A whole population of 'minimoons' - tiny natural satellites - may be quietly circling near Earth, scientists now believe, after discovering a second Moon-origin rock drifting close to our planet.
The newly observed object, named 2024 PT5, was spotted near Earth last August and is thought to be a lunar fragment, possibly blasted off the Moon during a massive impact long ago. This makes it only the second known piece of the Moon found orbiting in near-Earth space.
"If there were only one object, that would be interesting but an outlier," said Teddy Kareta, a planetary scientist at Lowell Observatory in Arizona. "If there's two, we're pretty confident that's a population."
The findings were published in The Astrophysical Journal Letters in January and presented in March at the 56th Lunar and Planetary Sciences Conference in Texas.
2024 PT5 stood out when it was first observed by astronomers in South Africa, thanks to its unusually slow movement, just 2 metres per second relative to Earth. That made it a promising candidate for the Mission Accessible Near-Earth Object Survey (MANOS), which focuses on finding asteroids that are easiest to reach with spacecraft.
The rock, just 26 to 39 feet (8 to 12 metres) wide, didn't look like a typical asteroid. Mr Kareta and his team believe it may have been ejected from the Moon's surface during a violent impact.
The discovery suggests that other such fragments might be hiding in near-Earth space. Earth regularly travels through a cloud of debris, both natural and man-made, as it orbits the Sun. Some of it is leftover rock from the early solar system, and scientists keep a close eye on these near-Earth objects (NEOs) in case any pose a threat.
Material that gets blasted off the Moon during an impact usually stays close to Earth's orbit, especially the slower pieces. 2024 PT5 was briefly called a minimoon in September because it moved alongside Earth for a short time. But it didn't stay for long and eventually drifted away. Scientists think it will come close to Earth again in 2055.
Within a week of the discovery, Mr Kareta and MANOS principal investigator Nick Moskovitz turned the Lowell Discovery Telescope toward 2024 PT5. After analysing the rock in visible and near-infrared light, they found its composition closely resembled moon rocks brought back by the Apollo missions and the Soviet Luna 24 mission.
"It's like realising a crime scene has a totally new kind of evidence you didn't know you had before," Mr Kareta told Space.com. "It might not help you solve the crime right away, but considering the importance of the task, new details to compare are always welcome."
This is only the second known lunar fragment spotted in near-Earth space. The first, called Kamo'oalewa, was discovered in 2016 and linked to the Moon in 2021.
