Starting at the end of September, Earth will have two moons, but don’t get too used to it. The situation is going to last less than two months, and it’s unlikely you’ll be able to actually see this transient second moon.
The mini-moon will actually be a small asteroid, named 2024 PT5, that was discovered on August 7 by the Asteroid Terrestrial-Impact Last Alert System (ATLAS). The object is on a trajectory in which it will be temporarily caught by Earth’s gravity. According to calculations made by Carlos and Raúl de la Fuente Marcos, both researchers at Spain’s Universidad Complutense de Madrid, the asteroid will orbit the Earth from September 29 until November 25. It will complete just one orbit over that time before careening away.
2024 PT5 is very tiny, measuring just 33 feet (10 meters) wide, so don’t expect to see it with the naked eye. Even those of you equipped with backyard telescopes probably won’t get a glimpse, as the asteroid will at best have a relatively dim magnitude value of 22, according to EarthSky (objects become visible to the unaided eye at magnitudes of 6 or lower)
In the paper published in Research Notes of the AAS, the pair wrote that the dual-moon scenario is rare for our planet but not unheard of. A similar thing happened in July 2006, when a mini-moon hung around Earth for about a year. Another orbited the planet for several years, before escaping Earth’s gravitational pull in May 2020. Other objects can sometimes get pulled by Earth’s gravity but escape before completing a single full orbit around the planet. In 2020, scientists said they discovered what they believed was a mini-moon, but the data was inconclusive.
As the duo wrote in their paper, the asteroid could have originated in the Arjuna asteroid belt—a collection of near-Earth objects that have orbits similar to our planet’s. “The object is unlikely to be artificial as its short-term dynamical evolution closely resembles that of 2022 NX1,” another asteroid that became a mini-moon in 1981, and again in 2022, the pair wrote.
Amateur astronomer Tony Dunn posted a simulation of the asteroid’s path on X, which is illuminating.
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
If you miss 2024 PT5 this time around, you’ll have another chance to catch it (well, at least catch its vibes—again, you won’t be able to see it). The astronomers calculated it will fly past Earth again on January 9, 2025. After that, it won’t be close again until 2055.
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