For the first time, scientists have traced an asteroid to its exact place of origin – a particular crater on the moon. Unlike most near-Earth asteroids, which are thought to hail from the main asteroid belt between the orbits of Mars and Jupiter, asteroid 2016 HO3, also known as Kamo'oalewa, was likely blasted from the Giordano Bruno crater on the moon's far side and has been hurtling through space for several million years, according to a study published in the journal Nature Astronomy. 

Selected as the target of China’s Tianwen-2 mission, Kamo'oalewa has been in space for several million years as one of a few of Earth’s co-orbital asteroids, meaning it travels around the sun on a similar orbit as Earth. Measuring between 150 and 190 feet in diameter, the asteroid is about half the size of the "London Eye" Ferris wheel. 

According to lead study author Yifei Jiao, a visting scholar at the University of Arizona Lunar and Planetary Laboratory who is also a doctoral student at Tsinghua University in Beijing, the report is the first account of a potentially hazardous near-Earth asteroid that has been linked to a specific crater on the moon. 

Previous research pointing to Kamo'oalewa likely originating from the moon included its reflectance spectrum, which is more compatible with lunar materials rather than the general population of near-Earth asteroids, and its low orbital velocity relative to Earth, suggesting it originated close to the Earth-moon system. However, scientists had not succeeded in pinpointing its likely point of origin until now. 

To shed light on the mystery, the research team used impact and dynamical modeling. The team included colleagues from Tsinghua University, UArizona, Beihang University in Beijing, the University of British Columbia and the Observatoire de la Côte d’Azur in France.

According to the simulations, it would have required an impactor of at least 1 kilometer (0.6 mile) in diameter to launch a large fragment like Kamo'oalewa beyond the moon's gravitational pull. According to the group's model, the impact would have dug up Kamo'oalewa from deep beneath the moon's surface, leaving behind an impact crater larger than 10 to 20 kilometers (6-12 miles) in diameter. Additionally, the crater would have to be younger than the average lifetime for near-Earth objects, which spans about 10 million to 100 million years, a very short and recent period in the history of the solar system. 

While the lunar surface is riddled with thousands of craters from impacts spanning the moon's 4.5 billion year-history, only Giordano Bruno with its 14-mile diameter and estimated 4 million years of age fits the bill in terms of size and age, making it the most probable source of Kamo'oalewa's origin. The team also showed that this scenario is feasible from an impact dynamics perspective. 

The discovery comes on the heels of two previous studies led by the UArizona Lunar and Planetary Laboratory: In 2021, a team obtained the first evidence suggesting that Kamo'oalewa was different from typical near-Earth asteroids and likely a fragment of the moon. Another team then concluded that there were indeed orbital pathways, albeit rare, for lunar crater fragments to reach an unusual orbit like Kamo'oalewa's.

"This was a surprise, and many were skeptical that it could come from the moon," said co-author and Lunar and Planetary Laboratory professor Erik Asphaug. "For 50 years we have been studying rocks collected by astronauts on the surface of the moon, as well as hundreds of small lunar meteorites that were ejected randomly by asteroid impacts from all over the moon that ended up on Earth. Kamo'oalewa is kind of a missing link that connects the two."

According to co-author and UArizona planetary sciences professor Renu Malhotra, the findings open up a source of near-Earth asteroids that has not been seriously studied until now, and they have revealed previously unknown orbital pathways for the transport of rocks from and between planetary bodies.

"Testing the new model of Kamo'oalewa's origin from a specific, young lunar crater paves the way for obtaining ground-truth knowledge of the damage that asteroid impacts can cause to planetary bodies," Malhotra said. In other words, it provides scientists with a natural laboratory to test ideas around asteroid impacts and get a better idea of what the consequences of such an event might be, should humankind ever experience one. 

For a strip mall-size rock to be blasted out of the moon at several miles per second and sent into orbit required very specific circumstances, Malhotra explained.  

"You’d think the impact event would pulverize and distribute the ejecta far and wide," Asphaug said. "But there it is. So, we turned the problem around and asked ourselves, 'How can we make this happen?'" 

According to Asphaug, the model provides more than just an explanation for the origin story of one particular asteroid. How massive rocks can be ejected from the surface of a planet and survive intact can be informative for fundamental questions, such as the origin of life in the universe. One such theory, known as panspermia, suggests that life – or its ingredients – could have been brought to planetary bodies from other sources across space, in the form of "organic hitchhikers" coming along for the ride, Asphaug explained. 

"While Kamo'oalewa comes from a lifeless planet, it demonstrates how rocks ejected from Mars could carry life – at least in principle," he said.

The Giordano Bruno impact event likely would have produced tens of hundreds of 10-meter-size ejecta fragments into space, according to Jiao. 

"While most of that debris would have impacted the Earth as lunar meteorites over the course of less than a million years," he said, "a few lucky objects can survive in heliocentric orbits as near-Earth asteroids, yet to be discovered or identified."

The upcoming Tianwen-2 mission aims to return samples from Kamo'oalewa, potentially confirming its lunar origin and enriching our understanding of lunar impact dynamics and space weathering effects. Additionally, NASA's NEO Surveyor mission is anticipated to identify more members of this lunar-derived near-Earth population.

"Fans of crime drama know the importance of arriving at the scene while all the evidence is fresh," Asphaug said, pointing out the importance of sample return missions such as the UArizona-led OSIRIS-REx mission. "They open up the unsolved cases surrounding the origins of meteorites in our collections, which are thought to have come from hundreds of other primitive asteroids."

Because Kamo'oalewa is not a surface rock but was ejected from much deeper than any mission had ever sampled, Asphaug has high hopes for the Kamo'oalewa sample Tianwen-2 is expected to bring to Earth: "It will be different in important ways from any of the specimens we have so far – one of those connecting pieces that help you solve the puzzle."