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

Earth is going to have its very own mini-moon from September 29 until November 25. The regular Moon’s new, temporary friend is 2024 PT₅, an asteroid captured from the Arjuna asteroid group (called the “Arjunas”).
| Photo Credit: AFP

Earth is going to have its very own mini-moon from September 29 until November 25. The regular Moon’s new, temporary friend is 2024 PT₅, an asteroid captured from the Arjuna asteroid group (called the “Arjunas”).

Our new mini-moon is approximately 10 metres in diameter and will be captured by Earth’s gravity for 57 days. It’s small and faint, so it won’t be visible by the eye or with small telescopes, but will be visible to larger telescopes.

What is a mini-moon?

NASA defines “moons” as “naturally formed bodies that orbit planets”. Intuitively, we think of moons as big rocks that stick around for a while, like our very own Moon.

Astronomers have defined mini-moons as asteroids or comets that are gravitationally captured by a planet for a temporary amount of time. Small mini-moons are very common, but ones big enough to spot are more rare.

2024 PT₅ is only the fifth ever detected mini-moon captured by Earth.

Orbiting or just flying by?

A mini-moon is a “temporarily captured orbiter” if it completes at least one full orbit of the Earth before returning to its usual orbit around the Sun.

If a mini-moon is captured by Earth’s gravity but doesn’t make it around for a full orbit, it’s a “temporarily captured flyby” instead.

Two of Earth’s mini-moons were “temporarily captured orbiters”, while the other three (including 2024 PT₅) fall into the “temporarily captured flybys” category.

Where do mini-moons come from?

Mini-moons are asteroids from the large population of near-Earth objects (or NEOs) that are temporarily grabbed from their orbit around the Sun.

Near-Earth objects are defined as natural physical objects floating in space, such as asteroids (space rocks) or comets (dirty space snowballs made of rock and ice), that are orbiting the Sun and approach to within 1.3 times Earth’s distance from the Sun at some point in their orbit.

Because these objects are around the same distance from the Sun as the Earth, they can sometimes be captured by Earth’s gravity.

2024 PT₅ and and a previous mini-moon called 2022 NX₁ were both captured from a group of asteroids orbiting the Sun at a similar distance from Earth, called the Arjunas. Arjuna is one of the main characters of the Hindu epic, Mahābhārata.

The first mini-moon – and a fake one

The first known mini-moon was called 1991 VG. It arrived in late 1991 and left in early 1992, and like 2024 PT₅ it was around 10 metres in diameter.

In 2002, amateur astronomer Bill Yeung found what he thought was a second mini-moon: J002E3.

However, on closer inspection the object displayed a spectrum of light suggesting it was coated in white paint containing titanium oxide. Then, a study of how the object’s brightness changed over time found its shape resembled something like the upper stage of a rocket.

Astronomers now believe J002E3 is the third stage of the Apollo Saturn V rocket (S-IVB) instead of a natural mini-moon.

More discoveries – with a note of caution

The plot twist of J002E3 made astronomers a bit more cautious. When another mini-moon (2006 RH₁₂₀) was spotted on September 14 2006, it was first classified as artificial. However, after more observations it turned out to be a natural mini-moon about 2–7 metres in diameter. 2006 RH₁₂₀ stuck around from July 2006 until July 2007.

The next mini-moon, 2020 CD₃, was gravitationally captured by Earth for more than two years, making it the longest-captured mini-moon observed to date. It escaped Earth’s orbit in May of 2020. This mini-moon was around 1–2 metres in diameter.

Amateur astronomers Grzegorz Duszanowicz and Jordi Camarasa discovered 2022 NX₁ using the Moonbase South Observatory in Namibia. Similar to 2006 RH₁₂₀, it was initially thought to be an artificial object from a past space mission.

It was later determined to originate from the Arjunas, just like 2024 PT₅. While it was discovered in 2022, it was temporarily captured by Earth’s gravity in January 1981 and June 2022. It’ll be captured again in December 2051.

2022 NX₁ was later found to be a natural mini-moon 5–15 metres in diameter.

Will we find more mini-moons?

Modelling suggests that, at any given time, Earth has at least one captured mini-moon less than 1 metre in diameter.

Even though astronomers think we always have a mini-moon, these bodies are challenging to detect. This is because they’re small and faint. They are usually found by projects specifically looking for asteroids near Earth.

2024 PT₅ was discovered using the Asteroid Terrestrial-impact Last Alert System (ATLAS), which is an ongoing project specifically designed to search for asteroids. 2006 RH₁₂₀ and 2020 CD₃ were discovered using the ongoing Catalina Sky Survey (CSS) for Near Earth Asteroids. 1991 VG was discovered by the SPACEWATCH group using the Spacewatch Telescope.

These projects will continue to search for asteroids, including mini-moons. We can also look forward to new discoveries and investigations when the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) starts observing in the next couple of years.

This enormous project will take images of the entire sky every few days for a period of ten years. First light of the Vera C. Rubin observatory is expected in mid-2025.

This article is republished from The Conversation under a Creative Commons license. Read the original article.