An extra moon may be orbiting Earth — and scientists think they know exactly where it came from

A fast-spinning asteroid that orbits in time with Earth may be a wayward chunk of the moon. Now, scientists think they know exactly which lunar crater it came from.

A new study, published April 19 in the journal Nature Astronomy, finds that the near-Earth asteroid 469219 Kamo'oalewa may have been flung into space when a mile-wide (1.6 kilometers) space rock hit the moon, creating the Giordano Bruno crater.

Kamo'oalewa's light reflectance matches that of weathered lunar rock, and its size, age and spin all match up with the 13.6-mile-wide (22 km) crater, which sits on the far side of the moon, the study researchers reported.

China plans to launch a sample-return mission to the asteroid in 2025. Called Tianwen-2, the mission will return pieces of Kamo'oalewa about 2.5 years later, according to Live Science's sister site Space.com.

"The possibility of a lunar-derived origin adds unexpected intrigue to the [Tianwen-2] mission and presents additional technical challenges for the sample return," Bin Cheng, a planetary scientist at Tsinghua University and a co-author of the new study, told Science.

Related: How many moons does Earth have?

Kamo'oalewa was discovered in 2016 by researchers at Haleakala Observatory in Hawaii. It has a diameter of about 100 to 200 feet (approximately 30 to 60 meters, or about the size of a large Ferris wheel) and spins at a rapid clip of one rotation every 28 minutes. The asteroid orbits the sun in a similar path to Earth, sometimes approaching within 10 million miles (16 million km).

Follow-up studies suggested that the light spectra reflected by Kamo'oalewa was very similar to the spectra reflected by samples brought back to Earth by lunar missions, as well as to meteorites known to come from the moon.

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Cheng and his colleagues first calculated what size object and what speed of impact would be necessary to eject a fragment like Kamo'oalewa from the lunar surface, as well as what size crater would be left behind. They figured out that the asteroid could have resulted from a 45-degree impact at about 420,000 mph (18 kilometers per second) and would have left a 6-to-12-mile-wide (10 to 20 km) crater.

There are tens of thousands of craters that size on the moon, but most are ancient, the researchers wrote in their paper. Near-Earth asteroids usually last only about 10 million years, or at most up to 100 million years before they crash into the sun or a planet or get flung out of the solar system entirely. By looking at young craters, the team narrowed down the contenders to a few dozen options.

The researchers focused on Giordano Bruno, which matched the requirements for both size and age. They found that the impact that formed Giordano Bruno could have created as many as three still-extant Kamo'oalewa-like objects. This makes Giordano Bruno crater the most likely source of the asteroid, the researchers concluded.

"It's like finding out which tree a fallen leaf on the ground came from in a vast forest," Cheng wrote on X, formerly known as Twitter.

Confirmation will come after the Tianwen-2 mission brings a piece of Kamo'oalewa back to Earth. Scientists already have a sample of what is believed to be ejecta from Giordano Bruno crater in the Luna 24 sample, a bit of moon rock brought back to Earth in a 1976 NASA mission. By comparing the two, researchers could verify Kamo'oalewa's origin.

Editor's note: This article's headline was updated on April 23 at 10 a.m. ET.