How did part of the moon become a near-Earth asteroid?

Kamo'oalewa is an uncommon near-Earth asteroid that takes an odd path around the Earth and is expected to remain a companion of our planet for millions of years. 

Given its "peculiar" path around the Earth, astronomers from the University of Arizona wanted to learn more about the near-Earth asteroid. What researchers figured out after two years of studying Kamo'oalewa is that the moon is a "likely source" of the asteroid, and there might be others like it. 

The researchers published their findings recently in the Nature journal Communications Earth and Environment. A previous study indicated that its composition was not like other near-Earth asteroids but was more like the moon's. The researchers believed it ejected from a meteoroid impact on the lunar surface. 

Researchers believed that it was unlikely for a knocked-off piece of the moon to enter near-Earth orbit. They added that moon fragments that have enough energy to escape the Earth-moon system generally have too much energy to end in a quasi-satellite orbit. 

What the researchers found, however, is that a small number of these fragments could end up in such orbits. Using simulations, the research says that about 93% of moon fragments from large meteoroid impacts would result in heliocentric orbits — ones that would cause the fragment to orbit the sun. But in 6.6% of simulations, fragments like Kamo'oalewa would take more horseshoe-like orbits, which could put them in quasi-satellite-like orbits. 

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"The existence of these outcomes lends credence to the hypothesis that Kamo'oalewa could indeed be lunar ejecta," the study says.

Renu Malhotra, professor of planetary sciences and a senior author on the paper, said some of the ejected lunar materials could land on Earth, but a small number of them could orbit the sun like other near-Earth asteroids. 

"We looked at Kamo'oalewa's spectrum only because it was in an unusual orbit," Malhotra said. "If it had been a typical near-Earth asteroid, no one would have thought to find its spectrum and we wouldn't have known Kamo'oalewa could be a lunar fragment."

According to NASA's Jet Propulsion Laboratory, Kamo'oalewa is expected to make its closest approach to the Earth this week. At its closest, it will be about eight times closer to the Earth than the sun. 

The small asteroid was first discovered by scientists in 2016.