For several weeks each April, the faint glow of a near-Earth asteroid is intriguingly visible in scientists’ telescopes. A group of astronomers, who followed the annual event almost ritually, made a surprising statement about this small piece: “It doesn’t look like any old space rock. It looks like a broken piece of the moon.”

The piece in the size of a Ferris wheel is called Kamo`oalewa, which comes from the Hawaiian language. Evidence for lunar identity was published Thursday in the journal Nature Communications Earth & Environment.

“I’ve looked at all the near-Earth asteroid spectra we have access to, and nothing matches,” said Ben Sharkey, a graduate student in the planetary sciences department at the University of Arizona and lead author of the paper.

Instead, Sharkey and other researchers noticed that the rock had a pattern of reflected light, or spectrum, very closely related to the moon rocks brought in on NASA’s Apollo missions.

“These challenging observations were made possible by the enormous light-gathering power of the Large Binocular Telescope’s twin 8.4-metre telescopes,” said Al Conrad, a scientist at the Large Binocular Telescope Observatory and co-author of the study.

But this is not the only evidence of Kamo`oalewa’s lunar origin. A subcategory of near-Earth asteroids that orbit both the sun and Earth, the semi-satellite orbits our planet with an unusual inclination, so it only appears in the night sky once a year.

“It is unlikely that a normal near-Earth meteorite would spontaneously enter semi-satellite orbit like that of Kamo`oalewa,” Renu Malhotra, professor of planetary sciences at the University of Arizona, said in a statement.

The piece is not expected to stay in this orbit for very long. Estimating that it arrived at its current location 500 years ago, Malhotra believes its orbit will change in about 300 years.

However, despite such powerful instruments and detailed orbital analysis, the team has come a long way in unraveling the mystery of Kamo`oalewa. Due to the rare occurrence of the dim sphere, a full examination of the extraterrestrial object was possible after several years of building the datasets, finding enough evidence to confirm that it belonged to the Moon.

“We doubted ourselves to death,” says study co-author Vishnu Reddy of the Moon and Planet lab at the University of Arizona.

The project began in 2016 and took several years, but in 2020 the team missed the asteroid’s viewport due to COVID-19 restrictions. Now, in 2021, they feel comfortable with the amount of information they’ve gathered to finally announce Kamo`oalewa’s unique past. “This spring, we got some much-needed follow-up observations,” Sharkey says, concluding: “We said, ‘Wow, it’s real.’ It’s easier to explain with the moon than other ideas.”

The only unanswered question: How did Kamo`oalewa leave the Moon?

Since this is the first near-Earth asteroid to display Moon features, it’s still unclear whether the space rock is an anomaly or whether there are other lunar fragments lurking in the solar system waiting to be found. In addition, there is no information yet about how the asteroid broke away from the Moon.