For a brief, but exciting, time last year, Earth had two moons. A small asteroid lingered close to our planet in its orbit around the Sun, temporarily caught by Earth’s gravity to become a far away mini-moon. A closer look at the space rock reveals a rather familiar origin: Earth’s mini-moon might actually be a broken-off piece of our regular Moon.
The small near-Earth object 2024 PT5 was first spotted on August 7, 2024 by the the Asteroid Terrestrial-Impact Last Alert System (ATLAS). The tiny asteroid measures a mere 33 feet (10 meters) across, and its orbit around the Sun closely matched that of Earth, suggesting that it may have originated nearby. In fact, the asteroid may be made up of rock that broke off from the Moon’s surface and was ejected into space in the aftermath of a large impact, according a recent study published in the Astrophysical Journal Letters.
“We had a general idea that this asteroid may have come from the Moon, but the smoking gun was when we found out that it was rich in silicate minerals — not the kind that are seen on asteroids but those that have been found in lunar rock samples,” Teddy Kareta, an astronomer at Lowell Observatory in Arizona, and lead author of the study, said in a statement.
Asteroids generally originate from the main asteroid belt between the orbits of Mars and Jupiter, but there is a small population of space rocks that were flung off the surface of the Moon following a massive impact. “This is a story about the Moon as told by asteroid scientists,” Kareta added. “It’s a rare situation where we’ve gone out to study an asteroid but then strayed into new territory in terms of the questions we can ask of 2024 PT5.”
When it was first discovered, the spectrum of reflected sunlight from the mini-moon’s surface didn’t match any other type of asteroid, and instead closely matched rock from our Moon. That, in addition to how the object moves, lead the scientists behind the study to conclude that the space rock sprung from our natural satellite.
Since it wasn’t made from the same material as asteroids, the planetary scientists wanted to rule out that the mini-moon was a piece of space junk in an Earth-like orbit (In 2020, Earth had another temporary mini-moon that turned out to be an old rocket booster). The difference between an asteroid and space junk is in how they move in space, as particles of light, or photons, from the Sun exert a tiny force on objects in space, speeding them up or slowing them down.
This force from solar radiation has a lesser effect on the movement of natural objects, such as asteroids, in space. “Human-made debris is usually relatively light and gets pushed around by the pressure of sunlight,” Oscar Fuentes-Muñoz, NASA postdoctoral fellow, and co-author of the new study, said in a statement. “That 2024 PT5 doesn’t move this way indicates it is much denser than [human-made] space debris.”
The scientists behind the study also observed a lack of space weathering on the surface of the mini-moon, suggesting that the impact behind its creation likely took place a few thousand years ago.
The discovery of Moon-related asteroids could increase as telescopes become sensitive to spotting smaller space rocks. If the lunar asteroids can be traced back to a specific impact crater on the Moon, it could help scientists learn more about the cratering process of the lunar surface, according to the study.