The rock is about 41 metres in diameter and orbits the Sun but remains close to Earth. It may have been the result of a collision striking the Moon, based on the light it reflects.
Astronomers discovered 469219 Kamo'oalewa, a small rock orbiting the Sun, in April 2016. Now they believe that instead of being a meteor, it may have been a broken off piece of the Moon.
According to a study published in the journal Nature on November 11, 2021, the rock, which is about 41 metres in diameter, gives off a spectrum close to the Moon’s: “This spectrum is indicative of a silicate-based composition, but with reddening beyond what is typically seen amongst asteroids in the inner solar system. We compare the spectrum to those of several material analogs and conclude that the best match is with lunar-like silicates.”
Based on this finding, the authors of the study write “This interpretation implies extensive space weathering and raises the prospect that Kamo’oalewa could comprise lunar material.”
According to a statement by the authors, “Kamo’oalewa was discovered by the PanSTARRS telescope in Hawaii in 2016, and the name – found in a Hawaiian creation chant – alludes to an offspring that travels on its own.”
Kamo’oalewa is classified as a quasi-satellite – “a subcategory of near-Earth asteroids that orbit the Sun but remain relatively close to Earth. Little is known about these objects because they are faint and difficult to observe,” the authors say in the news release.
Miriam Kramer for Axios writes “Originally, scientists thought Kamo'oalewa, which was discovered in 2016, was an asteroid captured by Earth's gravity at some point in the past, but this new data suggests otherwise.”
Because of its orbit, Kamo’oalewa can be observed from Earth “for a few weeks every April.” And because its size is relatively small, it can “only be seen with one of the largest telescopes on Earth.”
A team of astronomers led by UArizona planetary sciences graduate student Ben Sharkey used the UArizona-managed Large Binocular Telescope on Mount Graham in southern Arizona and found that its “pattern of reflected light, called a spectrum, matches lunar rocks from NASA’s Apollo missions, suggesting it originated from the Moon,” the news release explains.
“My first reaction to the observations in 2019 was that I probably had made a mistake,” Ben Sharkey, the study’s lead author, tells the New York Times.
Kamo'oalewa was expected to be composed of minerals typically found on asteroids. But additional observations this spring made it clear that “the data didn’t care what we thought,” Sharkey says. Kamo'oalewa really did resemble an extremely small version of the moon. Upon making that discovery, he says, “I was both excited and confused.”
"I looked through every near-Earth asteroid spectrum we had access to, and nothing matched," says Sharkey, in the news release.
Sharkey and his adviser, UArizona associate professor of lunar and planetary sciences Vishnu Reddy debated about the body’s origins which “led to another three years of hunting for a plausible explanation.”
"We doubted ourselves to death," says Reddy, a co-author who started the project in 2016. The team missed the chance to observe the asteroid in April 2020 because of the coronavirus shutdown, so they finally “found the final piece of the puzzle in 2021.”
"This spring, we got much needed follow-up observations and went, 'Wow it is real,'" Sharkey says. "It's easier to explain with the Moon than other ideas."
Study co-author Renu Malhotra, a UArizona planetary sciences professor who led the orbit analysis portion of the study also mentions that the asteroid’s orbit is similar to the Earth’s, with the slightest tilt, and not typical of near-Earth asteroids.
"It is very unlikely that a garden-variety near-Earth asteroid would spontaneously move into a quasi-satellite orbit like Kamo’oalewa's," says Malhotra, whose lab is working on a paper to further investigate the asteroid's origins, the news release mentions. "It will not remain in this particular orbit for very long, only about 300 years in the future, and we estimate that it arrived in this orbit about 500 years ago."
The authors do not yet know how Kamo'oalewa broke off from the Moon. Miriam Kramer from Axios suggests that it’s possible “a meteor impact cleaved some material from the Moon, allowing it to form the Ferris wheel-sized object.”
“The only way to be sure is to send a spacecraft to this small body,” Paul Byrne, a planetary scientist at Washington University in St. Louis who was not involved with the study, tells the New York Times. The New York Times reports that “China’s space agency plans to land on it and collect samples for return to Earth later this decade.”
“Until then, we’re left with the possibility that, on our journey through space, we’re accompanied by the remains of a collision that punched a hole in the moon,” Dr. Byrne says, “And that’s pretty cool.”
THUMBNAIL PHOTO: The unnamed Moon crater, just over a mile (1.8 kilometers) across, is too small to see from Earth with unaided eyes. NASA/GSFC/Arizona State University
HEADLINE PHOTO: The International Space Station, with a crew of six onboard, is seen in silhouette as it transits the Moon at roughly five miles per second, Saturday, Dec. 2, 2017, in Manchester Township, York County, Pennsylvania. NASA/Joel Kowsky