Earth Bids Goodbye to Its ‘Mini-Moon’ as Astronomers Investigate Where Our Planet’s Asteroid Companion Came From

After almost two months of companionship, Earth has just lost its “mini-moon.” On Monday, the asteroid dubbed 2024 PT5 was tugged away from our planet and resumed its course in the sun’s orbit once more.

NASA’s Asteroid Terrestrial-impact Last Alert System (ATLAS) first spotted the space rock in early August. By the end of September, Earth’s gravity had pulled it into what would become a temporary, horseshoe-shaped path around our planet.

The asteroid is roughly 33 feet long, which is about the length of a London bus, Jamie Carter reports for Forbes. Astronomers suggest it originated from the Arjuna asteroid belt, a ring of space rocks that orbit the sun on a similar path to Earth’s. 2024 PT5 traced about one-quarter of an orbital path around our planet at a distance of more than two million miles, which made it impossible to spot with the naked eye or even amateur telescopes.

Mini-moons are celestial objects that orbit Earth for a limited amount of time. They’re not real moons, because they are never captured by Earth’s gravity into a permanent orbit, and they also differ from quasi-moons, which orbit the sun but follow a close enough path to Earth that they might falsely look to be circling our planet.

Including 2024 PT5, astronomers have identified five total mini-moons that were once captured by Earth, as radio astronomer Laura Nicole Driessen of the University of Sydney in Australia writes for the Conversation. Technically, the asteroid—like two of the past mini-moons—is a “temporarily captured flyby,” meaning it will not complete a full orbit around our planet. But researchers have suggested our planet always has at least one mini-moon, even if it’s less than one meter across.

It's time to say farewell to Earth's temporary mini-moon, which leaves Earth orbit today after 57 days.

A 33-foot-wide asteroid that got a little caught up in Earth's gravitational field for a few months, the mini-moon is officially called 2024 PT5. pic.twitter.com/WvQ09ftnqM

— National Air and Space Museum (@airandspace) November 25, 2024

Even as 2024 PT5 leaves Earth’s orbit, astronomers are probing its origins. In a new preprint study published this month on arXiv and based on data from telescopes in the Canary Islands, researchers conclude that 2024 PT5 is a natural object and suggest it originated from our true moon.

The team—which includes two brothers who originally spotted 2024 PT5—revealed the asteroid’s chemical composition by spectral analysis. They found that it matches the signature of lunar samples brought back to Earth. Their work also suggests the rock rotates completely about once an hour, which aligns with how a large celestial body should behave after being knocked off the moon or another object by a violent collision.

“2024 PT5 could be a large boulder from the surface of the moon that was ejected into cislunar space after a cratering event, subsequently evolving dynamically towards an orbit within the Arjuna asteroid belt,” Carlos de la Fuente Marcos, lead author of the study and an astrophysicist at the Complutense University of Madrid, tells Space.com’s Robert Lea. “This object has helped the community to realize that lunar ejecta is probably a main source for the material that constitutes the Arjuna asteroid belt.”

In fact, the researchers note that a previous mini-moon from 2022 called 2022 NX1 had similar characteristics and might also have been broken off from Earth’s true moon. Astronomers have also previously claimed that the quasi-moon Kamo’oalewa, which has been orbiting the sun near Earth since 2016, might also have lunar origins, specifically from the moon’s far side.

“Just a brief look at the vast number of craters on the moon helps us to imagine how that might have happened,” Darren Baskill, an astrophysicist at the University of Sussex in England who was not involved with the research, tells Newsweek’s Tom Howarth.

If this is true, the leading moon origin theory would indicate that 2024 PT5 actually came from Earth originally: The giant-impact theory suggests that our moon formed from the debris following a giant collision of a Mars-sized body with Earth roughly 4.5 billion years ago.

So, in a way, “just as many of us will on Thursday (Nov. 28), it seems the asteroid was paying a visit to its relatives,” per Space.com.

Celestial bodies like mini-moons and quasi-moons also hold great interest for scientists searching for potential bases to store supplies and fuel as space missions become more ambitious, as Harry Baker writes for Live Science.

Unfortunately, 2024 PT5’s recent departure means that astronomers will not be able to gather new data that could confirm the observations of the preprint study. However, when the asteroid approaches Earth again in January, it will get as close as 1.1 million miles before heading off again. During that time, NASA will take the opportunity to observe the object with a radar antenna, per the Associated Press’ Marcia Dunn. Afterward, we won’t see 2024 PT5 make a close approach again until 2055.

“Mini-moons come and go as they wish,” Marcos says to Space.com. The good news, however, is that near-Earth object surveys have improved enough for scientists to spot them more frequently. That means scientists will be watching for when Earth catches another temporary companion, which, he adds, will likely happen in the next few months.

Margherita Bassi | READ MORE

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.