A huge rock orbiting near Earth appears to have originated from the moon, the second such object known to exist, with maybe more than a dozen awaiting discovery.
The asteroid, called 2024 PT5, is about 10 metres wide. Spotted in August, it was later snared by Earth鈥檚 gravitational pull, of our planet, referred to as a mini-moon, between September and November.
Re-examining the asteroid, at Lowell Observatory in Arizona and his colleagues have found that its appearance doesn鈥檛 match that of most other known asteroids. However, looking at light reflected by the asteroid to deduce its composition 鈥� rich in pyroxene but low in olivine 鈥� revealed that it does match samples from the moon collected by the Apollo 14 mission in 1971.
Advertisement
鈥淎pollo 14 primarily sampled the lunar highlands,鈥� says Kareta, the lighter portions of the moon that we can see from Earth. This suggests something slammed into this region in relatively recent history 鈥� maybe the past 200,000 years 鈥� and blasted rocks into space including 2024 PT5, he says. 鈥淪o, if we had to bet which part of the moon this thing came from, it probably came from the highlands.鈥�
This would make 2024 PT5 the second known asteroid to have originated from the moon, after an object called Kamo鈥榦alewa was also found to have a lunar origin in 2021. Both objects are known as quasi-satellites, because they share a similar orbit to Earth, and are also red from their exposure to the sun. 鈥淚t鈥檚 like rocks getting a sunburn,鈥� says Kareta.
The existence of a second moon asteroid is 鈥渞eally exciting鈥�, says Kareta. 鈥淚t means there鈥檚 a bunch of these things out there. Nature doesn鈥檛 just make two of something.鈥� Kareta predicts there are about 16 moon asteroids orbiting near Earth, based on how often they might be produced and how long they should stay in space.
at the University of Arizona, part of the team that deduced the lunar origin of Kamo鈥榦alewa, says that moon asteroids probably only remain noticeable near Earth for a few million years, until their orbits are 鈥渄iffused into bigger space鈥�. Then they vanish among the thousands of other near-Earth asteroids. 鈥淚t would be very hard to tell where they came from,鈥� she says, because they are so small and faint.
Such lunar asteroids are unlikely to be seen too frequently because they would need just the right conditions to form. If an impact on the moon is too low energy, the resultant debris would simply fall back to the lunar surface. Too much energy, and the debris is flung into the wider solar system.
鈥淭he preferred location is the trailing hemisphere,鈥� says Malhotra, the half of the moon pointing backwards in its orbit, which would reduce the velocity of the debris and prevent it leaving the Earth-moon system.
China plans to launch a mission to visit Kamo鈥榦alewa in 2025 called Tianwen-2, which could give us insights on these objects. 鈥淭hese would be the most fresh, pristine asteroids generated in recent times,鈥� says Malhotra. 鈥淭hey might contain fingerprints of their impact history.鈥�
arXiv
