Astronomers identify small 'quasi‑moon' that has trailed Earth since the 1960s

Astronomers working with the Pan‑STARRS observatory in Hawaii have identified an asteroid, designated 2025 PN7, that has been moving in a Sun‑centred orbit closely linked to Earth’s trajectory for roughly six decades. The object, about 19 metres wide, has been classified as a quasi‑moon — a small body that appears to follow Earth but is not gravitationally bound to the planet.

Researchers analysed orbital data and backward‑propagated the asteroid’s trajectory to conclude that 2025 PN7 entered its Earth‑related, or quasi‑satellite, configuration in the 1960s and will likely remain in a similar state for about another 60 years before diverging. While it can appear to loop around Earth from certain perspectives, the asteroid continues to orbit the Sun rather than orbiting Earth itself, and its distance from Earth varies widely between roughly 2.8 million miles (4.5 million kilometres) and 37 million miles (59 million kilometres).

The find, reported in Research Notes of the American Astronomical Society, adds 2025 PN7 to a growing list of known Earth quasi‑satellites. It is described by the discovery team as the smallest and least stable of the group. Quasi‑moons belong to a dynamical class of near‑Earth objects known as Arjunas, which move in similar orbital periods and paths to Earth and can resonate with the planet for timescales ranging from years to centuries.

Astronomers have known about quasi‑satellites since the early 1990s, when the object 1991 VG was first detected. Another well‑studied example, Kamo'oalewa (provisionally 2016 HO3), is a more stable quasi‑moon whose Earth‑related orbital configuration persists on the order of several centuries. By comparison, 2025 PN7’s residence in the Earth‑linked region is comparatively short and subject to perturbations that make its current state less stable.

"This new quasi‑moon is small, faint and visibility windows from Earth are rather unfavourable, so it is not surprising that it went unnoticed for that long," co‑author Carlos de la Fuente Marcos of Complutense University of Madrid said in comments relayed to the reporting outlets. The object’s small size and episodic proximity to Earth keep it below the detection threshold of many surveys until favourable viewing geometries occur.

The commissioning of wide, deep surveys such as the Vera C. Rubin Observatory in Chile is expected to improve discovery rates for similar objects. The Rubin facility’s large, rapid sky surveys can uncover faint near‑Earth objects during limited windows, potentially revealing more quasi‑moons and short‑lived Earth companions.

Quasi‑moons differ from "minimoons," which are temporarily captured into actual orbit around Earth. Only four minimoons have been discovered to date, and none remain bound to Earth. Both quasi‑moons and minimoons are of scientific interest because they carry information about the small‑body population near Earth and the origins of those bodies: possibilities include the main asteroid belt, fragments from lunar impacts, or breakup products of larger objects on similar orbits.

Planetary scientists say studying these near‑Earth companions can improve understanding of asteroid composition, dynamical evolution and potential impact hazards. Small bodies like 2025 PN7 are too small to pose significant planetary risk, but tracking their orbits refines models of how objects move between regions of the inner solar system and how transient Earth companions form and disperse.

The discovery of 2025 PN7 illustrates the increasingly detailed picture emerging of Earth’s near‑space environment as survey capabilities improve. Continued observations and refined orbital modeling will determine how long 2025 PN7 remains in a quasi‑satellite relationship with Earth and whether additional, similarly faint companions are active participants in the planet’s local dynamical neighborhood.