Earth's new temporary mini-moon will arrive this weekend – Sky & Telescope

We're about to have some company! Starting September 29, our planet will be joined by another kind of moon. But this house-sized asteroid is only a temporary companion, and unfortunately, it's out of reach of home telescopes.

Newly discovered #asteroid 2024 PT5 is about to experience a “mini-moon event” when its geocentric energy will turn negative from September 29 to November 25.https://t.co/sAo1qSRu3J image.twitter.com/pVYAmSbkCF

— Tony Dunn (@tony873004) September 10, 2024

A set of telescopes in South Africa, known as the Asteroid Terrestrial-Impact Last Alert System, discovered this temporary mini-moon, named 2024 PT₅ — on August 7. With a magnitude of 27.6, scientists estimate it is about 10 meters (33 feet) in diameter — roughly the size of a two-story house. The rock will orbit Earth for nearly two months and will cease to be a minimoon on November 25.

Until now, only four other temporary Earth moonlets have been discovered, and there are currently no others in similar orbits. Richard Binzel (MIT), who has long specialized in determining the properties of asteroids, describes moonlets this way: “It's like if you were taking a long walk in the woods and a dog came up and kept you company for a while, before walking away. During that time, you had a companion, but it wasn't your dog.”

(Earth also has at least two “quasi-moons,” but although these appear to orbit our planet, they are much more distant and not within its gravitational influence.)

Following the discovery of the object, brothers Carlos and Raúl de la Fuente Marcos (both from the Complutense University of Madrid) determined that this particular asteroid, although not “our dog,” will be our companion for almost two months. AAS Research NotesThey found that after the asteroid leaves its mini-moon orbit in November, it will return to a horseshoe-shaped orbit, in which it sometimes approaches Earth but then moves away again.

Extensive numerical simulations show that this orbital period around the Earth is essentially a certainty, says Carlos. Sky and telescope. Unfortunately, he says, it's too faint to be seen even by most large amateur telescopes: you'd need a primary mirror of at least 30 inches and a CCD or CMOS sensor to capture it.

“Theoretical estimates suggest that brief engagements occur every few years, and long episodes may take place every one to two decades,” he says. Only one minimoon has been found so far, 2020 CD₃It remained around Earth for only a few years. “The truth is that we have very limited knowledge of this population of objects that could be our closest cosmic neighbors,” he says.

This particular object, 2024 PT₅It will return to Earth's vicinity and become a temporary minimoon again in 2055, albeit for an even shorter period. Then, after moving away again, it could return in 2084, he says. “This third one is less certain,” he says, but “encounters with minimoons tend to be recurrent.”

The origin of this object is unknown, but Alan W. Harris (Space Science Institute) says that given its velocity relative to Earth, “it is much more likely to be lunar ejecta (or space debris).” In fact, it is more likely to originate on the Moon than on Earth. 469219 Kamo'oalewaa quasi-moon that astronomers suggested earlier this year was created by a crater on the Moon's far side; Harris says it is moving too fast for this scenario to be likely.

“(2024 PT₅) is not really gravitationally bound to Earth, so in that sense it’s not really a ‘moon,’” Harris says. “It will drift away after just one orbit… it’s just passing through.”

Binzel, who was not involved in this discovery, is excited by the find. “Minimoons could literally be used as stepping stones to Mars,” he says. “A mission to a minimoon offers an opportunity for a test cruise before committing a human crew and a new class of spacecraft to a much longer-duration, much more distant mission to Mars.”

Specifically, he adds, “2024 PT₅ “It is one of the most accessible near-Earth objects yet discovered” in terms of the acceleration needed to reach it and the total duration of the mission. Of course, it won’t be around long enough to plan a mission, but the discovery of similar objects is accelerating. The first such object, 1991 VG, was discovered in 1991 by James Scotti (Spacewatch), and the second, 2006 RH₁₂₀was found in 2006. Then two (including the 2020 CD3, shown above) were found in 2020.

“The increasing discovery of minimoons is a tribute to the growing capabilities of our research efforts,” Binzel said. “There are many more to come. We just have to keep looking.”