The Earth has just added its seventh confirmed quasilunar moon. It is 2025 PN7, a small Apollo-type asteroid detected in August solely by its brightness, thanks to the Hawaiian Pan-STARRS 1 telescope.
After analyzing its trajectory, scientists concluded that the object maintains a 1:1 resonance with the Earth. In other words, it orbits the sun at the same time as our planet. From a distant perspective, this synchronicity makes it look as if the Earth is accompanied by a tiny asteroid—as if it had an additional moon.
Unlike the moon, quasilunar moons are not gravitationally bound to the Earth. They are ephemeral companions, in cosmological terms, following their own path around the sun. Only at certain times do they come close enough to appear bound. In the case of 2025 PN7, its minimum distance is 299,000 km, while at its farthest point it can reach 17 million km. For comparison, the moon remains at an average distance of 384,000 km from Earth.
According to the article published in Research Notes of the AAS, the asteroid has been in a quasi-satellite phase since 1965, and is expected to remain so for 128 years. Some researchers estimate that 2025 PN7 will finally move away in 2083.
Why Does the Earth Have Quasilunar Moons?
So far, seven bodies have been confirmed that appear to accompany the planet in its orbit. Astronomers believe that more may be discovered in the future. Earth is a natural reservoir of quasilunars because the Earth’s orbit is similar to that of certain nearby objects that inhabit the so-called Arjuna group of asteroids, a population that has only recently begun to be studied in greater detail.
The Arjuna group does not form a ring like the asteroid belt between Mars and Jupiter, but comprises a legion of near-Earth rocks that orbit the sun on a similar path as our planet. Occasionally, some of these asteroids coincide with our trajectory and, depending on their orbital dynamics, are classified as quasilunar or mini-moons.
The quasilunar moon 2025 PN7 sits in the Arjuna asteroid group not far from Earth.Illustration: WIRED