Earth Is Getting a New ‘Mini Moon’ for the Next Two Months, Astronomers Say

In just over one week’s time, Earth will gain a new celestial companion that will stay for two months in the planet’s gravitational cradle. Meet asteroid 2024 PT5, Earth’s latest mini-moon.

“It is pretty cool,” Federica Spoto, an asteroid dynamics researcher at the Center for Astrophysics, Harvard and Smithsonian, tells Robin George Andrews of the New York Times.

A plethora of objects chart their courses around the sun in different orbits. Most of them wander far beyond our perception, but a few come close enough to Earth to raise our hackles. Many skirt past us, and occasionally, they make landfall—but in rare cases, an asteroid of just the right size and speed will be captured by Earth’s gravity and temporarily go into orbit, becoming a mini-moon.

The next space rock that will earn that distinction, asteroid 2024 PT5, was first detected on August 7 using the Asteroid Terrestrial-impact Last Alert System (ATLAS), funded by NASA. ATLAS is a cluster of telescopes around the world that scan the skies in search of potential asteroid threats. Thanks to their vigilance, these sentinels also pick out scientific curiosities from the night skies.

The program’s recent sighting, 2024 PT5, originated from the Arjuna asteroid belt, a halo of objects with Earth-like orbits around the sun. In a new report in the Research Notes of the American Astronomical Society, scientists announced that 2024 PT5 is just 33 feet long—on the order of one thousand times punier than the Chicxulub impactor that eviscerated the dinosaurs. The asteroid’s sojourn around Earth will last from September 29 to November 25.

Newly-discovered #asteroid 2024 PT5 is about to undergo a "mini-moon event" when its geocentric energy becomes negative from September 29 - November 25.https://t.co/sAo1qSRu3J pic.twitter.com/pVYAmSbkCF

— Tony Dunn (@tony873004) September 10, 2024

Technically, 2024 PT5’s loop around the Earth is incomplete. The waylaid space rock will take a horseshoe path around our planet, but it won’t make a full orbit. For that reason, “I’m not sure I would classify it as a mini-moon,” Lance Benner, the principal investigator of the asteroid radar research program at the Jet Propulsion Laboratory, tells the New York Times.

Still, it’s easy to appreciate the delicate physics involved in making mini-moons possible, whether they’re legitimately lunar or not. While in its original orbit around the sun, the object needs to come close enough to Earth to get diverted by our planet’s gravity. At the same time, it must approach relatively slowly, at a speed of 2,200 miles per hour—too fast, and the asteroid might escape Earth’s gravitational leash entirely. For the object to become a permanent satellite, its velocity needs to be a lot lower, Derek Richardson, an astronomer at the University of Maryland, tells Kasha Patel of the Washington Post.

This isn’t the first time mini-moons have graced Earth’s skies. Scientists recorded two such cosmic catch-and-release events in 2006 and 2020, with each mini-moon hanging around for at least a year. In 1992 and 2022, two other mini-moons rendezvoused with Earth for mere days or weeks, like 2024 PT5 will. According to Robert Lea of Space.com, mini-moons coming and going are relatively common, with several drop-ins occurring every decade.

Sometimes, objects that loop around Earth aren’t even cosmic in origin. In separate incidents in the past, scientists mistook rocket stages and the Gaia spacecraft as mini-moons. But researchers say this isn’t the case for 2024 PT5, which appears to be a natural object—perhaps a fragment of a far-flung moon.

When the new mini-moon comes around, however, most people will probably not get to witness it: “The object is too small and dim for typical amateur telescopes and binoculars,” study co-author Carlos de la Fuente Marcos, an astronomer at the Complutense University of Madrid, tells Space.com. “However, the object is well within the brightness range of typical telescopes used by professional astronomers.”

Though astronomers say the latest mini-moon is benign, researchers are training their eyes on the skies for more dramatic asteroids. For one, scientists are constantly on the alert for larger bodies that can cause potential damage. In 2022, NASA successfully deflected a roughly 530-foot-long asteroid from its path by crashing a spacecraft into it, demonstrating that humankind has at least one solution to deal with threats of a planetary scale.

Additionally, asteroids stoke interest as mining targets. These space rocks may be rich with metals, and mini-moons captured by Earth may be relatively easy to access.

After asteroid 2024 PT5 departs its terrestrial orbit, it will continue swinging around the sun, making its next pit stop at Earth in 2055. If you miss these two visits, the next mini-moon might often be just a few years away.

“Captures like this must occur frequently,” Richardson tells the Washington Post. “They are just hard to detect.”

Shi En Kim | | READ MORE

Shi En Kim is a Washington, D.C.-based freelance science journalist. Her work has appeared in National Geographic, Scientific American, the Atlantic, Popular Science and others. In 2021, she interned at Smithsonian magazine as an AAAS Mass Media Fellow.