Astronomers have announced that they have discovered a new neighbour in the earth’s skies: asteroid 2025 PN7, confirmed in September 2025 to be the planet’s latest quasi-satellite. (The prefix ‘quasi’ means almost.)
First spotted on August 2 by the Pan-STARRS 1 telescope in Hawaii, 2025 PN7 is a member of the Arjuna asteroid class — named for the protagonist of the ‘Mahabharata’ who is believed to be fast-moving and difficult to catch. 2025 PN7 follows an orbit very similar to that of the earth, making it appear to ‘hover’ nearby even though it’s not gravitationally bound to the planet.
Not bound, and yet…
Researchers Carlos de la Fuente Marcos and Raúl de la Fuente Marcos, of the Complutense University of Madrid in Spain, reported in Research Notes of the American Astronomical Society earlier this month that 2025 PN7 is also the earth’s seventh known quasi-satellite. Its presence adds to astronomers’ understanding of the shifting population of asteroids hanging out near the earth in space.
In fact, astronomers are motivated by two important questions when they look for and study bodies like 2025 PN7: how stable are near-earth companions? And what do they reveal about planetary dynamics?
Arjuna asteroid class
Their broader interest stems from the Arjuna asteroid class, first hinted at with the discovery of asteroid 1991 VG more than three decades ago. At that time, its earth-like orbit was so unusual that some experts speculated it might be an alien probe. While that suggestion faded soon after, the discovery prompted a systematic search for asteroids moving almost in step with the earth. Today, astronomers are aware of more than 100 such Arjunas, which together form a secondary asteroid belt.
Arjunas come closer to the earth than objects of almost any other asteroid family, and could occasionally become temporary mini-moons. Others slip into resonant orbits such as quasi-satellite orbits, where they loop around the sun but stay near the earth for decades or even centuries. Studying these objects can sharpen astronomical models of orbital mechanics, orbital resonance, and gravitational perturbations.
There are practical reasons as well. Because of their earth-like paths and relatively low approach speeds, Arjunas are potential future spacecraft targets, moreover because they’re cheaper to reach and can be useful to test mining or asteroid redirection technologies. Arjunas also factor into planetary defense because understanding their movements can help astronomers better estimate the chances of one of them eventually colliding with the earth.
Taken together, 2025 PN7 is more than just a curiosity: it’s another piece in the puzzle of how the earth interacts with its small celestial companions in its corner of the Solar System.
Joining a small club
For astronomers to classify 2025 PN7 as a quasi-satellite required meticulous observations and analysis. The asteroid was first flagged on August 2 by the Pan-STARRS survey. With a brightness magnitude of about 26.4, it’s small and faint and not easily tracked. By the end of August, however, astronomers had assembled 27 observations spanning more than 11 years of orbital data, enough to pin its path down with confidence.
The semi-major axis of 2025 PN7 is 1.003 astronomical unit (AU), almost identical to that of the earth. Astronomical unit is equal to the earth-sun distance. Its eccentricity is around 0.108, indicating a slightly oval orbit; and its inclination is just under 2 degrees, keeping it close to the earth’s orbital plane. These traits neatly match the Arjuna criteria: semimajor axis between 0.985 and 1.013 AU, low eccentricity, and low inclination.
To make sense of its orbit, astronomers combined numerical tools provided by NASA’s Jet Propulsion Laboratory (JPL) Horizons system with orbital statistics data from JPL’s Small-Body Database. They focused in particular on a set of more than 400 earth-like asteroids.
One critical marker was the asteroid’s mean longitude relative to the earth — which is a value that oscillates when an object is in a 1:1 resonance, i.e. when the object completes one orbit every time the earth completes one orbit. The mean longitude of 2025 PN7 was found to exhibit the precise behaviour expected of a quasi-satellite.
The researchers also checked for non-gravitational forces like the Yarkovsky effect, which can gradually shift an asteroid’s orbit due to thermal radiation. They found that 2025 PN7 is indeed the earth’s newest quasi-satellite, joining a small club that includes Cardea, Kamo‘oalewa, and 2013 LX28, among others. Like its peers, PN7 is not gravitationally bound to the earth but in fact it orbits the sun, whereas its orbital resonance keeps it close to the earth, making it a kind of temporary companion.
Natural laboratory
Astronomers also determined that 2025 PN7 will remain in this quasi-satellite state for about 128 years, shorter than Kamo‘oalewa’s time of residence of nearly four centuries — yet long enough to be scientifically valuable. The evolution of its orbit also mirrors Kamo‘oalewa’s in the short term.
Perhaps most importantly, 2025 PN7 lies in the innermost part of the Arjuna belt, which reinforces the idea in astronomy that quasi-satellites tend to cluster near the most earth-like orbital paths. For scientists, this means 2025 PN7 is especially intriguing both as a natural laboratory where they can study orbital resonance in action and as an accessible waypoint for further exploration.
Beyond the scientific and practical considerations lies a philosophy of astronomy, so to speak: 2025 PN7 is a reminder that the universe isn’t as empty as it often seems. Our own planet is surrounded by an ephemeral coterie of rocks that drop in and out of resonance with the earth. Yet even if their companionship is temporary, the knowledge they yield can be lasting. Just as Arjuna embodied skill and focus in the Indian epic tradition, so too do the Arjunas draw our attention and science.
Published – September 18, 2025 10:46 am IST