Earth’s Mini-Moon Linked to Farside Lunar Crater

Researchers might have located the birthplace of 469219 Kamo‘oalewa, a small asteroid that has been described as Earth’s “mini-moon.” By analyzing Kamo‘oalewa’s geology and simulating different formation scenarios, they traced it back to a specific impact crater on the far side of the Moon.

On its voyage around the Sun, Earth is accompanied not only by the Moon but by quasi-satellites, objects that — despite not being bound by our planet’s gravity — co-orbit the Sun with us over long time periods. The closest and most stable of these is Kamo‘oalewa.

This neighbor of ours loops out as far as 100 times the distance of the Moon, and has a rapid spin rate, rotating every 28 minutes. At around 36 to 60 meters (118 to 197 feet) in diameter, it is barely bigger than a boulder. Indeed, after it was discovered in 2016 by astronomers at Haleakalā Observatory in Hawai‘i, who gave the object its Hawaiian name, some even speculated that it might be a piece of space junk, leftover from some unknown mission; it has since been established as natural.

Asteroids like Kamo‘oalewa are of interest to planetary geologists, for they contain clues about the history of the solar system. Kamo‘oalewa is a sort of floating Rosetta stone: a slab of rock that, once deciphered, can unlock age-old mysteries.

“Small-sized asteroids in the range of Kamo‘oalewa are the least–well understood portion of the near-Earth asteroid population,” explains Sarah Greenstreet (NSF’s NOIRLab and University of Washington). Studying the formation and evolution of these small bodies, she says, “will provide important links to their better-known, larger counterparts and benefit our understanding of the formation and evolution of the asteroid population as a whole.”

In Nature Astronomy, Yifei Jiao and Bin Cheng (Tsinghua University, China) as well as colleagues in China, Canada, France, and the U.S. based their study on previous ground-based observations of the mini-moon's reflectance — the light reflected back from Kamo‘oalewa’s surface. Ben Sharkey (University of Arizona) and others had previously pointed out that Kamo‘oalewa has the most in common with lunar samples as opposed to observations of near-Earth asteroids. Like the Moon, the asteroid also appears to be made up of olivine, pyroxene, or a combination of those minerals, and shows the effects of space weathering.

A lunar origin means that millions of years ago, a large body would have slammed into the Moon. Besides leaving behind a crater, it would also have ejected a few large fragments such as Kamo‘oalewa.

The Moon is awash with craters, so the team narrowed down the possibilities using simulations to reconstruct lunar impact events, estimating what kind of impact could have produced an asteroid of Kamo‘oalewa’s size and orbit, and what size the resulting crater would have been.

The team further whittled down candidate craters of the required size based on their age. Kamo‘oalewa is younger than most of the craters on the Moon, and it turns out only one crater could have been forged in the same impact event: a 22-kilometer-wide crater on the far side of the Moon called Giordano Bruno. Observations indicate that its mineral properties match those of the asteroid.

That scientists have been able to learn so much about an asteroid using only spectroscopy and advanced modeling techniques is testament to the power of this discipline. Sherry Fieber-Beyer (University of North Dakota) pays tribute to the contributions that different teams made to the research, noting that the previous work from Sharkey’s team “laid the groundwork” for Cheng’s team “to map the dynamics straight back to the Moon.” That connection, she underscores, “is equivalent to finding a needle in a haystack.”

Two upcoming missions offer opportunities to study Kamo‘oalewa in more detail and verify its origin. In 2025 China will launch Tianwen-2. This spacecraft will escort Kamo‘oalewa for a few months, taking up-close measurements, before dropping a lander to retrieve samples and bring them back to Earth. Then, in 2027, NASA’s NEO Surveyor mission is due to launch. As it studies near-Earth asteroids, with the primary objective of identifying any hazards, it could also turn up further space-borne flotsam from the Giordano Bruno impact event.

As Fieber-Beyer puts it, “We are on the precipice of expanding insights into the blueprint of the inner solar system.”