Astronomers suspect an unusual near-Earth rocky object is not a typical escapee from the Solar System’s asteroid belt, but is instead a chunk of the Moon blasted into space eons ago by a spectacular impact. Now, a team of researchers has modeled what sort of lunar impact could have ejected such a gobbet of Moon and deposit it in a stable, nearby orbit. Surprisingly, only one strong candidate emerged: the asteroid strike that created the famous Giordano Bruno crater, the youngest large crater on the Moon, the group reports today in Nature Astronomy.
“The authors’ modeling techniques are solid and well established,” says geophysicist Ronald Ballouz of Johns Hopkins University. “They are able to show that ejecta from a crater the size of the Giordano Bruno … could survive for a long enough timescale in a co-orbital zone around Earth.”
The odd asteroid, known as 469219 Kamo‘oalewa, was discovered in 2016 by Pan-STARRS, a telescope system in Hawaii designed to identify potentially threatening space rocks. Kamo‘oalewa measures between 40 and 100 meters across and rotates particularly fast—once every 28 minutes. It follows an elliptical orbit around the Sun that moves in sync with Earth, giving the impression that the asteroid orbits Earth, even though it is outside the planet’s gravitational influence. The asteroid’s curious orbit and small size led to it being chosen as the first target for China’s sample return mission Tianwen-2, set for launch in 2025.
Interest in the asteroid heightened in 2021, when studies by the Large Binocular Telescope Observatory in Arizona first suggested its composition more closely resembles a Moon rock than a typical asteroid. The spectrum of the light reflected off Kamo‘oalewa revealed silicates more typical of a lunar sample. “The possibility of a lunar-derived origin adds unexpected intrigue to the [Tianwen-2] mission and presents additional technical challenges for the sample return,” says planetary scientist Bin Cheng of Tsinghua University.
Cheng and a group of international colleagues set out to calculate what could have produced such a quasi-satellite. First, they modeled what sort of impact could eject a blob of that size at the Moon’s escape velocity. The effort involved “exploring vast parameter spaces and simulating millions of particles for long timescales, often requiring weeks of computation on supercomputers,” Cheng says. The researchers calculated that ejecting a fragment of at least 36 meters across would require a projectile between 0.8 and 1.4 kilometers in size, a collision that would leave behind a crater 10 to 20 kilometers wide.
Tens of thousands of craters larger than 10 kilometers pock the Moon, but the researchers also figured the collision had to be relatively recent and the resulting crater particularly young. Asteroids in busy near-Earth space don’t typically last very long before they collide, are swallowed up, or are ejected from the Solar System. The average life span is 10 million years. Such considerations winnowed the candidate craters to a few dozen. The team then zeroed in on Giordano Bruno, 22 kilometers wide, formed by the impact of a 1.7-kilometer-wide asteroid, and by far the youngest crater of the lot.
Lying just beyond the limb of the Moon on its far side, Giordano Bruno must be young because long light-colored “rays” still radiate from it, the signature of blast debris that gets covered over relatively quickly by smaller impacts. Estimates put the crater’s age at between 1 million and 10 million years. In 1976, Russia’s Luna 24 mission—a robotic lander—returned samples to Earth thought to contain debris from the formation of Giordano Bruno. Cheng’s team noted spectral similarities between the Luna 24 samples and Kamo‘oalewa. Observations of the crater’s sides and rim also show it is made of pyroxene, a mineral detected on Kamo‘oalewa as well.
Finally, the team estimated that the crater-forming collision threw out as many as 400 Kamo‘oalewa-size fragments. Modeling their trajectories over millions of years, the team calculated that a tiny fraction of them would survive in near-Earth orbits. But, if the Giordano Bruno impact was nearer to 1 million years ago than 10 million, there could be as many as three Kamo‘oalewa-like objects still orbiting near Earth today.
If the researchers are right, and Tianwen-2 brings some of the asteroid back to Earth, it would be the first time scientists would be able to study a piece of lunar ejecta, Cheng says. The stuff that makes up Kamo‘oalewa may have come from several kilometers below the surface, making it astronomers’ only relatively recent sample of material from the lunar interior. Tianwen-2’s samples would be hugely valuable for impact science, says hypervelocity impact physicist Angela Stickle of Johns Hopkins. “If a piece of Moon ejecta is linked to a particular crater, that’s very exciting.”
Cheng says he hopes future missions, such as NASA’s NEO Surveyor, set to launch in 2027, may be able to find a larger population of lunar debris still orbiting in our neighborhood. “We can learn a lot about the crater history of the Moon, and its contribution to Earth’s environment.”
