Earth’s quasi-moon Asteroid Kamo'oalewa likely blasted out of this giant Moon crater

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Less than a decade after discovering a near-Earth asteroid with lunar origins, an international group of scientists says they have determined where this mysterious object came from on the Moon's surface. 

The asteroid Kamo'oalewa was discovered in 2016 using a telescope at the Haleakala Observatory in Hawaii. The 50-meter-long asteroid is considered potentially hazardous because of its near-Earth orbit. Its proximity and orbit caught the attention of orbital dynamics researchers like Renu Malhotra, regent professor of planetary science at the University of Arizona.

"It stays close to Earth. It has been what we call a quasi-moon or a quasi-satellite of the Earth for the last almost 500 years, and probably in the future as well, for another few hundred years," Malhotra said. "It's going to stay there, and then it's going to leave this orbit for some time and then come back to it. It's in this very unusual orbit."

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The orbit was off from a typical near-Earth asteroid, and its physical properties were also unlike those of the rocky asteroids in the main asteroid belt.

In 2021, Malhotra and her fellow scientists revealed that the football-field-length asteroid had more than likely come from the Moon. To do this, they had to request observation time for the Large Binocular Telescope (LBT) and the Lowell Discovery Telescope in Arizona. 

"To get time on a large telescope is very competitive," she said. "We had to make a case, a convincing case, that it was worthwhile to spend large telescope time on a dinky little rock in the Earth's neighborhood."

Asteroid Kamo`oalewa's approximate orbit via NASA JPL's small-body database.

The data from the LBT helped provide light spectrum in the visible light range and also in infrared light, which gave the information needed to confirm this asteroid was something unusual. 

"It’s the reflected light from this asteroid was unlike other asteroids, and the closest similarity that we could find was to the spectrum, the reflected light, from the Moon," Malhotra said. "And so that led us to hypothesize that it might be, it might be not from the asteroid belt, but it might be a piece of the Moon that had gotten knocked out through an impact on the Moon."

Malhotra asked a graduate student to run computer simulations of rocks ejected from the Moon that might meet the special conditions needed to reach an orbit like Kamo'oalewa's. Last year, Arizona graduate student Jose Daniel Castro-Cisneros was the lead author of a study explaining how Kamo'oalewa got into its funky orbit. 

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The "how" became the next clue in the puzzle of Earth's mini-moon. 

These simulations helped the team narrow their candidates to the Moon's trailing hemisphere, facing away from Earth. Once they honed in on that area, the search narrowed even further to a few lunar craters.

Find an explosive match for the asteroid

The type of impact needed to create a massive space object like Kamo'oalewa could be the equivalent of a nuclear explosion on Earth. The computer programs scientists used to determine the asteroid’s potential origin were initially designed for nuclear weapons simulations because impacts from space rocks happen at high speeds, around tens of thousands of miles per hour. 

A wide view of Giordano Bruno crater on the Moon and its ejecta blanket. 

"Those codes were later adopted for the purpose of understanding the impacts of asteroids on planets," she said. "It's actually very similar physics … When there's an impact of a space rock on a planet, it really has similar energies and actually even exceeds energies of nuclear weapons."

In a study published in April, researchers say they found their match with Giordano Bruno, a young crater on the Moon that formed no later than 10 million years ago. It's also the right size and depth. Malhotra said the rocks ejected from this crater that enter orbit would need to come from a significant depth, not just the surface of the Moon. 

"The crater has to be big enough to have launched something at high enough speed to get out of the Earth's, gravity, the Earth-Moon gravity field, to actually get into this kind of orbit," Malhotra said. "It can't be too small, and it can't be too big. So there's a Goldilocks size range. It also has to be young enough because this asteroid has not been hanging around in this orbit for more than a few million years."

Sample mission planned to Asteroid Kamo'oalewa

China plans to launch a sample collection Tianwan-2 mission to the asteroid next year. With NASA missions to the Moon in the coming years, scientists hope to collect samples from Giordano Bruno crater and Kamo'oalewa, providing definitive evidence that the two are a match.

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A view of Giordano Bruno crater (21 km diameter) on the Moon taken by NASA's Lunar Reconnaissance Orbiter Camera.

(NASA/GSFC/ Arizona State University)

There are also likely many more near-Earth asteroids created from the blast that caused Giordano Bruno.

While the asteroid Kamo'oalewa was only discovered in 2016, Malhotra said it was found because it was only visible to telescope surveys for about half of its orbital period when it reflected sunlight. It is also most visible in an area with reduced coverage for survey telescopes. 

"For every one asteroid that we discover in this kind of orbit, from the same impact, there were probably several hundred that were launched during the impact that we haven't seen yet," Malhotra said. "So they are out there, and we haven't discovered them yet."

Small asteroid detections are happening more often as telescope technology increases.