For several months last fall, a school bus-sized space rock named 2024 PT5 gained fame as Earth's new “minimoon.” Astronomers rushed to study the objects while they were still close enough to Earth to observe them in detail.
One team has now concluded that PT5 in 2024 is part of a real moon. The rock was most likely sent into space by an impact on the moon within the past tens of thousands of years, according to a study published earlier this month in The Astrophysical Journal Letters.
This is the second time scientists have linked a near-Earth object to the moon, after a 2021 study identified object 469219 Kamooarewa as having possible lunar origins. Taken together, this study has uncovered a previously hidden population of exiled moon rocks, or “lunar ejecta.”
“We knew that lunar ejecta could potentially be hidden in these near-Earth orbits,” said Teddy Caleta, a postdoctoral fellow at the Lowell Observatory in Arizona who led the study. Ta.
Earth often encounters drifting objects such as asteroids and meteorites that can become temporary satellites or “semi-satellites.” Advanced observatories such as the Asteroid Terrestrial Impact Last Alert System (ATLAS) are designed to discover these fellow travelers, who are typically only visible for weeks or months.
2024 PT5 was first detected by ATLAS on August 7, and the discovery sparked follow-up observations around the world. On August 16, Dr. Kaleta and colleagues used the Lowell Discovery Telescope in Arizona and the NASA Infrared Telescope Facility in Hawaii to scout for celestial objects.
Dr. Kaleta compared the pattern of light reflecting off the rocks to how light is observed reflecting off other extraterrestrial surfaces. He found strong matches with silicate-rich lunar particles collected by Apollo crews and Soviet robotic landers. The object also followed a characteristic “horseshoe” trajectory during its encounter with Earth, a fact that suggests a local origin.
The rocky, winding path prevented it from entering Earth's orbit, sparking debate over whether the object should really be called a minimoon. However, the term is still used loosely in both academic and popular contexts.
“In the official sense, this object wasn't orbiting the Earth, but it was actually very close,” Dr Kaleta said. “We don't have a good vocabulary to describe the exact scenario of this object moving very little relative to Earth for a short period of time.”
Vishnu Reddy, a professor of planetary science at the University of Arizona who investigated the study, said the research team had done an “excellent job” in evaluating the possible origins of PT5 in 2024.
“Given the circumstantial evidence, the moon option is the most logical,” Dr Reddy said. He also previously served as a doctoral advisor to two of the study's authors. “But we can't be sure this is part of the moon until we go and bring back samples.”
There are currently no plans to collect 2024 PT5 fragments on Earth for research on Earth. However, China is developing a mission called Tianwen-2 that will collect samples from 469219 Kamoo Alewa, another object fixed as a moon rock.
Raul de la Fuente Marcos, an astronomer at Madrid Complutense University who was not involved in the study, called this a “compelling case” for a lunar origin for 2024 PT5. This explanation is consistent with his own team's findings in an upcoming paper that identifies the third rock, 2022 NX1, as another possible part of the moon.
This growing family of moon rocks on Earth is packed with insights into our planet's backyard. A study has already attempted to trace the origin of 469219 Kamooarewa to the moon's Giordano Bruno crater, raising the possibility that future rocks will match a specific location on the moon. These efforts could shed light on the processes by which lunar rocks form on and off the moon, while also satisfying the general demand for new minimoons.
“It's good that the public is interested in asteroids that won't kill us,” Dr. Kaleta said. “What a joy!”
