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Astronomers discover previously unknown quasi-moon near Earth

Astronomers have spotted a quasi-moon near Earth — and the small space rock has likely been hanging out near our planet unseen by telescopes for about 60 years, according to new research.

The newly discovered celestial object, named 2025 PN7, is a type of near-Earth asteroid that orbits the sun but sticks close to our planet. Like our world, 2025 PN7 takes one year to complete an orbit around the sun.

Quasi-moons differ from temporary mini-moons that occasionally orbit Earth such as 2024 PT5, which circled the planet for two months in 2024 and could be an ancient fragment that was blasted off of our primary moon.

The newly found 2025 PN7 is just one of a handful of known quasi-moons with orbits near our planet, including Kamo‘oalewa, which is also thought to be an ancient lunar fragment. Kamo‘oalewa is one of the destinations of China’s Tianwen-2 mission launched in May, which aims to collect and return samples from the space rock in 2027.

The Pan-STARRS observatory located on the Haleakala volcano in Hawaii captured observations of 2025 PN7 on August 29. Archival data revealed that the object has been in an Earth-like orbit for decades.

The quasi-moon managed to escape the notice of astronomers for so long because it is small and faint, said Carlos de la Fuente Marcos, a researcher on the faculty of mathematical sciences at the Complutense University of Madrid who recently authored a paper about the space rock. The paper was published on September 2 in the journal Research Notes of the American Astronomical Society, which is for timely non-peer-reviewed astronomical observations.

The space rock swings within 186,000 miles (299,337 kilometers) of us during its closest pass of our planet, de la Fuente Marcos said. For reference, 2025 PN7 is 238,855 miles (384,400 kilometers) away from Earth on average, according to NASA.

“It can only be detected by currently available telescopes when it gets close to our planet as it did this summer,” de la Fuente Marcos explained. “Its visibility windows are few and far between. It is a challenging object.”

Finding and studying quasi-moons can reveal more about our corner of the universe, de la Fuente Marcos said.

“The Solar System is full of surprises so we keep looking,” he wrote in an email. “For Earth’s neighborhood, the existence of 2025 PN7 suggests that there may not exist a lower limit to the size of a quasi-satellite.”

A temporary companion

Astronomers are still trying to figure out 2025 PN7’s size. About 98 feet (30 meters) across is a reasonable estimate, de la Fuente Marcos said. It also has the potential to be 62 feet (19 meters) in diameter, according to EarthSky.

The space rock is currently the smallest-known quasi-moon to have orbited near Earth, de la Fuente Marcos said.

Over time, 2025 PN7 switches between a closer, more circular Earthlike orbit and a horseshoe orbit, similar to Kamoʻoalewa’s. During the horseshoe orbit, the space rock can reach a distance of 185 million miles (297 million kilometers) from Earth.

The quasi-moon is expected to remain in its current near-Earth orbit for about another 60 years before the gravitational tug of the sun pulls it back into a horseshoe orbit.

Questions remain about the space rock’s composition, which is currently unknown.

“Based on what little we know so far, it’s almost certainly a rocky and natural object — sometimes old satellites and rocket junk end up in these very-near-Earth kinds of orbits, but we can often tell ‘natural’ (e.g., asteroidal) from ‘artificial’ (e.g., satellite) based on how their orbits evolve on short timescales,” wrote Dr. Teddy Kareta, assistant professor in the department of astrophysics and planetary science at Villanova University, in an email.

Kareta, who has previously studied mini-moons and quasi-moons, was not involved in the research and said bad weather has thus far interfered with his plans to observe it.

While 2025 PN7, like Kamoʻoalewa, may also be a piece of the moon, more data is needed to confirm that, de la Fuente Marcos said.

De la Fuente Marcos believes that 2025 PN7 came from the Arjuna asteroid belt. Unlike the main asteroid belt, located between the orbits of Mars and Jupiter, the Arjuna asteroid is not a distinct belt. Instead, they are a group of small space rocks that have orbits around the sun similar to Earth’s.

Previously, de la Fuente Marcos had proposed that mini-moon 2024 PT5 was also an Arjuna asteroid that could have come from Earth’s moon before becoming part of the belt.

“Now we know that material ejected during lunar impacts may contribute members to the Arjuna secondary asteroid belt,” de la Fuente Marcos said.

Even though quasi-moons and mini-moons can closely approach our planet, they don’t typically pose an impact risk for Earth, so 2025 PN7 is no threat, he added.

Sending missions to investigate quasi-moons, as Tianwen-2 is expected to do with Kamoʻoalewa, could reveal the true origin and history of these intriguing space rocks — and provide other opportunities as well, he said.

“These asteroids are relatively easy to access for unmanned missions and can be used to test planetary exploration technologies with a relatively modest investment,” de la Fuente Marcos said.

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