively observed in 2018, their existence remains debated.
Quasi-moons are not unique to Earth. Venus, Jupiter, Saturn, Neptune, Pluto, and even the dwarf planet Ceres all host quasi-moons. The first quasi-moon ever discovered, Zoozve (2002 VE68), orbits Venus and received its unusual name after a misreading of its provisional designation.
Why it went unnoticed
2025 PN7’s small size, faint magnitude, and unfavorable visibility from Earth explain why it remained undetected for decades. Even modern telescopes have difficulty identifying such dim objects against the stellar background.
The newly operational Vera C. Rubin Observatory in Chile is expected to change this. Its wide-field survey will scan the sky every few nights, likely revealing many more faint quasi-moons and minimoons.
Risks and impacts
Minimoons, by contrast, frequently end their lifetimes by falling into Earth’s atmosphere. About 1% eventually impact Earth, though most are small enough to burn up harmlessly. A very large minimoon, about 1 km (0.6 mi) in size, is expected to strike Earth only once every billion years.
Current orbital models show no collision risk between Earth and any known quasi-moon, including 2025 PN7. Their resonant orbits keep them close to Earth but predictably stable.
If an asteroid the size of 2025 PN7 were to strike Earth, most of it would burn up in the atmosphere creating spectacular light show, with possibility of some localized consequences, depending on its composition. The Chelyabinsk impact in 2013, caused by an object of similar size, injured more than 1 500 people and damaged over 7 000 buildings.
Scientific significance
Quasi-moons provide valuable insight into the dynamics of near-Earth space. They are easier to reach than main-belt asteroids, making them attractive exploration targets.
China’s Tianwen-2 mission, scheduled to launch in 2025, will target Kamoʻoalewa, aiming to collect samples and probe its internal structure with a small explosive device. Missions to other quasi-moons, including small objects like 2025 PN7, may follow.
Studying quasi-moons and minimoons can reveal their origins, whether from the main asteroid belt, lunar ejecta, or fragments of larger objects. These faint companions are stepping stones for planetary defense strategies and potential resource use in future space exploration.
References:
1 Meet Arjuna 2025 PN7, the Newest Quasi-satellite of Earth – Carlos de la Fuente Marcos et al. – Research Notes of the AAS – September 2025 – https://doi.org/10.3847/2515-5172/ae028f – OPEN ACCESS
2 Earth’s quasi-moons, minimoons, and ghost moons – The Planetary Society – May 21, 2024
I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.
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